snitfaq(n)              Snit's Not Incr Tcl, OO system              snitfaq(n)



______________________________________________________________________________

NAME
       snitfaq - Snit Frequently Asked Questions

DESCRIPTION
OVERVIEW
   WHAT IS THIS DOCUMENT?
       This is an atypical FAQ list, in that few of the questions are
       frequently asked.  Rather, these are the questions I think a newcomer
       to Snit should be asking.  This file is not a complete reference to
       Snit, however; that information is in the snit man page.

   WHAT IS SNIT?
       Snit is a framework for defining abstract data types and megawidgets in
       pure Tcl.  The name "Snit" stands for "Snit's Not Incr Tcl", signifying
       that Snit takes a different approach to defining objects than does Incr
       Tcl, the best known object framework for Tcl.  Had I realized that Snit
       would become at all popular, I'd probably have chosen something else.

       The primary purpose of Snit is to be object glue--to help you compose
       diverse objects from diverse sources into types and megawidgets with
       clean, convenient interfaces so that you can more easily build your
       application.

       Snit isn't about theoretical purity or minimalist design; it's about
       being able to do powerful things easily and consistently without having
       to think about them--so that you can concentrate on building your
       application.

       Snit isn't about implementing thousands of nearly identical carefully-
       specified lightweight thingamajigs--not as individual Snit objects.
       Traditional Tcl methods will be much faster, and not much more
       complicated.  But Snit is about implementing a clean interface to
       manage a collection of thousands of nearly identical carefully-
       specified lightweight thingamajigs (e.g., think of the text widget and
       text tags, or the canvas widget and canvas objects).  Snit lets you
       hide the details of just how those thingamajigs are stored--so that you
       can ignore it, and concentrate on building your application.

       Snit isn't a way of life, a silver bullet, or the Fountain of Youth.
       It's just a way of managing complexity--and of managing some of the
       complexity of managing complexity--so that you can concentrate on
       building your application.

   WHAT VERSION OF TCL DOES SNIT REQUIRE?
       Snit 1.3 requires Tcl 8.3 or later; Snit 2.2 requires Tcl 8.5 or later.
       See SNIT VERSIONS for the differences between Snit 1.3 and Snit 2.2.

   WHERE CAN I DOWNLOAD SNIT?
       Snit is part of Tcllib, the standard Tcl library, so you might already
       have it.  It's also available at the Snit Home Page,
       http://www.wjduquette.com/snit.

   WHAT ARE SNIT'S GOALS?
       o      A Snit object should be at least as efficient as a hand-coded
              Tcl object (see http://www.wjduquette.com/tcl/objects.html).

       o      The fact that Snit was used in an object's implementation should
              be transparent (and irrelevant) to clients of that object.

       o      Snit should be able to encapsulate objects from other sources,
              particularly Tk widgets.

       o      Snit megawidgets should be (to the extent possible)
              indistinguishable in interface from Tk widgets.

       o      Snit should be Tclish--that is, rather than trying to emulate
              C++, Smalltalk, or anything else, it should try to emulate Tcl
              itself.

       o      It should have a simple, easy-to-use, easy-to-remember syntax.

   HOW IS SNIT DIFFERENT FROM OTHER OO FRAMEWORKS?
       Snit is unique among Tcl object systems in that it is based not on
       inheritance but on delegation.  Object systems based on inheritance
       only allow you to inherit from classes defined using the same system,
       and that's a shame.  In Tcl, an object is anything that acts like an
       object; it shouldn't matter how the object was implemented.  I designed
       Snit to help me build applications out of the materials at hand; thus,
       Snit is designed to be able to incorporate and build on any object,
       whether it's a hand-coded object, a Tk widget, an Incr Tcl object, a
       BWidget or almost anything else.

       Note that you can achieve the effect of inheritance using COMPONENTS
       and DELEGATION--and you can inherit from anything that looks like a Tcl
       object.

   WHAT CAN I DO WITH SNIT?
       Using Snit, a programmer can:

       o      Create abstract data types and Tk megawidgets.

       o      Define instance variables, type variables, and Tk-style options.

       o      Define constructors, destructors, instance methods, type
              methods, procs.

       o      Assemble a type out of component types.  Instance methods and
              options can be delegated to the component types automatically.

SNIT VERSIONS
   WHICH VERSION OF SNIT SHOULD I USE?
       The current Snit distribution includes two versions, Snit 1.3 and Snit
       2.2.  The reason that both are included is that Snit 2.2 takes
       advantage of a number of new features of Tcl 8.5 to improve run-time
       efficiency; as a side-effect, the ugliness of Snit's error messages and
       stack traces has been reduced considerably.  The cost of using Snit
       2.2, of course, is that you must target Tcl 8.5.

       Snit 1.3, on the other hand, lacks Snit 2.2's optimizations, but
       requires only Tcl 8.3 and later.

       In short, if you're targetting Tcl 8.3 or 8.4 you should use Snit 1.3.
       If you can afford to target Tcl 8.5, you should definitely use Snit
       2.2.  If you will be targetting both, you can use Snit 1.3 exclusively,
       or (if your code is unaffected by the minor incompatibilities between
       the two versions) you can use Snit 1.3 for Tcl 8.4 and Snit 2.2 for Tcl
       8.5.

   HOW DO I SELECT THE VERSION OF SNIT I WANT TO USE?
       To always use Snit 1.3 (or a later version of Snit 1.x), invoke Snit as
       follows:

              package require snit 1.3

       To always use Snit 2.2 (or a later version of Snit 2.x), say this
       instead:

              package require snit 2.2

       Note that if you request Snit 2.2 explicitly, your application will
       halt with Tcl 8.4, since Snit 2.2 is unavailable for Tcl 8.4.

       If you wish your application to always use the latest available version
       of Snit, don't specify a version number:

              package require snit

       Tcl will find and load the latest version that's available relative to
       the version of Tcl being used.  In this case, be careful to avoid using
       any incompatible features.

   HOW ARE SNIT 1.3 AND SNIT 2.2 INCOMPATIBLE?
       To the extent possible, Snit 2.2 is intended to be a drop-in
       replacement for Snit 1.3. Unfortunately, some incompatibilities were
       inevitable because Snit 2.2 uses Tcl 8.5's new namespace ensemble
       mechanism to implement subcommand dispatch.  This approach is much
       faster than the mechanism used in Snit 1.3, and also results in much
       better error messages; however, it also places new constraints on the
       implementation.

       There are four specific incompatibilities between Snit 1.3 and Snit
       2.2.

       o      Snit 1.3 supports implicit naming of objects.  Suppose you
              define a new snit::type called dog.  You can create instances of
              dog in three ways:

              dog spot               ;# Explicit naming
              set obj1 [dog %AUTO%]  ;# Automatic naming
              set obj2 [dog]         ;# Implicit naming


              In Snit 2.2, type commands are defined using the namespace
              ensemble mechanism; and namespace ensemble doesn't allow an
              ensemble command to be called without a subcommand.  In short,
              using namespace ensemble there's no way to support implicit
              naming.

              All is not lost, however.  If the type has no type methods, then
              the type command is a simple command rather than an ensemble,
              and namespace ensemble is not used.  In this case, implicit
              naming is still possible.

              In short, you can have implicit naming if you're willing to do
              without type methods (including the standard type methods, like
              $type info).  To do so, use the -hastypemethods pragma:

              pragma -hastypemethods 0

       o      Hierarchical methods and type methods are implemented
              differently in Snit 2.2.

              A hierarchical method is an instance method which has
              subcommands; these subcommands are themselves methods.  The Tk
              text widget's tag command and its subcommands are examples of
              hierarchical methods.  You can implement such subcommands in
              Snit simply by including multiple words in the method names:

              method {tag configure} {tag args} { ... }

              method {tag cget} {tag option} {...}


              Here we've implicitly defined a tag method which has two
              subcommands, configure and cget.

              In Snit 1.3, hierarchical methods could be called in two ways:

              $obj tag cget -myoption      ;# The good way
              $obj {tag cget} -myoption    ;# The weird way


              In the second call, we see that a hierarchical method or type
              method is simply one whose name contains multiple words.

              In Snit 2.2 this is no longer the case, and the "weird" way of
              calling hierarchical methods and type methods no longer works.

       o      The third incompatibility derives from the second.  In Snit 1.3,
              hierarchical methods were also simply methods whose name
              contains multiple words.  As a result, $obj info methods
              returned the full names of all hierarchical methods.  In the
              example above, the list returned by $obj info methods would
              include tag configure and tag cget but not tag, since tag is
              defined only implicitly.

              In Snit 2.2, hierarchical methods and type methods are no longer
              simply ones whose name contains multiple words; in the above
              example, the list returned by $obj info methods would include
              tag but not tag configure or tag cget.

       o      The fourth incompatibility is due to a new feature.  Snit 2.2
              uses the new namespace path command so that a type's code can
              call any command defined in the type's parent namespace without
              qualification or importation.  For example, suppose you have a
              package called mypackage which defines a number of commands
              including a type, ::mypackage::mytype.  Thanks to namespace
              path, the type's code can call any of the other commands defined
              in ::mypackage::.

              This is extremely convenient.  However, it also means that
              commands defined in the parent namespace, ::mypackage:: can
              block the type's access to identically named commands in the
              global namespace.  This can lead to bugs.  For example, Tcllib
              includes a type called ::tie::std::file.  This type's code calls
              the standard file command.  When run with Snit 2.2, the code
              broke-- the type's command, ::tie::std::file, is itself a
              command in the type's parent namespace, and so instead of
              calling the standard file command, the type found itself calling
              itself.

   ARE THERE OTHER DIFFERENCES BETWEEN SNIT 1.X AND SNIT 2.2?
       Yes.

       o      Method dispatch is considerably faster.

       o      Many error messages and stack traces are cleaner.

       o      The -simpledispatch pragma is obsolete, and ignored if present.
              In Snit 1.x, -simpledispatch substitutes a faster mechanism for
              method dispatch, at the cost of losing certain features.  Snit
              2.2 method dispatch is faster still in all cases, so
              -simpledispatch is no longer needed.

       o      In Snit 2.2, a type's code (methods, type methods, etc.) can
              call commands from the type's parent namespace without
              qualifying or importing them, i.e., type ::parentns::mytype's
              code can call ::parentns::someproc as just someproc.

              This is extremely useful when a type is defined as part of a
              larger package, and shares a parent namespace with the rest of
              the package; it means that the type can call other commands
              defined by the package without any extra work.

              This feature depends on the new Tcl 8.5 namespace path command,
              which is why it hasn't been implemented for V1.x.  V1.x code can
              achieve something similar by placing

              namespace import [namespace parent]::*

              in a type constructor.  This is less useful, however, as it
              picks up only those commands which have already been exported by
              the parent namespace at the time the type is defined.

OBJECTS
   WHAT IS AN OBJECT?
       A full description of object-oriented programming is beyond the scope
       of this FAQ, obviously.  In simple terms, an object is an instance of
       an abstract data type--a coherent bundle of code and data.  There are
       many ways to represent objects in Tcl/Tk; the best known examples are
       the Tk widgets.

       A Tk widget is an object; it is represented by a Tcl command.  The
       object's methods are subcommands of the Tcl command.  The object's
       properties are options accessed using the configure and cget methods.
       Snit uses the same conventions as Tk widgets do.

   WHAT IS AN ABSTRACT DATA TYPE?
       In computer science terms, an abstract data type is a complex data
       structure along with a set of operations--a stack, a queue, a binary
       tree, etc--that is to say, in modern terms, an object.  In systems that
       include some form of inheritance the word class is usually used instead
       of abstract data type, but as Snit doesn't implement inheritance as
       it's ordinarily understood the older term seems more appropriate.
       Sometimes this is called object-based programming as opposed to object-
       oriented programming.  Note that you can easily create the effect of
       inheritance using COMPONENTS and DELEGATION.

       In Snit, as in Tk, a type is a command that creates instances --
       objects -- which belong to the type.  Most types define some number of
       options which can be set at creation time, and usually can be changed
       later.

       Further, an instance is also a Tcl command--a command that gives access
       to the operations which are defined for that abstract data type.
       Conventionally, the operations are defined as subcommands of the
       instance command.  For example, to insert text into a Tk text widget,
       you use the text widget's insert subcommand:

                  # Create a text widget and insert some text in it.
                  text .mytext -width 80 -height 24
                  .mytext insert end "Howdy!"


       In this example, text is the type command and .mytext is the instance
       command.

       In Snit, object subcommands are generally called INSTANCE METHODS.

   WHAT KINDS OF ABSTRACT DATA TYPES DOES SNIT PROVIDE?
       Snit allows you to define three kinds of abstract data type:

       o      snit::type

       o      snit::widget

       o      snit::widgetadaptor

   WHAT IS A SNIT::TYPE?
       A snit::type is a non-GUI abstract data type, e.g., a stack or a queue.
       snit::types are defined using the snit::type command.  For example, if
       you were designing a kennel management system for a dog breeder, you'd
       need a dog type.

              % snit::type dog {
                  # ...
              }
              ::dog
              %


       This definition defines a new command (::dog, in this case) that can be
       used to define dog objects.

       An instance of a snit::type can have INSTANCE METHODS, INSTANCE
       VARIABLES, OPTIONS, and COMPONENTS.  The type itself can have TYPE
       METHODS, TYPE VARIABLES, TYPE COMPONENTS, and PROCS.

   WHAT IS A SNIT::WIDGET?, THE SHORT STORY
       A snit::widget is a Tk megawidget built using Snit; it is very similar
       to a snit::type.  See WIDGETS.

   WHAT IS A SNIT::WIDGETADAPTOR?, THE SHORT STORY
       A snit::widgetadaptor uses Snit to wrap an existing widget type (e.g.,
       a Tk label), modifying its interface to a lesser or greater extent.  It
       is very similar to a snit::widget.  See WIDGET ADAPTORS.

   HOW DO I CREATE AN INSTANCE OF A SNIT::TYPE?
       You create an instance of a snit::type by passing the new instance's
       name to the type's create method.  In the following example, we create
       a dog object called spot.

              % snit::type dog {
                  # ....
              }
              ::dog
              % dog create spot
              ::spot
              %


       In general, the create method name can be omitted so long as the
       instance name doesn't conflict with any defined TYPE METHODS. (See TYPE
       COMPONENTS for the special case in which this doesn't work.)  So the
       following example is identical to the previous example:

              % snit::type dog {
                  # ....
              }
              ::dog
              % dog spot
              ::spot
              %


       This document generally uses the shorter form.

       If the dog type defines OPTIONS, these can usually be given defaults at
       creation time:

              % snit::type dog {
                  option -breed mongrel
                  option -color brown

                  method bark {} { return "$self barks." }
              }
              ::dog
              % dog create spot -breed dalmation -color spotted
              ::spot
              % spot cget -breed
              dalmation
              % spot cget -color
              spotted
              %


       Once created, the instance name now names a new Tcl command that is
       used to manipulate the object.  For example, the following code makes
       the dog bark:

              % spot bark
              ::spot barks.
              %


   HOW DO I REFER TO AN OBJECT INDIRECTLY?
       Some programmers prefer to save the object name in a variable, and
       reference it that way.  For example,

              % snit::type dog { ... }
              ::dog
              % set d [dog spot -breed dalmation -color spotted]
              ::spot
              % $d cget -breed
              dalmation
              % $d bark
              ::spot barks.
              %


       If you prefer this style, you might prefer to have Snit generate the
       instance's name automatically.

   HOW CAN I GENERATE THE OBJECT NAME AUTOMATICALLY?
       If you'd like Snit to generate an object name for you, use the %AUTO%
       keyword as the requested name:

              % snit::type dog { ... }
              ::dog
              % set d [dog %AUTO%]
              ::dog2
              % $d bark
              ::dog2 barks.
              %


       The %AUTO% keyword can be embedded in a longer string:

              % set d [dog obj_%AUTO%]
              ::obj_dog4
              % $d bark
              ::obj_dog4 barks.
              %


   CAN TYPES BE RENAMED?
       Tcl's rename command renames other commands.  It's a common technique
       in Tcl to modify an existing command by renaming it and defining a new
       command with the original name; the new command usually calls the
       renamed command.

       snit::type commands, however, should never be renamed; to do so breaks
       the connection between the type and its objects.

   CAN OBJECTS BE RENAMED?
       Tcl's rename command renames other commands.  It's a common technique
       in Tcl to modify an existing command by renaming it and defining a new
       command with the original name; the new command usually calls the
       renamed command.

       All Snit objects (including widgets and widgetadaptors) can be renamed,
       though this flexibility has some consequences:

       o      In an instance method, the implicit argument self will always
              contain the object's current name, so instance methods can
              always call other instance methods using $self.

       o      If the object is renamed, however, then $self's value will
              change.  Therefore, don't use $self for anything that will break
              if $self changes. For example, don't pass a callback command to
              another object like this:


                  .btn configure -command [list $self ButtonPress]


              You'll get an error if .btn calls your command after your object
              is renamed.

       o      Instead, your object should define its callback command like
              this:


                  .btn configure -command [mymethod ButtonPress]


              The mymethod command returns code that will call the desired
              method safely; the caller of the callback can add additional
              arguments to the end of the command as usual.

       o      Every object has a private namespace; the name of this namespace
              is available in method bodies, etc., as the value of the
              implicit argument selfns.  This value is constant for the life
              of the object.  Use $selfns instead of $self if you need a
              unique token to identify the object.

       o      When a snit::widget's instance command is renamed, its Tk window
              name remains the same -- and is still extremely important.
              Consequently, the Tk window name is available in method bodies
              as the value of the implicit argument win.  This value is
              constant for the life of the object.  When creating child
              windows, it's best to use $win.child rather than $self.child as
              the name of the child window.

   HOW DO I DESTROY A SNIT OBJECT?
       Any Snit object of any type can be destroyed by renaming it to the
       empty string using the Tcl rename command.

       Snit megawidgets (i.e., instances of snit::widget and
       snit::widgetadaptor) can be destroyed like any other widget: by using
       the Tk destroy command on the widget or on one of its ancestors in the
       window hierarchy.

       Every instance of a snit::type has a destroy method:

              % snit::type dog { ... }
              ::dog
              % dog spot
              ::spot
              % spot bark
              ::spot barks.
              % spot destroy
              % spot barks
              invalid command name "spot"
              %


       Finally, every Snit type has a type method called destroy; calling it
       destroys the type and all of its instances:

              % snit::type dog { ... }
              ::dog
              % dog spot
              ::spot
              % spot bark
              ::spot barks.
              % dog destroy
              % spot bark
              invalid command name "spot"
              % dog fido
              invalid command name "dog"
              %


INSTANCE METHODS
   WHAT IS AN INSTANCE METHOD?
       An instance method is a procedure associated with a specific object and
       called as a subcommand of the object's command.  It is given free
       access to all of the object's type variables, instance variables, and
       so forth.

   HOW DO I DEFINE AN INSTANCE METHOD?
       Instance methods are defined in the type definition using the method
       statement.  Consider the following code that might be used to add dogs
       to a computer simulation:

              % snit::type dog {
                  method bark {} {
                      return "$self barks."
                  }

                  method chase {thing} {
                      return "$self chases $thing."
                  }
              }
              ::dog
              %


       A dog can bark, and it can chase things.

       The method statement looks just like a normal Tcl proc, except that it
       appears in a snit::type definition.  Notice that every instance method
       gets an implicit argument called self; this argument contains the
       object's name.  (There's more on implicit method arguments below.)

   HOW DOES A CLIENT CALL AN INSTANCE METHOD?
       The method name becomes a subcommand of the object.  For example, let's
       put a simulated dog through its paces:

              % dog spot
              ::spot
              % spot bark
              ::spot barks.
              % spot chase cat
              ::spot chases cat.
              %


   HOW DOES AN INSTANCE METHOD CALL ANOTHER INSTANCE METHOD?
       If method A needs to call method B on the same object, it does so just
       as a client does: it calls method B as a subcommand of the object
       itself, using the object name stored in the implicit argument self.

       Suppose, for example, that our dogs never chase anything without
       barking at them:

              % snit::type dog {
                  method bark {} {
                      return "$self barks."
                  }

                  method chase {thing} {
                      return "$self chases $thing.  [$self bark]"
                  }
              }
              ::dog
              % dog spot
              ::spot
              % spot bark
              ::spot barks.
              % spot chase cat
              ::spot chases cat.  ::spot barks.
              %


   ARE THERE ANY LIMITATIONS ON INSTANCE METHOD NAMES?
       Not really, so long as you avoid the standard instance method names:
       configure, configurelist, cget, destroy, and info.  Also, method names
       consisting of multiple words define hierarchical methods.

   WHAT IS A HIERARCHICAL METHOD?
       An object's methods are subcommands of the object's instance command.
       Hierarchical methods allow an object's methods to have subcommands of
       their own; and these can in turn have subcommands, and so on.  This
       allows the programmer to define a tree-shaped command structure, such
       as is used by many of the Tk widgets--the subcommands of the Tk text
       widget's tag method are hierarchical methods.

   HOW DO I DEFINE A HIERARCHICAL METHOD?
       Define methods whose names consist of multiple words.  These words
       define the hierarchy implicitly.  For example, the following code
       defines a tag method with subcommands cget and configure:

              snit::widget mytext {
                  method {tag configure} {tag args} { ... }

                  method {tag cget} {tag option} {...}
              }

       Note that there is no explicit definition for the tag method; it is
       implicit in the definition of tag configure and tag cget.  If you tried
       to define tag explicitly in this example, you'd get an error.

   HOW DO I CALL HIERARCHICAL METHODS?
       As subcommands of subcommands.

              % mytext .text
              % .text tag configure redtext -foreground red -background black
              % .text tag cget redtext -foreground
              red
              %


   HOW DO I MAKE AN INSTANCE METHOD PRIVATE?
       It's often useful to define private methods, that is, instance methods
       intended to be called only by other methods of the same object.

       Snit doesn't implement any access control on instance methods, so all
       methods are de facto public.  Conventionally, though, the names of
       public methods begin with a lower-case letter, and the names of private
       methods begin with an upper-case letter.

       For example, suppose our simulated dogs only bark in response to other
       stimuli; they never bark just for fun.  So the bark method becomes Bark
       to indicate that it is private:

              % snit::type dog {
                  # Private by convention: begins with uppercase letter.
                  method Bark {} {
                      return "$self barks."
                  }

                  method chase {thing} {
                      return "$self chases $thing. [$self Bark]"
                  }
              }
              ::dog
              % dog fido
              ::fido
              % fido chase cat
              ::fido chases cat. ::fido barks.
              %


   ARE THERE ANY LIMITATIONS ON INSTANCE METHOD ARGUMENTS?
       Method argument lists are defined just like normal Tcl proc argument
       lists; in particular, they can include arguments with default values
       and the args argument.

       However, every method also has a number of implicit arguments provided
       by Snit in addition to those explicitly defined.  The names of these
       implicit arguments may not used to name explicit arguments.

   WHAT IMPLICIT ARGUMENTS ARE PASSED TO EACH INSTANCE METHOD?
       The arguments implicitly passed to every method are type, selfns, win,
       and self.

   WHAT IS $TYPE?
       The implicit argument type contains the fully qualified name of the
       object's type:

              % snit::type thing {
                  method mytype {} {
                      return $type
                  }
              }
              ::thing
              % thing something
              ::something
              % something mytype
              ::thing
              %


   WHAT IS $SELF?
       The implicit argument self contains the object's fully qualified name.

       If the object's command is renamed, then $self will change to match in
       subsequent calls.  Thus, your code should not assume that $self is
       constant unless you know for sure that the object will never be
       renamed.

              % snit::type thing {
                  method myself {} {
                      return $self
                  }
              }
              ::thing
              % thing mutt
              ::mutt
              % mutt myself
              ::mutt
              % rename mutt jeff
              % jeff myself
              ::jeff
              %


   WHAT IS $SELFNS?
       Each Snit object has a private namespace in which to store its INSTANCE
       VARIABLES and OPTIONS.  The implicit argument selfns contains the name
       of this namespace; its value never changes, and is constant for the
       life of the object, even if the object's name changes:

              % snit::type thing {
                  method myNameSpace {} {
                      return $selfns
                  }
              }
              ::thing
              % thing jeff
              ::jeff
              % jeff myNameSpace
              ::thing::Snit_inst3
              % rename jeff mutt
              % mutt myNameSpace
              ::thing::Snit_inst3
              %


       The above example reveals how Snit names an instance's private
       namespace; however, you should not write code that depends on the
       specific naming convention, as it might change in future releases.

   WHAT IS $WIN?
       The implicit argument win is defined for all Snit methods, though it
       really makes sense only for those of WIDGETS and WIDGET ADAPTORS.  $win
       is simply the original name of the object, whether it's been renamed or
       not.  For widgets and widgetadaptors, it is also therefore the name of
       a Tk window.

       When a snit::widgetadaptor is used to modify the interface of a widget
       or megawidget, it must rename the widget's original command and replace
       it with its own.

       Thus, using win whenever the Tk window name is called for means that a
       snit::widget or snit::widgetadaptor can be adapted by a
       snit::widgetadaptor.  See WIDGETS for more information.

   HOW DO I PASS AN INSTANCE METHOD AS A CALLBACK?
       It depends on the context.

       Suppose in my application I have a dog object named fido, and I want
       fido to bark when a Tk button called .bark is pressed.  In this case, I
       create the callback command in the usual way, using list:

                  button .bark -text "Bark!" -command [list fido bark]


       In typical Tcl style, we use a callback to hook two independent
       components together.  But suppose that the dog object has a graphical
       interface and owns the button itself?  In this case, the dog must pass
       one of its own instance methods to the button it owns.  The obvious
       thing to do is this:

              % snit::widget dog {
                  constructor {args} {
                      #...
                      button $win.barkbtn -text "Bark!" -command [list $self bark]
                      #...
                  }
              }
              ::dog
              %


       (Note that in this example, our dog becomes a snit::widget, because it
       has GUI behavior.  See WIDGETS for more.)  Thus, if we create a dog
       called .spot, it will create a Tk button called .spot.barkbtn; when
       pressed, the button will call $self bark.

       Now, this will work--provided that .spot is never renamed to something
       else.  But surely renaming widgets is abnormal?  And so it is--unless
       .spot is the hull component of a snit::widgetadaptor.  If it is, then
       it will be renamed, and .spot will become the name of the
       snit::widgetadaptor object.  When the button is pressed, the command
       $self bark will be handled by the snit::widgetadaptor, which might or
       might not do the right thing.

       There's a safer way to do it, and it looks like this:

              % snit::widget dog {
                  constructor {args} {
                      #...
                      button $win.barkbtn -text "Bark!" -command [mymethod bark]
                      #...
                  }
              }
              ::dog
              %


       The command mymethod takes any number of arguments, and can be used
       like list to build up a callback command; the only difference is that
       mymethod returns a form of the command that won't change even if the
       instance's name changes.

       On the other hand, you might prefer to allow a widgetadaptor to
       override a method such that your renamed widget will call the
       widgetadaptor's method instead of its own.  In this case, using [list
       $self bark] will do what you want...but this is a technique which
       should be used only in carefully controlled circumstances.

   HOW DO I DELEGATE INSTANCE METHODS TO A COMPONENT?
       See DELEGATION.

INSTANCE VARIABLES
   WHAT IS AN INSTANCE VARIABLE?
       An instance variable is a private variable associated with some
       particular Snit object.  Instance variables can be scalars or arrays.

   HOW IS A SCALAR INSTANCE VARIABLE DEFINED?
       Scalar instance variables are defined in the type definition using the
       variable statement.  You can simply name it, or you can initialize it
       with a value:

              snit::type mytype {
                  # Define variable "greeting" and initialize it with "Howdy!"
                  variable greeting "Howdy!"
              }


   HOW IS AN ARRAY INSTANCE VARIABLE DEFINED?
       Array instance variables are also defined in the type definition using
       the variable command.  You can initialize them at the same time by
       specifying the -array option:

              snit::type mytype {
                  # Define array variable "greetings"
                  variable greetings -array {
                      formal "Good Evening"
                      casual "Howdy!"
                  }
              }


   WHAT HAPPENS IF I DON'T INITIALIZE AN INSTANCE VARIABLE?
       Variables do not really exist until they are given values.  If you do
       not initialize a variable when you define it, then you must be sure to
       assign a value to it (in the constructor, say, or in some method)
       before you reference it.

   ARE THERE ANY LIMITATIONS ON INSTANCE VARIABLE NAMES?
       Just a few.

       First, every Snit object has a built-in instance variable called
       options, which should never be redefined.

       Second, all names beginning with "Snit_" are reserved for use by Snit
       internal code.

       Third, instance variable names containing the namespace delimiter (::)
       are likely to cause great confusion.

   DO I NEED TO DECLARE MY INSTANCE VARIABLES IN MY METHODS?
       No. Once you've defined an instance variable in the type definition, it
       can be used in any instance code (instance methods, the constructor,
       and the destructor) without declaration.  This differs from normal Tcl
       practice, in which all non-local variables in a proc need to be
       declared.

       There is a speed penalty to having all instance variables implicitly
       available in all instance code.  Even though your code need not declare
       the variables explicitly, Snit must still declare them, and that takes
       time.  If you have ten instance variables, a method that uses none of
       them must still pay the declaration penalty for all ten.  In most
       cases, the additional runtime cost is negligible.  If extreme cases,
       you might wish to avoid it; there are two methods for doing so.

       The first is to define a single instance variable, an array, and store
       all of your instance data in the array.  This way, you're only paying
       the declaration penalty for one variable--and you probably need the
       variable most of the time anyway.  This method breaks down if your
       instance variables include multiple arrays; in Tcl 8.5, however, the
       dict command might come to your rescue.

       The second method is to declare your instance variables explicitly in
       your instance code, while not including them in the type definition:

              snit::type dog {
                  constructor {} {
                      variable mood

                      set mood happy
                  }

                  method setmood {newMood} {
                      variable mood

                      set mood $newMood
                  }

                  method getmood {} {
                      variable mood

                      return $mood
                  }
              }

       This allows you to ensure that only the required variables are included
       in each method, at the cost of longer code and run-time errors when you
       forget to declare a variable you need.

   HOW DO I PASS AN INSTANCE VARIABLE'S NAME TO ANOTHER OBJECT?
       In Tk, it's common to pass a widget a variable name; for example, Tk
       label widgets have a -textvariable option which names the variable
       which will contain the widget's text.  This allows the program to
       update the label's value just by assigning a new value to the variable.

       If you naively pass the instance variable name to the label widget,
       you'll be confused by the result; Tk will assume that the name names a
       global variable.  Instead, you need to provide a fully-qualified
       variable name.  From within an instance method or a constructor, you
       can fully qualify the variable's name using the myvar command:

              snit::widget mywidget {
                  variable labeltext ""

                  constructor {args} {
                      # ...

                      label $win.label -textvariable [myvar labeltext]

                      # ...
                  }
              }


   HOW DO I MAKE AN INSTANCE VARIABLE PUBLIC?
       Practically speaking, you don't.  Instead, you'll implement public
       variables as OPTIONS.  Alternatively, you can write INSTANCE METHODS to
       set and get the variable's value.

OPTIONS
   WHAT IS AN OPTION?
       A type's options are the equivalent of what other object-oriented
       languages would call public member variables or properties: they are
       data values which can be retrieved and (usually) set by the clients of
       an object.

       Snit's implementation of options follows the Tk model fairly exactly,
       except that snit::type objects usually don't interact with THE TK
       OPTION DATABASE; snit::widget and snit::widgetadaptor objects, on the
       other hand, always do.

   HOW DO I DEFINE AN OPTION?
       Options are defined in the type definition using the option statement.
       Consider the following type, to be used in an application that manages
       a list of dogs for a pet store:

              snit::type dog {
                  option -breed -default mongrel
                  option -color -default brown
                  option -akc   -default 0
                  option -shots -default 0
              }


       According to this, a dog has four notable properties: a breed, a color,
       a flag that says whether it's pedigreed with the American Kennel Club,
       and another flag that says whether it has had its shots.  The default
       dog, evidently, is a brown mutt.

       There are a number of options you can specify when defining an option;
       if -default is the only one, you can omit the word -default as follows:

              snit::type dog {
                  option -breed mongrel
                  option -color brown
                  option -akc   0
                  option -shots 0
              }


       If no -default value is specified, the option's default value will be
       the empty string (but see THE TK OPTION DATABASE).

       The Snit man page refers to options like these as "locally defined"
       options.

   HOW CAN A CLIENT SET OPTIONS AT OBJECT CREATION?
       The normal convention is that the client may pass any number of options
       and their values after the object's name at object creation.  For
       example, the ::dog command defined in the previous answer can now be
       used to create individual dogs.  Any or all of the options may be set
       at creation time.

              % dog spot -breed beagle -color "mottled" -akc 1 -shots 1
              ::spot
              % dog fido -shots 1
              ::fido
              %


       So ::spot is a pedigreed beagle; ::fido is a typical mutt, but his
       owners evidently take care of him, because he's had his shots.

       Note: If the type defines a constructor, it can specify a different
       object-creation syntax.  See CONSTRUCTORS for more information.

   HOW CAN A CLIENT RETRIEVE AN OPTION'S VALUE?
       Retrieve option values using the cget method:

              % spot cget -color
              mottled
              % fido cget -breed
              mongrel
              %


   HOW CAN A CLIENT SET OPTIONS AFTER OBJECT CREATION?
       Any number of options may be set at one time using the configure
       instance method.  Suppose that closer inspection shows that ::fido is
       not a brown mongrel, but rather a rare Arctic Boar Hound of a lovely
       dun color:

              % fido configure -color dun -breed "Arctic Boar Hound"
              % fido cget -color
              dun
              % fido cget -breed
              Arctic Boar Hound


       Alternatively, the configurelist method takes a list of options and
       values; occasionally this is more convenient:

              % set features [list -color dun -breed "Arctic Boar Hound"]
              -color dun -breed {Arctic Boar Hound}
              % fido configurelist $features
              % fido cget -color
              dun
              % fido cget -breed
              Arctic Boar Hound
              %


       In Tcl 8.5, the * keyword can be used with configure in this case:

              % set features [list -color dun -breed "Arctic Boar Hound"]
              -color dun -breed {Arctic Boar Hound}
              % fido configure {*}$features
              % fido cget -color
              dun
              % fido cget -breed
              Arctic Boar Hound
              %


       The results are the same.

   HOW SHOULD AN INSTANCE METHOD ACCESS AN OPTION VALUE?
       There are two ways an instance method can set and retrieve an option's
       value.  One is to use the configure and cget methods, as shown below.

              % snit::type dog {
                  option -weight 10

                  method gainWeight {} {
                      set wt [$self cget -weight]
                      incr wt
                      $self configure -weight $wt
                  }
              }
              ::dog
              % dog fido
              ::fido
              % fido cget -weight
              10
              % fido gainWeight
              % fido cget -weight
              11
              %


       Alternatively, Snit provides a built-in array instance variable called
       options.  The indices are the option names; the values are the option
       values.  The method gainWeight can thus be rewritten as follows:

                  method gainWeight {} {
                      incr options(-weight)
                  }


       As you can see, using the options variable involves considerably less
       typing and is the usual way to do it.  But if you use -configuremethod
       or -cgetmethod (described in the following answers), you might wish to
       use the configure and cget methods anyway, just so that any special
       processing you've implemented is sure to get done.  Also, if the option
       is delegated to a component then configure and cget are the only way to
       access it without accessing the component directly.  See DELEGATION for
       more information.

   HOW CAN I MAKE AN OPTION READ-ONLY?
       Define the option with -readonly yes.

       Suppose you've got an option that determines how instances of your type
       are constructed; it must be set at creation time, after which it's
       constant.  For example, a dog never changes its breed; it might or
       might not have had its shots, and if not can have them at a later time.
       -breed should be read-only, but -shots should not be.

              % snit::type dog {
                  option -breed -default mongrel -readonly yes
                  option -shots -default no
              }
              ::dog
              % dog fido -breed retriever
              ::fido
              % fido configure -shots yes
              % fido configure -breed terrier
              option -breed can only be set at instance creation
              %


   HOW CAN I CATCH ACCESSES TO AN OPTION'S VALUE?
       Define a -cgetmethod for the option.

   WHAT IS A -CGETMETHOD?
       A -cgetmethod is a method that's called whenever the related option's
       value is queried via the cget instance method.  The handler can compute
       the option's value, retrieve it from a database, or do anything else
       you'd like it to do.

       Here's what the default behavior would look like if written using a
       -cgetmethod:

              snit::type dog {
                  option -color -default brown -cgetmethod GetOption

                  method GetOption {option} {
                      return $options($option)
                  }
              }


       Any instance method can be used, provided that it takes one argument,
       the name of the option whose value is to be retrieved.

   HOW CAN I CATCH CHANGES TO AN OPTION'S VALUE?
       Define a -configuremethod for the option.

   WHAT IS A -CONFIGUREMETHOD?
       A -configuremethod is a method that's called whenever the related
       option is given a new value via the configure or configurelist instance
       methods. The method can pass the value on to some other object, store
       it in a database, or do anything else you'd like it to do.

       Here's what the default configuration behavior would look like if
       written using a -configuremethod:

              snit::type dog {
                  option -color -default brown -configuremethod SetOption

                  method SetOption {option value} {
                      set options($option) $value
                  }
              }


       Any instance method can be used, provided that it takes two arguments,
       the name of the option and the new value.

       Note that if your method doesn't store the value in the options array,
       the options array won't get updated.

   HOW CAN I VALIDATE AN OPTION'S VALUE?
       Define a -validatemethod.

   WHAT IS A -VALIDATEMETHOD?
       A -validatemethod is a method that's called whenever the related option
       is given a new value via the configure or configurelist instance
       methods.  It's the method's responsibility to determine whether the new
       value is valid, and throw an error if it isn't.  The -validatemethod,
       if any, is called before the value is stored in the options array; in
       particular, it's called before the -configuremethod, if any.

       For example, suppose an option always takes a Boolean value.  You can
       ensure that the value is in fact a valid Boolean like this:

              % snit::type dog {
                  option -shots -default no -validatemethod BooleanOption

                  method BooleanOption {option value} {
                      if {![string is boolean -strict $value]} {
                          error "expected a boolean value, got \"$value\""
                      }
                  }
              }
              ::dog
              % dog fido
              % fido configure -shots yes
              % fido configure -shots NotABooleanValue
              expected a boolean value, got "NotABooleanValue"
              %

       Note that the same -validatemethod can be used to validate any number
       of boolean options.

       Any method can be a -validatemethod provided that it takes two
       arguments, the option name and the new option value.

TYPE VARIABLES
   WHAT IS A TYPE VARIABLE?
       A type variable is a private variable associated with a Snit type
       rather than with a particular instance of the type.  In C++ and Java,
       the term static member variable is used for the same notion.  Type
       variables can be scalars or arrays.

   HOW IS A SCALAR TYPE VARIABLE DEFINED?
       Scalar type variables are defined in the type definition using the
       typevariable statement.  You can simply name it, or you can initialize
       it with a value:

              snit::type mytype {
                  # Define variable "greeting" and initialize it with "Howdy!"
                  typevariable greeting "Howdy!"
              }


       Every object of type mytype now has access to a single variable called
       greeting.

   HOW IS AN ARRAY-VALUED TYPE VARIABLE DEFINED?
       Array-valued type variables are also defined using the typevariable
       command; to initialize them, include the -array option:

              snit::type mytype {
                  # Define typearray variable "greetings"
                  typevariable greetings -array {
                      formal "Good Evening"
                      casual "Howdy!"
                  }
              }


   WHAT HAPPENS IF I DON'T INITIALIZE A TYPE VARIABLE?
       Variables do not really exist until they are given values.  If you do
       not initialize a variable when you define it, then you must be sure to
       assign a value to it (in the type constructor, say) before you
       reference it.

   ARE THERE ANY LIMITATIONS ON TYPE VARIABLE NAMES?
       Type variable names have the same restrictions as the names of INSTANCE
       VARIABLES do.

   DO I NEED TO DECLARE MY TYPE VARIABLES IN MY METHODS?
       No. Once you've defined a type variable in the type definition, it can
       be used in INSTANCE METHODS or TYPE METHODS without declaration.  This
       differs from normal Tcl practice, in which all non-local variables in a
       proc need to be declared.

       Type variables are subject to the same speed/readability tradeoffs as
       instance variables; see Do I need to declare my instance variables in
       my methods?

   HOW DO I PASS A TYPE VARIABLE'S NAME TO ANOTHER OBJECT?
       In Tk, it's common to pass a widget a variable name; for example, Tk
       label widgets have a -textvariable option which names the variable
       which will contain the widget's text.  This allows the program to
       update the label's value just by assigning a new value to the variable.

       If you naively pass a type variable name to the label widget, you'll be
       confused by the result; Tk will assume that the name names a global
       variable.  Instead, you need to provide a fully-qualified variable
       name.  From within an instance method or a constructor, you can fully
       qualify the type variable's name using the mytypevar command:

              snit::widget mywidget {
                  typevariable labeltext ""

                  constructor {args} {
                      # ...

                      label $win.label -textvariable [mytypevar labeltext]

                      # ...
                  }
              }


   HOW DO I MAKE A TYPE VARIABLE PUBLIC?
       There are two ways to do this.  The preferred way is to write a pair of
       TYPE METHODS to set and query the type variable's value.

       Type variables are stored in the type's namespace, which has the same
       name as the type itself.  Thus, you can also publicize the type
       variable's name in your documentation so that clients can access it
       directly.  For example,

              snit::type mytype {
                  typevariable myvariable
              }

              set ::mytype::myvariable "New Value"


TYPE METHODS
   WHAT IS A TYPE METHOD?
       A type method is a procedure associated with the type itself rather
       than with any specific instance of the type, and called as a subcommand
       of the type command.

   HOW DO I DEFINE A TYPE METHOD?
       Type methods are defined in the type definition using the typemethod
       statement:

              snit::type dog {
                  # List of pedigreed dogs
                  typevariable pedigreed

                  typemethod pedigreedDogs {} {
                      return $pedigreed
                  }
              }


       Suppose the dog type maintains a list of the names of the dogs that
       have pedigrees.  The pedigreedDogs type method returns this list.

       The typemethod statement looks just like a normal Tcl proc, except that
       it appears in a snit::type definition.  Notice that every type method
       gets an implicit argument called type, which contains the fully-
       qualified type name.

   HOW DOES A CLIENT CALL A TYPE METHOD?
       The type method name becomes a subcommand of the type's command.  For
       example, assuming that the constructor adds each pedigreed dog to the
       list of pedigreedDogs,

              snit::type dog {
                  option -pedigreed 0

                  # List of pedigreed dogs
                  typevariable pedigreed

                  typemethod pedigreedDogs {} {
                      return $pedigreed
                  }

                  # ...
              }

              dog spot -pedigreed 1
              dog fido

              foreach dog [dog pedigreedDogs] { ... }


   ARE THERE ANY LIMITATIONS ON TYPE METHOD NAMES?
       Not really, so long as you avoid the standard type method names:
       create, destroy, and info.

   HOW DO I MAKE A TYPE METHOD PRIVATE?
       It's sometimes useful to define private type methods, that is, type
       methods intended to be called only by other type or instance methods of
       the same object.

       Snit doesn't implement any access control on type methods; by
       convention, the names of public methods begin with a lower-case letter,
       and the names of private methods begin with an upper-case letter.

       Alternatively, a Snit proc can be used as a private type method; see
       PROCS.

   ARE THERE ANY LIMITATIONS ON TYPE METHOD ARGUMENTS?
       Method argument lists are defined just like normal Tcl proc argument
       lists; in particular, they can include arguments with default values
       and the args argument.

       However, every type method is called with an implicit argument called
       type that contains the name of the type command.  In addition, type
       methods should by convention avoid using the names of the arguments
       implicitly defined for INSTANCE METHODS.

   HOW DOES AN INSTANCE OR TYPE METHOD CALL A TYPE METHOD?
       If an instance or type method needs to call a type method, it should
       use $type to do so:

              snit::type dog {

                  typemethod pedigreedDogs {} { ... }

                  typemethod printPedigrees {} {
                      foreach obj [$type pedigreedDogs] { ... }
                  }
              }


   HOW DO I PASS A TYPE METHOD AS A CALLBACK?
       It's common in Tcl to pass a snippet of code to another object, for it
       to call later.  Because types cannot be renamed, you can just use the
       type name, or, if the callback is registered from within a type method,
       type.  For example, suppose we want to print a list of pedigreed dogs
       when a Tk button is pushed:

              button .btn -text "Pedigrees" -command [list dog printPedigrees]
              pack .btn

       Alternatively, from a method or type method you can use the
       mytypemethod command, just as you would use mymethod to define a
       callback command for INSTANCE METHODS.

   CAN TYPE METHODS BE HIERARCHICAL?
       Yes, you can define hierarchical type methods in just the same way as
       you can define hierarchical instance methods.  See INSTANCE METHODS for
       more.

PROCS
   WHAT IS A PROC?
       A Snit proc is really just a Tcl proc defined within the type's
       namespace.  You can use procs for private code that isn't related to
       any particular instance.

   HOW DO I DEFINE A PROC?
       Procs are defined by including a proc statement in the type definition:

              snit::type mytype {
                  # Pops and returns the first item from the list stored in the
                  # listvar, updating the listvar
                 proc pop {listvar} { ... }

                 # ...
              }


   ARE THERE ANY LIMITATIONS ON PROC NAMES?
       Any name can be used, so long as it does not begin with Snit_; names
       beginning with Snit_ are reserved for Snit's own use.  However, the
       wise programmer will avoid proc names (set, list, if, etc.) that would
       shadow standard Tcl command names.

       proc names, being private, should begin with a capital letter according
       to convention; however, as there are typically no public procs in the
       type's namespace it doesn't matter much either way.

   HOW DOES A METHOD CALL A PROC?
       Just like it calls any Tcl command.  For example,

              snit::type mytype {
                  # Pops and returns the first item from the list stored in the
                  # listvar, updating the listvar
                  proc pop {listvar} { ... }

                  variable requestQueue {}

                  # Get one request from the queue and process it.
                  method processRequest {} {
                      set req [pop requestQueue]
                  }
              }


   HOW CAN I PASS A PROC TO ANOTHER OBJECT AS A CALLBACK?
       The myproc command returns a callback command for the proc, just as
       mymethod does for a method.

TYPE CONSTRUCTORS
   WHAT IS A TYPE CONSTRUCTOR?
       A type constructor is a body of code that initializes the type as a
       whole, rather like a C++ static initializer.  The body of a type
       constructor is executed once when the type is defined, and never again.

       A type can have at most one type constructor.

   HOW DO I DEFINE A TYPE CONSTRUCTOR?
       A type constructor is defined by using the typeconstructor statement in
       the type definition.  For example, suppose the type uses an array-
       valued type variable as a look-up table, and the values in the array
       have to be computed at start-up.

              % snit::type mytype {
                  typevariable lookupTable

                  typeconstructor {
                      array set lookupTable {key value...}
                  }
              }


CONSTRUCTORS
   WHAT IS A CONSTRUCTOR?
       In object-oriented programming, an object's constructor is responsible
       for initializing the object completely at creation time. The
       constructor receives the list of options passed to the snit::type
       command's create method and can then do whatever it likes.  That might
       include computing instance variable values, reading data from files,
       creating other objects, updating type and instance variables, and so
       forth.

       The constructor's return value is ignored (unless it's an error, of
       course).

   HOW DO I DEFINE A CONSTRUCTOR?
       A constructor is defined by using the constructor statement in the type
       definition.  Suppose that it's desired to keep a list of all pedigreed
       dogs.  The list can be maintained in a type variable and retrieved by a
       type method.  Whenever a dog is created, it can add itself to the
       list--provided that it's registered with the American Kennel Club.

              % snit::type dog {
                  option -akc 0

                  typevariable akcList {}

                  constructor {args} {
                      $self configurelist $args

                      if {$options(-akc)} {
                          lappend akcList $self
                      }
                  }

                  typemethod akclist {} {
                      return $akcList
                  }
              }
              ::dog
              % dog spot -akc 1
              ::spot
              % dog fido
              ::fido
              % dog akclist
              ::spot
              %


   WHAT DOES THE DEFAULT CONSTRUCTOR DO?
       If you don't provide a constructor explicitly, you get the default
       constructor, which is identical to the explicitly-defined constructor
       shown here:

              snit::type dog {
                  constructor {args} {
                      $self configurelist $args
                  }
              }


       When the constructor is called, args will be set to the list of
       arguments that follow the object's name.  The constructor is allowed to
       interpret this list any way it chooses; the normal convention is to
       assume that it's a list of option names and values, as shown in the
       example above.  If you simply want to save the option values, you
       should use the configurelist method, as shown.

   CAN I CHOOSE A DIFFERENT SET OF ARGUMENTS FOR THE CONSTRUCTOR?
       Yes, you can.  For example, suppose we wanted to be sure that the breed
       was explicitly stated for every dog at creation time, and couldn't be
       changed thereafter.  One way to do that is as follows:

              % snit::type dog {
                  variable breed

                  option -color brown
                  option -akc 0

                  constructor {theBreed args} {
                      set breed $theBreed
                      $self configurelist $args
                  }

                  method breed {} { return $breed }
              }
              ::dog
              % dog spot dalmatian -color spotted -akc 1
              ::spot
              % spot breed
              dalmatian


       The drawback is that this syntax is non-standard, and may limit the
       compatibility of your new type with other people's code.  For example,
       Snit assumes that it can create COMPONENTS using the standard creation
       syntax.

   ARE THERE ANY LIMITATIONS ON CONSTRUCTOR ARGUMENTS?
       Constructor argument lists are subject to the same limitations as those
       on instance method argument lists.  It has the same implicit arguments,
       and can contain default values and the args argument.

   IS THERE ANYTHING SPECIAL ABOUT WRITING THE CONSTRUCTOR?
       Yes.  Writing the constructor can be tricky if you're delegating
       options to components, and there are specific issues relating to
       snit::widgets and snit::widgetadaptors.  See DELEGATION, WIDGETS,
       WIDGET ADAPTORS, and THE TK OPTION DATABASE.

DESTRUCTORS
   WHAT IS A DESTRUCTOR?
       A destructor is a special kind of method that's called when an object
       is destroyed.  It's responsible for doing any necessary clean-up when
       the object goes away: destroying COMPONENTS, closing files, and so
       forth.

   HOW DO I DEFINE A DESTRUCTOR?
       Destructors are defined by using the destructor statement in the type
       definition.

       Suppose we're maintaining a list of pedigreed dogs; then we'll want to
       remove dogs from it when they are destroyed.

              snit::type dog {
                  option -akc 0

                  typevariable akcList {}

                  constructor {args} {
                      $self configurelist $args

                      if {$options(-akc)} {
                          lappend akcList $self
                      }
                  }

                  destructor {
                      set ndx [lsearch $akcList $self]

                      if {$ndx != -1} {
                          set akcList [lreplace $akcList $ndx $ndx]
                      }
                  }

                  typemethod akclist {} {
                      return $akcList
                  }
              }


   ARE THERE ANY LIMITATIONS ON DESTRUCTOR ARGUMENTS?
       Yes; a destructor has no explicit arguments.

   WHAT IMPLICIT ARGUMENTS ARE PASSED TO THE DESTRUCTOR?
       The destructor gets the same implicit arguments that are passed to
       INSTANCE METHODS: type, selfns, win, and self.

   MUST COMPONENTS BE DESTROYED EXPLICITLY?
       Yes and no.

       Any Tk widgets created by a snit::widget or snit::widgetadaptor will be
       destroyed automatically by Tk when the megawidget is destroyed, in
       keeping with normal Tk behavior (destroying a parent widget destroys
       the whole tree).

       Components of normal snit::types, on the other hand, are never
       destroyed automatically, nor are non-widget components of Snit
       megawidgets.  If your object creates them in its constructor, then it
       should generally destroy them in its destructor.

   IS THERE ANY SPECIAL ABOUT WRITING A DESTRUCTOR?
       Yes.  If an object's constructor throws an error, the object's
       destructor will be called to clean up; this means that the object might
       not be completely constructed when the destructor is called.  This can
       cause the destructor to throw its own error; the result is usually
       misleading, confusing, and unhelpful.  Consequently, it's important to
       write your destructor so that it's fail-safe.

       For example, a dog might create a tail component; the component will
       need to be destroyed.  But suppose there's an error while processing
       the creation options--the destructor will be called, and there will be
       no tail to destroy.  The simplest solution is generally to catch and
       ignore any errors while destroying components.

              snit::type dog {
                  component tail

                  constructor {args} {
                      $self configurelist $args

                      set tail [tail %AUTO%]
                  }

                  destructor {
                      catch {$tail destroy}
                  }
              }


COMPONENTS
   WHAT IS A COMPONENT?
       Often an object will create and manage a number of other objects.  A
       Snit megawidget, for example, will often create a number of Tk widgets.
       These objects are part of the main object; it is composed of them, so
       they are called components of the object.

       But Snit also has a more precise meaning for COMPONENT.  The components
       of a Snit object are those objects to which methods or options can be
       delegated.  (See DELEGATION for more information about delegation.)

   HOW DO I DECLARE A COMPONENT?
       First, you must decide what role a component plays within your object,
       and give the role a name.  Then, you declare the component using its
       role name and the component statement.  The component statement
       declares an instance variable which is used to store the component's
       command name when the component is created.

       For example, suppose your dog object creates a tail object (the better
       to wag with, no doubt):

              snit::type dog {
                  component mytail

                  constructor {args} {
                      # Create and save the component's command
                      set mytail [tail %AUTO% -partof $self]
                      $self configurelist $args
                  }

                  method wag {} {
                      $mytail wag
                  }
              }


       As shown here, it doesn't matter what the tail object's real name is;
       the dog object refers to it by its component name.

       The above example shows one way to delegate the wag method to the
       mytail component; see DELEGATION for an easier way.

   HOW IS A COMPONENT NAMED?
       A component has two names.  The first name is that of the component
       variable; this represents the role the component object plays within
       the Snit object.  This is the component name proper, and is the name
       used to refer to the component within Snit code.  The second name is
       the name of the actual component object created by the Snit object's
       constructor.  This second name is always a Tcl command name, and is
       referred to as the component's object name.

       In the example in the previous question, the component name is mytail;
       the mytail component's object name is chosen automatically by Snit
       since %AUTO% was used when the component object was created.

   ARE THERE ANY LIMITATIONS ON COMPONENT NAMES?
       Yes.  snit::widget and snit::widgetadaptor objects have a special
       component called the hull component; thus, the name hull should be used
       for no other purpose.

       Otherwise, since component names are in fact instance variable names
       they must follow the rules for INSTANCE VARIABLES.

   WHAT IS AN OWNED COMPONENT?
       An owned component is a component whose object command's lifetime is
       controlled by the snit::type or snit::widget.

       As stated above, a component is an object to which our object can
       delegate methods or options.  Under this definition, our object will
       usually create its component objects, but not necessarily.  Consider
       the following: a dog object has a tail component; but tail knows that
       it's part of the dog:

              snit::type dog {
                  component mytail

                  constructor {args} {
                      set mytail [tail %AUTO% -partof $self]
                      $self configurelist $args
                  }

                  destructor {
                      catch {$mytail destroy}
                  }

                  delegate method wagtail to mytail as wag

                  method bark {} {
                      return "$self barked."
                  }
              }

               snit::type tail {
                   component mydog
                   option -partof -readonly yes

                   constructor {args} {
                       $self configurelist $args
                       set mydog $options(-partof)
                   }

                   method wag {} {
                       return "Wag, wag."
                   }

                   method pull {} {
                       $mydog bark
                   }
               }

       Thus, if you ask a dog to wag its tail, it tells its tail to wag; and
       if you pull the dog's tail, the tail tells the dog to bark.  In this
       scenario, the tail is a component of the dog, and the dog is a
       component of the tail, but the dog owns the tail and not the other way
       around.

   WHAT DOES THE INSTALL COMMAND DO?
       The install command creates an owned component using a specified
       command, and assigns the result to the component's instance variable.
       For example:

              snit::type dog {
                  component mytail

                  constructor {args} {
                      # set mytail [tail %AUTO% -partof $self]
                      install mytail using tail %AUTO% -partof $self
                      $self configurelist $args
                  }
              }

       In a snit::type's code, the install command shown above is equivalent
       to the set mytail command that's commented out.  In a snit::widget's or
       snit::widgetadaptor's, code, however, the install command also queries
       THE TK OPTION DATABASE and initializes the new component's options
       accordingly.  For consistency, it's a good idea to get in the habit of
       using install for all owned components.

   MUST OWNED COMPONENTS BE CREATED IN THE CONSTRUCTOR?
       No, not necessarily.  In fact, there's no reason why an object can't
       destroy and recreate a component multiple times over its own lifetime.

   ARE THERE ANY LIMITATIONS ON COMPONENT OBJECT NAMES?
       Yes.

       Component objects which are Tk widgets or megawidgets must have valid
       Tk window names.

       Component objects which are not widgets or megawidgets must have fully-
       qualified command names, i.e., names which include the full namespace
       of the command.  Note that Snit always creates objects with fully
       qualified names.

       Next, the object names of components and owned by your object must be
       unique.  This is no problem for widget components, since widget names
       are always unique; but consider the following code:

              snit::type tail { ... }

              snit::type dog {
                  delegate method wag to mytail

                  constructor {} {
                      install mytail using tail mytail
                  }
              }


       This code uses the component name, mytail, as the component object
       name.  This is not good, and here's why: Snit instance code executes in
       the Snit type's namespace.  In this case, the mytail component is
       created in the ::dog:: namespace, and will thus have the name
       ::dog::mytail.

       Now, suppose you create two dogs.  Both dogs will attempt to create a
       tail called ::dog::mytail.  The first will succeed, and the second will
       fail, since Snit won't let you create an object if its name is already
       a command.  Here are two ways to avoid this situation:

       First, if the component type is a snit::type you can specify %AUTO% as
       its name, and be guaranteed to get a unique name.  This is the safest
       thing to do:

                  install mytail using tail %AUTO%


       If the component type isn't a snit::type you can create the component
       in the object's instance namespace:

                  install mytail using tail ${selfns}::mytail


       Make sure you pick a unique name within the instance namespace.

   MUST I DESTROY THE COMPONENTS I OWN?
       That depends.  When a parent widget is destroyed, all child widgets are
       destroyed automatically. Thus, if your object is a snit::widget or
       snit::widgetadaptor you don't need to destroy any components that are
       widgets, because they will generally be children or descendants of your
       megawidget.

       If your object is an instance of snit::type, though, none of its owned
       components will be destroyed automatically, nor will be non-widget
       components of a snit::widget be destroyed automatically.  All such
       owned components must be destroyed explicitly, or they won't be
       destroyed at all.

   CAN I EXPOSE A COMPONENT'S OBJECT COMMAND AS PART OF MY INTERFACE?
       Yes, and there are two ways to do it.  The most appropriate way is
       usually to use DELEGATION.  Delegation allows you to pass the options
       and methods you specify along to particular components.  This
       effectively hides the components from the users of your type, and
       ensures good encapsulation.

       However, there are times when it's appropriate, not to mention simpler,
       just to make the entire component part of your type's public interface.

   HOW DO I EXPOSE A COMPONENT'S OBJECT COMMAND?
       When you declare the component, specify the component statement's
       -public option.  The value of this option is the name of a method which
       will be delegated to your component's object command.

       For example, supposed you've written a combobox megawidget which owns a
       listbox widget, and you want to make the listbox's entire interface
       public.  You can do it like this:

              snit::widget combobox {
                   component listbox -public listbox

                   constructor {args} {
                       install listbox using listbox $win.listbox ....
                   }
              }

              combobox .mycombo


       Your comobox widget, .mycombo, now has a listbox method which has all
       of the same subcommands as the listbox widget itself.  Thus, the above
       code sets the listbox component's width to 30.

       Usually you'll let the method name be the same as the component name;
       however, you can name it anything you like.

TYPE COMPONENTS
   WHAT IS A TYPE COMPONENT?
       A type component is a component that belongs to the type itself instead
       of to a particular instance of the type.  The relationship between
       components and type components is the same as the relationship between
       INSTANCE VARIABLES and TYPE VARIABLES.  Both INSTANCE METHODS and TYPE
       METHODS can be delegated to type components.

       Once you understand COMPONENTS and DELEGATION, type components are just
       more of the same.

   HOW DO I DECLARE A TYPE COMPONENT?
       Declare a type component using the typecomponent statement.  It takes
       the same options (-inherit and -public) as the component statement
       does, and defines a type variable to hold the type component's object
       command.

       Suppose in your model you've got many dogs, but only one veterinarian.
       You might make the veterinarian a type component.

              snit::type veterinarian { ... }

              snit::type dog {
                  typecomponent vet

                  # ...
              }


   HOW DO I INSTALL A TYPE COMPONENT?
       Just use the set command to assign the component's object command to
       the type component.  Because types (even snit::widget types) are not
       widgets, and do not have options anyway, the extra features of the
       install command are not needed.

       You'll usually install type components in the type constructor, as
       shown here:

              snit::type veterinarian { ... }

              snit::type dog {
                  typecomponent vet

                  typeconstructor {
                      set vet [veterinarian %AUTO%]
                  }
              }


   ARE THERE ANY LIMITATIONS ON TYPE COMPONENT NAMES?
       Yes, the same as on INSTANCE VARIABLES, TYPE VARIABLES, and normal
       COMPONENTS.

DELEGATION
   WHAT IS DELEGATION?
       Delegation, simply put, is when you pass a task you've been given to
       one of your assistants.  (You do have assistants, don't you?)  Snit
       objects can do the same thing.  The following example shows one way in
       which the dog object can delegate its wag method and its -taillength
       option to its tail component.

              snit::type dog {
                  variable mytail

                  option -taillength -configuremethod SetTailOption -cgetmethod GetTailOption


                  method SetTailOption {option value} {
                       $mytail configure $option $value
                  }

                  method GetTailOption {option} {
                       $mytail cget $option
                  }

                  method wag {} {
                      $mytail wag
                  }

                  constructor {args} {
                      install mytail using tail %AUTO% -partof $self
                      $self configurelist $args
                  }

              }


       This is the hard way to do it, by it demonstrates what delegation is
       all about.  See the following answers for the easy way to do it.

       Note that the constructor calls the configurelist method after it
       creates its tail; otherwise, if -taillength appeared in the list of
       args we'd get an error.

   HOW CAN I DELEGATE A METHOD TO A COMPONENT OBJECT?
       Delegation occurs frequently enough that Snit makes it easy. Any method
       can be delegated to any component or type component by placing a single
       delegate statement in the type definition.  (See COMPONENTS and TYPE
       COMPONENTS for more information about component names.)

       For example, here's a much better way to delegate the dog object's wag
       method:

              % snit::type dog {
                  delegate method wag to mytail

                  constructor {} {
                      install mytail using tail %AUTO%
                  }
              }
              ::dog
              % snit::type tail {
                  method wag {} { return "Wag, wag, wag."}
              }
              ::tail
              % dog spot
              ::spot
              % spot wag
              Wag, wag, wag.


       This code has the same effect as the code shown under the previous
       question: when a dog's wag method is called, the call and its arguments
       are passed along automatically to the tail object.

       Note that when a component is mentioned in a delegate statement, the
       component's instance variable is defined implicitly.  However, it's
       still good practice to declare it explicitly using the component
       statement.

       Note also that you can define a method name using the method statement,
       or you can define it using delegate; you can't do both.

   CAN I DELEGATE TO A METHOD WITH A DIFFERENT NAME?
       Suppose you wanted to delegate the dog's wagtail method to the tail's
       wag method.  After all you wag the tail, not the dog.  It's easily
       done:

              snit::type dog {
                  delegate method wagtail to mytail as wag

                  constructor {args} {
                      install mytail using tail %AUTO% -partof $self
                      $self configurelist $args
                  }
              }


   CAN I DELEGATE TO A METHOD WITH ADDITIONAL ARGUMENTS?
       Suppose the tail's wag method takes as an argument the number of times
       the tail should be wagged.  You want to delegate the dog's wagtail
       method to the tail's wag method, specifying that the tail should be
       wagged exactly three times.  This is easily done, too:

              snit::type dog {
                  delegate method wagtail to mytail as {wag 3}
                  # ...
              }

              snit::type tail {
                  method wag {count} {
                      return [string repeat "Wag " $count]
                  }
                  # ...
              }


   CAN I DELEGATE A METHOD TO SOMETHING OTHER THAN AN OBJECT?
       Normal method delegation assumes that you're delegating a method (a
       subcommand of an object command) to a method of another object (a
       subcommand of a different object command).  But not all Tcl objects
       follow Tk conventions, and not everything you'd to which you'd like to
       delegate a method is necessary an object.  Consequently, Snit makes it
       easy to delegate a method to pretty much anything you like using the
       delegate statement's using clause.

       Suppose your dog simulation stores dogs in a database, each dog as a
       single record.  The database API you're using provides a number of
       commands to manage records; each takes the record ID (a string you
       choose) as its first argument.  For example, saverec saves a record.
       If you let the record ID be the name of the dog object, you can
       delegate the dog's save method to the saverec command as follows:

              snit::type dog {
                  delegate method save using {saverec %s}
              }

       The %s is replaced with the instance name when the save method is
       called; any additional arguments are the appended to the resulting
       command.

       The using clause understands a number of other %-conversions; in
       addition to the instance name, you can substitute in the method name
       (%m), the type name (%t), the instance namespace (%n), the Tk window
       name (%w), and, if a component or typecomponent name was given in the
       delegate statement, the component's object command (%c).

   HOW CAN I DELEGATE A METHOD TO A TYPE COMPONENT OBJECT?
       Just exactly as you would to a component object.  The delegate method
       statement accepts both component and type component names in its to
       clause.

   HOW CAN I DELEGATE A TYPE METHOD TO A TYPE COMPONENT OBJECT?
       Use the delegate typemethod statement.  It works like delegate method,
       with these differences: first, it defines a type method instead of an
       instance method; second, the using clause ignores the %s, %n, and %w
       %-conversions.

       Naturally, you can't delegate a type method to an instance
       component...Snit wouldn't know which instance should receive it.

   HOW CAN I DELEGATE AN OPTION TO A COMPONENT OBJECT?
       The first question in this section (see DELEGATION) shows one way to
       delegate an option to a component; but this pattern occurs often enough
       that Snit makes it easy.  For example, every tail object has a -length
       option; we want to allow the creator of a dog object to set the tail's
       length.  We can do this:

              % snit::type dog {
                  delegate option -length to mytail

                  constructor {args} {
                      install mytail using tail %AUTO% -partof $self
                      $self configurelist $args
                  }
              }
              ::dog
              % snit::type tail {
                  option -partof
                  option -length 5
              }
              ::tail
              % dog spot -length 7
              ::spot
              % spot cget -length
              7


       This produces nearly the same result as the -configuremethod and
       -cgetmethod shown under the first question in this section: whenever a
       dog object's -length option is set or retrieved, the underlying tail
       object's option is set or retrieved in turn.

       Note that you can define an option name using the option statement, or
       you can define it using delegate; you can't do both.

   CAN I DELEGATE TO AN OPTION WITH A DIFFERENT NAME?
       In the previous answer we delegated the dog's -length option down to
       its tail.  This is, of course, wrong.  The dog has a length, and the
       tail has a length, and they are different.  What we'd really like to do
       is give the dog a -taillength option, but delegate it to the tail's
       -length option:

              snit::type dog {
                  delegate option -taillength to mytail as -length

                  constructor {args} {
                      set mytail [tail %AUTO% -partof $self]
                      $self configurelist $args
                  }
              }


   HOW CAN I DELEGATE ANY UNRECOGNIZED METHOD OR OPTION TO A COMPONENT OBJECT?
       It may happen that a Snit object gets most of its behavior from one of
       its components.  This often happens with snit::widgetadaptors, for
       example, where we wish to slightly the modify the behavior of an
       existing widget.  To carry on with our dog example, however, suppose
       that we have a snit::type called animal that implements a variety of
       animal behaviors--moving, eating, sleeping, and so forth.  We want our
       dog objects to inherit these same behaviors, while adding dog-like
       behaviors of its own.  Here's how we can give a dog methods and options
       of its own while delegating all other methods and options to its animal
       component:

              snit::type dog {
                  delegate option * to animal
                  delegate method * to animal

                  option -akc 0

                  constructor {args} {
                      install animal using animal %AUTO% -name $self
                      $self configurelist $args
                  }

                  method wag {} {
                      return "$self wags its tail"
                  }
              }


       That's it.  A dog is now an animal that has a -akc option and can wag
       its tail.

       Note that we don't need to specify the full list of method names or
       option names that animal will receive.  It gets anything dog doesn't
       recognize--and if it doesn't recognize it either, it will simply throw
       an error, just as it should.

       You can also delegate all unknown type methods to a type component
       using delegate typemethod *.

   HOW CAN I DELEGATE ALL BUT CERTAIN METHODS OR OPTIONS TO A COMPONENT?
       In the previous answer, we said that every dog is an animal by
       delegating all unknown methods and options to the animal component. But
       what if the animal type has some methods or options that we'd like to
       suppress?

       One solution is to explicitly delegate all the options and methods, and
       forgo the convenience of delegate method * and delegate option *.  But
       if we wish to suppress only a few options or methods, there's an easier
       way:

              snit::type dog {
                  delegate option * to animal except -numlegs
                  delegate method * to animal except {fly climb}

                  # ...

                  constructor {args} {
                      install animal using animal %AUTO% -name $self -numlegs 4
                      $self configurelist $args
                  }

                  # ...
              }


       Dogs have four legs, so we specify that explicitly when we create the
       animal component, and explicitly exclude -numlegs from the set of
       delegated options.  Similarly, dogs can neither fly nor climb, so we
       exclude those animal methods as shown.

   CAN A HIERARCHICAL METHOD BE DELEGATED?
       Yes; just specify multiple words in the delegated method's name:

              snit::type tail {
                  method wag {} {return "Wag, wag"}
                  method droop {} {return "Droop, droop"}
              }


              snit::type dog {
                  delegate method {tail wag} to mytail
                  delegate method {tail droop} to mytail

                  # ...

                  constructor {args} {
                      install mytail using tail %AUTO%
                      $self configurelist $args
                  }

                  # ...
              }


       Unrecognized hierarchical methods can also be delegated; the following
       code delegates all subcommands of the "tail" method to the "mytail"
       component:

              snit::type dog {
                  delegate method {tail *} to mytail

                  # ...
              }


WIDGETS
   WHAT IS A SNIT::WIDGET?
       A snit::widget is the Snit version of what Tcl programmers usually call
       a megawidget: a widget-like object usually consisting of one or more Tk
       widgets all contained within a Tk frame.

       A snit::widget is also a special kind of snit::type.  Just about
       everything in this FAQ list that relates to snit::types also applies to
       snit::widgets.

   HOW DO I DEFINE A SNIT::WIDGET?
       snit::widgets are defined using the snit::widget command, just as
       snit::types are defined by the snit::type command.

       The body of the definition can contain all of the same kinds of
       statements, plus a couple of others which will be mentioned below.

   HOW DO SNIT::WIDGETS DIFFER FROM SNIT::TYPES?
       o      The name of an instance of a snit::type can be any valid Tcl
              command name, in any namespace.  The name of an instance of a
              snit::widget must be a valid Tk widget name, and its parent
              widget must already exist.

       o      An instance of a snit::type can be destroyed by calling its
              destroy method.  Instances of a snit::widget have no destroy
              method; use the Tk destroy command instead.

       o      Every instance of a snit::widget has one predefined component
              called its hull component.  The hull is usually a Tk frame or
              toplevel widget; any other widgets created as part of the
              snit::widget will usually be contained within the hull.

       o      snit::widgets can have their options receive default values from
              THE TK OPTION DATABASE.

   WHAT IS A HULL COMPONENT?
       Snit can't create a Tk widget object; only Tk can do that.  Thus, every
       instance of a snit::widget must be wrapped around a genuine Tk widget;
       this Tk widget is called the hull component.  Snit effectively
       piggybacks the behavior you define (methods, options, and so forth) on
       top of the hull component so that the whole thing behaves like a
       standard Tk widget.

       For snit::widgets the hull component must be a Tk widget that defines
       the -class option.

       snit::widgetadaptors differ from snit::widgets chiefly in that any kind
       of widget can be used as the hull component; see WIDGET ADAPTORS.

   HOW CAN I SET THE HULL TYPE FOR A SNIT::WIDGET?
       A snit::widget's hull component will usually be a Tk frame widget;
       however, it may be any Tk widget that defines the -class option.  You
       can explicitly choose the hull type you prefer by including the
       hulltype command in the widget definition:

              snit::widget mytoplevel {
                  hulltype toplevel

                  # ...
              }


       If no hulltype command appears, the hull will be a frame.

       By default, Snit recognizes the following hull types: the Tk widgets
       frame, labelframe, toplevel, and the Tile widgets ttk::frame,
       ttk::labelframe, and ttk::toplevel.  To enable the use of some other
       kind of widget as the hull type, you can lappend the widget command to
       the variable snit::hulltypes (always provided the widget defines the
       -class option.  For example, suppose Tk gets a new widget type called a
       prettyframe:

              lappend snit::hulltypes prettyframe

              snit::widget mywidget {
                  hulltype prettyframe

                  # ...
              }


   HOW SHOULD I NAME WIDGETS WHICH ARE COMPONENTS OF A SNIT::WIDGET?
       Every widget, whether a genuine Tk widget or a Snit megawidget, has to
       have a valid Tk window name.  When a snit::widget is first created, its
       instance name, self, is a Tk window name; however, if the snit::widget
       is used as the hull component by a snit::widgetadaptor its instance
       name will be changed to something else.  For this reason, every
       snit::widget method, constructor, destructor, and so forth is passed
       another implicit argument, win, which is the window name of the
       megawidget.  Any children should be named using win as the root.

       Thus, suppose you're writing a toolbar widget, a frame consisting of a
       number of buttons placed side-by-side.  It might look something like
       this:

              snit::widget toolbar {
                  delegate option * to hull

                  constructor {args} {
                      button $win.open -text Open -command [mymethod open]
                      button $win.save -text Save -command [mymethod save]

                      # ....

                      $self configurelist $args

                  }
              }


       See also the question on renaming objects, toward the top of this file.

WIDGET ADAPTORS
   WHAT IS A SNIT::WIDGETADAPTOR?
       A snit::widgetadaptor is a kind of snit::widget.  Whereas a
       snit::widget's hull is automatically created and is always a Tk frame,
       a snit::widgetadaptor can be based on any Tk widget--or on any Snit
       megawidget, or even (with luck) on megawidgets defined using some other
       package.

       It's called a widget adaptor because it allows you to take an existing
       widget and customize its behavior.

   HOW DO I DEFINE A SNIT::WIDGETADAPTOR?
       Use the snit::widgetadaptor command.  The definition for a
       snit::widgetadaptor looks just like that for a snit::type or
       snit::widget, except that the constructor must create and install the
       hull component.

       For example, the following code creates a read-only text widget by the
       simple device of turning its insert and delete methods into no-ops.
       Then, we define new methods, ins and del, which get delegated to the
       hull component as insert and delete.  Thus, we've adapted the text
       widget and given it new behavior while still leaving it fundamentally a
       text widget.

              ::snit::widgetadaptor rotext {

                  constructor {args} {
                      # Create the text widget; turn off its insert cursor
                      installhull using text -insertwidth 0

                      # Apply any options passed at creation time.
                      $self configurelist $args
                  }

                  # Disable the text widget's insert and delete methods, to
                  # make this readonly.
                  method insert {args} {}
                  method delete {args} {}

                  # Enable ins and del as synonyms, so the program can insert and
                  # delete.
                  delegate method ins to hull as insert
                  delegate method del to hull as delete

                  # Pass all other methods and options to the real text widget, so
                  # that the remaining behavior is as expected.
                  delegate method * to hull
                  delegate option * to hull
              }


       The most important part is in the constructor.  Whereas snit::widget
       creates the hull for you, snit::widgetadaptor cannot -- it doesn't know
       what kind of widget you want.  So the first thing the constructor does
       is create the hull component (a Tk text widget in this case), and then
       installs it using the installhull command.

       Note: There is no instance command until you create one by installing a
       hull component.  Any attempt to pass methods to $self prior to calling
       installhull will fail.

   CAN I ADAPT A WIDGET CREATED ELSEWHERE IN THE PROGRAM?
       Yes.

       At times, it can be convenient to adapt a pre-existing widget instead
       of creating your own.  For example, the Bwidget PagesManager widget
       manages a set of frame widgets, only one of which is visible at a time.
       The application chooses which frame is visible.  All of the These
       frames are created by the PagesManager itself, using its add method.
       It's convenient to adapt these frames to do what we'd like them to do.

       In a case like this, the Tk widget will already exist when the
       snit::widgetadaptor is created.  Snit provides an alternate form of the
       installhull command for this purpose:

              snit::widgetadaptor pageadaptor {
                  constructor {args} {
                      # The widget already exists; just install it.
                      installhull $win

                      # ...
                  }
              }


   CAN I ADAPT ANOTHER MEGAWIDGET?
       Maybe. If the other megawidget is a snit::widget or
       snit::widgetadaptor, then yes.  If it isn't then, again, maybe.  You'll
       have to try it and see.  You're most likely to have trouble with widget
       destruction--you have to make sure that your megawidget code receives
       the <Destroy> event before the megawidget you're adapting does.

THE TK OPTION DATABASE
   WHAT IS THE TK OPTION DATABASE?
       The Tk option database is a database of default option values
       maintained by Tk itself; every Tk application has one.  The concept of
       the option database derives from something called the X Windows
       resource database; however, the option database is available in every
       Tk implementation, including those which do not use the X Windows
       system (e.g., Microsoft Windows).

       Full details about the Tk option database are beyond the scope of this
       document; both Practical Programming in Tcl and Tk by Welch, Jones, and
       Hobbs, and Effective Tcl/Tk Programming by Harrison and McClennan.,
       have good introductions to it.

       Snit is implemented so that most of the time it will simply do the
       right thing with respect to the option database, provided that the
       widget developer does the right thing by Snit.  The body of this
       section goes into great deal about what Snit requires.  The following
       is a brief statement of the requirements, for reference.

       o      If the widget's default widget class is not what is desired, set
              it explicitly using the widgetclass statement in the widget
              definition.

       o      When defining or delegating options, specify the resource and
              class names explicitly when necessary.

       o      Use the installhull using command to create and install the hull
              for snit::widgetadaptors.

       o      Use the install command to create and install all components
              which are widgets.

       o      Use the install command to create and install components which
              aren't widgets if you'd like them to receive option values from
              the option database.

       The interaction of Tk widgets with the option database is a complex
       thing; the interaction of Snit with the option database is even more
       so, and repays attention to detail.

   DO SNIT::TYPES USE THE TK OPTION DATABASE?
       No, they don't; querying the option database requires a Tk window name,
       and snit::types don't have one.

       If you create an instance of a snit::type as a component of a
       snit::widget or snit::widgetadaptor, on the other hand, and if any
       options are delegated to the component, and if you use install to
       create and install it, then the megawidget will query the option
       database on the snit::type's behalf.  This might or might not be what
       you want, so take care.

   WHAT IS MY SNIT::WIDGET'S WIDGET CLASS?
       Every Tk widget has a "widget class": a name that is used when adding
       option settings to the database.  For Tk widgets, the widget class is
       the same as the widget command name with an initial capital.  For
       example, the widget class of the Tk button widget is Button.

       Similarly, the widget class of a snit::widget defaults to the
       unqualified type name with the first letter capitalized.  For example,
       the widget class of

              snit::widget ::mylibrary::scrolledText { ... }


       is ScrolledText.

       The widget class can also be set explicitly using the widgetclass
       statement within the snit::widget definition:

              snit::widget ::mylibrary::scrolledText {
                  widgetclass Text

                  # ...
              }


       The above definition says that a scrolledText megawidget has the same
       widget class as an ordinary text widget.  This might or might not be a
       good idea, depending on how the rest of the megawidget is defined, and
       how its options are delegated.

   WHAT IS MY SNIT::WIDGETADAPTOR'S WIDGET CLASS?
       The widget class of a snit::widgetadaptor is just the widget class of
       its hull widget; Snit has no control over this.

       Note that the widget class can be changed only for frame and toplevel
       widgets, which is why these are the valid hull types for snit::widgets.

       Try to use snit::widgetadaptors only to make small modifications to
       another widget's behavior.  Then, it will usually not make sense to
       change the widget's widget class anyway.

   WHAT ARE OPTION RESOURCE AND CLASS NAMES?
       Every Tk widget option has three names: the option name, the resource
       name, and the class name.  The option name begins with a hyphen and is
       all lowercase; it's used when creating widgets, and with the configure
       and cget commands.

       The resource and class names are used to initialize option default
       values by querying the option database.  The resource name is usually
       just the option name minus the hyphen, but may contain uppercase
       letters at word boundaries; the class name is usually just the resource
       name with an initial capital, but not always.  For example, here are
       the option, resource, and class names for several Tk text widget
       options:

                  -background         background         Background
                  -borderwidth        borderWidth        BorderWidth
                  -insertborderwidth  insertBorderWidth  BorderWidth
                  -padx               padX               Pad


       As is easily seen, sometimes the resource and class names can be
       inferred from the option name, but not always.

   WHAT ARE THE RESOURCE AND CLASS NAMES FOR MY MEGAWIDGET'S OPTIONS?
       For options implicitly delegated to a component using delegate option
       *, the resource and class names will be exactly those defined by the
       component.  The configure method returns these names, along with the
       option's default and current values:

              % snit::widget mytext {
                  delegate option * to text

                  constructor {args} {
                      install text using text .text
                      # ...
                  }

                  # ...
              }
              ::mytext
              % mytext .text
              % .text configure -padx
              -padx padX Pad 1 1
              %


       For all other options (whether locally defined or explicitly
       delegated), the resource and class names can be defined explicitly, or
       they can be allowed to have default values.

       By default, the resource name is just the option name minus the hyphen;
       the the class name is just the option name with an initial capital
       letter.  For example, suppose we explicitly delegate "-padx":

              % snit::widget mytext {
                  option -myvalue 5

                  delegate option -padx to text
                  delegate option * to text

                  constructor {args} {
                      install text using text .text
                      # ...
                  }

                  # ...
              }
              ::mytext
              % mytext .text
              % .text configure -myvalue
              -myvalue myvalue Myvalue 5 5
              % .text configure -padx
              -padx padx Padx 1 1
              %


       Here the resource and class names are chosen using the default rules.
       Often these rules are sufficient, but in the case of "-padx" we'd most
       likely prefer that the option's resource and class names are the same
       as for the built-in Tk widgets.  This is easily done:

              % snit::widget mytext {
                  delegate option {-padx padX Pad} to text

                  # ...
              }
              ::mytext
              % mytext .text
              % .text configure -padx
              -padx padX Pad 1 1
              %


   HOW DOES SNIT INITIALIZE MY MEGAWIDGET'S LOCALLY-DEFINED OPTIONS?
       The option database is queried for each of the megawidget's locally-
       defined options, using the option's resource and class name.  If the
       result isn't "", then it replaces the default value given in widget
       definition.  In either case, the default can be overridden by the
       caller.  For example,

              option add *Mywidget.texture pebbled

              snit::widget mywidget {
                  option -texture smooth
                  # ...
              }

              mywidget .mywidget -texture greasy


       Here, -texture would normally default to "smooth", but because of the
       entry added to the option database it defaults to "pebbled".  However,
       the caller has explicitly overridden the default, and so the new widget
       will be "greasy".

   HOW DOES SNIT INITIALIZE DELEGATED OPTIONS?
       That depends on whether the options are delegated to the hull, or to
       some other component.

   HOW DOES SNIT INITIALIZE OPTIONS DELEGATED TO THE HULL?
       A snit::widget's hull is a widget, and given that its class has been
       set it is expected to query the option database for itself.  The only
       exception concerns options that are delegated to it with a different
       name.  Consider the following code:

              option add *Mywidget.borderWidth 5
              option add *Mywidget.relief sunken
              option add *Mywidget.hullbackground red
              option add *Mywidget.background green

              snit::widget mywidget {
                  delegate option -borderwidth to hull
                  delegate option -hullbackground to hull as -background
                  delegate option * to hull
                  # ...
              }

              mywidget .mywidget

              set A [.mywidget cget -relief]
              set B [.mywidget cget -hullbackground]
              set C [.mywidget cget -background]
              set D [.mywidget cget -borderwidth]


       The question is, what are the values of variables A, B, C and D?

       The value of A is "sunken".  The hull is a Tk frame which has been
       given the widget class Mywidget; it will automatically query the option
       database and pick up this value.  Since the -relief option is
       implicitly delegated to the hull, Snit takes no action.

       The value of B is "red".  The hull will automatically pick up the value
       "green" for its -background option, just as it picked up the -relief
       value.  However, Snit knows that -hullbackground is mapped to the
       hull's -background option; hence, it queries the option database for
       -hullbackground and gets "red" and updates the hull accordingly.

       The value of C is also "red", because -background is implicitly
       delegated to the hull; thus, retrieving it is the same as retrieving
       -hullbackground.  Note that this case is unusual; the -background
       option should probably have been excluded using the delegate
       statement's except clause, or (more likely) delegated to some other
       component.

       The value of D is "5", but not for the reason you think.  Note that as
       it is defined above, the resource name for -borderwidth defaults to
       borderwidth, whereas the option database entry is borderWidth, in
       accordance with the standard Tk naming for this option.  As with
       -relief, the hull picks up its own -borderwidth option before Snit does
       anything.  Because the option is delegated under its own name, Snit
       assumes that the correct thing has happened, and doesn't worry about it
       any further.  To avoid confusion, the -borderwidth option should have
       been delegated like this:

                  delegate option {-borderwidth borderWidth BorderWidth} to hull


       For snit::widgetadaptors, the case is somewhat altered.  Widget
       adaptors retain the widget class of their hull, and the hull is not
       created automatically by Snit.  Instead, the snit::widgetadaptor must
       call installhull in its constructor.  The normal way to do this is as
       follows:

              snit::widgetadaptor mywidget {
                  # ...
                  constructor {args} {
                      # ...
                      installhull using text -foreground white
                      # ...
                  }
                  # ...
              }


       In this case, the installhull command will create the hull using a
       command like this:

                  set hull [text $win -foreground white]


       The hull is a text widget, so its widget class is Text.  Just as with
       snit::widget hulls, Snit assumes that it will pick up all of its normal
       option values automatically, without help from Snit.  Options delegated
       from a different name are initialized from the option database in the
       same way as described above.

       In earlier versions of Snit, snit::widgetadaptors were expected to call
       installhull like this:

                  installhull [text $win -foreground white]


       This form still works--but Snit will not query the option database as
       described above.

   HOW DOES SNIT INITIALIZE OPTIONS DELEGATED TO OTHER COMPONENTS?
       For hull components, Snit assumes that Tk will do most of the work
       automatically.  Non-hull components are somewhat more complicated,
       because they are matched against the option database twice.

       A component widget remains a widget still, and is therefore initialized
       from the option database in the usual way.  A text widget remains a
       text widget whether it is a component of a megawidget or not, and will
       be created as such.

       But then, the option database is queried for all options delegated to
       the component, and the component is initialized accordingly--provided
       that the install command is used to create it.

       Before option database support was added to Snit, the usual way to
       create a component was to simply create it in the constructor and
       assign its command name to the component variable:

              snit::widget mywidget {
                  delegate option -background to myComp

                  constructor {args} {
                      set myComp [text $win.text -foreground black]
                  }
              }


       The drawback of this method is that Snit has no opportunity to
       initialize the component properly.  Hence, the following approach is
       now used:

              snit::widget mywidget {
                  delegate option -background to myComp

                  constructor {args} {
                      install myComp using text $win.text -foreground black
                  }
              }


       The install command does the following:

       o      Builds a list of the options explicitly included in the install
              command--in this case, -foreground.

       o      Queries the option database for all options delegated explicitly
              to the named component.

       o      Creates the component using the specified command, after
              inserting into it a list of options and values read from the
              option database.  Thus, the explicitly included options (like
              -foreground) will override anything read from the option
              database.

       o      If the widget definition implicitly delegated options to the
              component using delegate option *, then Snit calls the newly
              created component's configure method to receive a list of all of
              the component's options.  From this Snit builds a list of
              options implicitly delegated to the component which were not
              explicitly included in the install command.  For all such
              options, Snit queries the option database and configures the
              component accordingly.

       You don't really need to know all of this; just use install to install
       your components, and Snit will try to do the right thing.

   WHAT HAPPENS IF I INSTALL A NON-WIDGET AS A COMPONENT OF WIDGET?
       A snit::type never queries the option database.  However, a
       snit::widget can have non-widget components.  And if options are
       delegated to those components, and if the install command is used to
       install those components, then they will be initialized from the option
       database just as widget components are.

       However, when used within a megawidget, install assumes that the
       created component uses a reasonably standard widget-like creation
       syntax.  If it doesn't, don't use install.

ENSEMBLE COMMANDS
   WHAT IS AN ENSEMBLE COMMAND?
       An ensemble command is a command with subcommands.  Snit objects are
       all ensemble commands; however, the term more usually refers to
       commands like the standard Tcl commands string, file, and clock.  In a
       sense, these are singleton objects--there's only one instance of them.

   HOW CAN I CREATE AN ENSEMBLE COMMAND USING SNIT?
       There are two ways--as a snit::type, or as an instance of a snit::type.

   HOW CAN I CREATE AN ENSEMBLE COMMAND USING AN INSTANCE OF A SNIT::TYPE?
       Define a type whose INSTANCE METHODS are the subcommands of your
       ensemble command.  Then, create an instance of the type with the
       desired name.

       For example, the following code uses DELEGATION to create a work-alike
       for the standard string command:

              snit::type ::mynamespace::mystringtype {
                  delegate method * to stringhandler

                  constructor {} {
                      set stringhandler string
                  }
              }

              ::mynamespace::mystringtype mystring

       We create the type in a namespace, so that the type command is hidden;
       then we create a single instance with the desired name-- mystring, in
       this case.

       This method has two drawbacks.  First, it leaves the type command
       floating about.  More seriously, your shiny new ensemble command will
       have info and destroy subcommands that you probably have no use for.
       But read on.

   HOW CAN I CREATE AN ENSEMBLE COMMAND USING A SNIT::TYPE?
       Define a type whose TYPE METHODS are the subcommands of your ensemble
       command.

       For example, the following code uses DELEGATION to create a work-alike
       for the standard string command:

              snit::type mystring {
                  delegate typemethod * to stringhandler

                  typeconstructor {
                      set stringhandler string
                  }
              }

       Now the type command itself is your ensemble command.

       This method has only one drawback, and though it's major, it's also
       surmountable.  Your new ensemble command will have create, info and
       destroy subcommands you don't want.  And worse yet, since the create
       method can be implicit, users of your command will accidentally be
       creating instances of your mystring type if they should mispell one of
       the subcommands.  The command will succeed--the first time--but won't
       do what's wanted.  This is very bad.

       The work around is to set some PRAGMAS, as shown here:

              snit::type mystring {
                  pragma -hastypeinfo    no
                  pragma -hastypedestroy no
                  pragma -hasinstances   no

                  delegate typemethod * to stringhandler

                  typeconstructor {
                      set stringhandler string
                  }
              }

       Here we've used the pragma statement to tell Snit that we don't want
       the info typemethod or the destroy typemethod, and that our type has no
       instances; this eliminates the create typemethod and all related code.
       As a result, our ensemble command will be well-behaved, with no
       unexpected subcommands.

PRAGMAS
   WHAT IS A PRAGMA?
       A pragma is an option you can set in your type definitions that affects
       how the type is defined and how it works once it is defined.

   HOW DO I SET A PRAGMA?
       Use the pragma statement.  Each pragma is an option with a value; each
       time you use the pragma statement you can set one or more of them.

   HOW CAN I GET RID OF THE  INFO" TYPE METHOD?"
       Set the -hastypeinfo pragma to no:

              snit::type dog {
                  pragma -hastypeinfo no
                  # ...
              }

       Snit will refrain from defining the info type method.

   HOW CAN I GET RID OF THE  DESTROY" TYPE METHOD?"
       Set the -hastypedestroy pragma to no:

              snit::type dog {
                  pragma -hastypedestroy no
                  # ...
              }

       Snit will refrain from defining the destroy type method.

   HOW CAN I GET RID OF THE  CREATE" TYPE METHOD?"
       Set the -hasinstances pragma to no:

              snit::type dog {
                  pragma -hasinstances no
                  # ...
              }

       Snit will refrain from defining the create type method; if you call the
       type command with an unknown method name, you'll get an error instead
       of a new instance of the type.

       This is useful if you wish to use a snit::type to define an ensemble
       command rather than a type with instances.

       Pragmas -hastypemethods and -hasinstances cannot both be false (or
       there'd be nothing left).

   HOW CAN I GET RID OF TYPE METHODS ALTOGETHER?
       Normal Tk widget type commands don't have subcommands; all they do is
       create widgets--in Snit terms, the type command calls the create type
       method directly.  To get the same behavior from Snit, set the
       -hastypemethods pragma to no:

              snit::type dog {
                  pragma -hastypemethods no
                  #...
              }

              # Creates ::spot
              dog spot

              # Tries to create an instance called ::create
              dog create spot

       Pragmas -hastypemethods and -hasinstances cannot both be false (or
       there'd be nothing left).

   WHY CAN'T I CREATE AN OBJECT THAT REPLACES AN OLD OBJECT WITH THE SAME
       NAME?
       Up until Snit 0.95, you could use any name for an instance of a
       snit::type, even if the name was already in use by some other object or
       command.  You could do the following, for example:

              snit::type dog { ... }

              dog proc

       You now have a new dog named "proc", which is probably not something
       that you really wanted to do.  As a result, Snit now throws an error if
       your chosen instance name names an existing command.  To restore the
       old behavior, set the -canreplace pragma to yes:

              snit::type dog {
                  pragma -canreplace yes
                  # ...
              }


   HOW CAN I MAKE MY SIMPLE TYPE RUN FASTER?
       In Snit 1.x, you can set the -simpledispatch pragma to yes.

       Snit 1.x method dispatch is both flexible and fast, but the flexibility
       comes with a price.  If your type doesn't require the flexibility, the
       -simpledispatch pragma allows you to substitute a simpler dispatch
       mechanism that runs quite a bit faster.  The limitations are these:

       o      Methods cannot be delegated.

       o      uplevel and upvar do not work as expected: the caller's scope is
              two levels up rather than one.

       o      The option-handling methods (cget, configure, and configurelist)
              are very slightly slower.

       In Snit 2.2, the -simpledispatch macro is obsolete, and ignored; all
       Snit 2.2 method dispatch is faster than Snit 1.x's -simpledispatch.

MACROS
   WHAT IS A MACRO?
       A Snit macro is nothing more than a Tcl proc that's defined in the Tcl
       interpreter used to compile Snit type definitions.

   WHAT ARE MACROS GOOD FOR?
       You can use Snit macros to define new type definition syntax, and to
       support conditional compilation.

   HOW DO I DO CONDITIONAL COMPILATION?
       Suppose you want your type to use a fast C extension if it's available;
       otherwise, you'll fallback to a slower Tcl implementation.  You want to
       define one set of methods in the first case, and another set in the
       second case.  But how can your type definition know whether the fast C
       extension is available or not?

       It's easily done.  Outside of any type definition, define a macro that
       returns 1 if the extension is available, and 0 otherwise:

              if {$gotFastExtension} {
                  snit::macro fastcode {} {return 1}
              } else {
                  snit::macro fastcode {} {return 0}
              }

       Then, use your macro in your type definition:

              snit::type dog {

                  if {[fastcode]} {
                      # Fast methods
                      method bark {} {...}
                      method wagtail {} {...}
                  } else {
                      # Slow methods
                      method bark {} {...}
                      method wagtail {} {...}
                  }
              }


   HOW DO I DEFINE NEW TYPE DEFINITION SYNTAX?
       Use a macro.  For example, your snit::widget's -background option
       should be propagated to a number of component widgets.  You could
       implement that like this:

              snit::widget mywidget {
                  option -background -default white -configuremethod PropagateBackground

                  method PropagateBackground {option value} {
                      $comp1 configure $option $value
                      $comp2 configure $option $value
                      $comp3 configure $option $value
                  }
              }

       For one option, this is fine; if you've got a number of options, it
       becomes tedious and error prone.  So package it as a macro:

              snit::macro propagate {option "to" components} {
                  option $option -configuremethod Propagate$option

                  set body "\n"

                  foreach comp $components {
                      append body "\$$comp configure $option \$value\n"
                  }

                  method Propagate$option {option value} $body
              }

       Then you can use it like this:

              snit::widget mywidget {
                  option -background default -white
                  option -foreground default -black

                  propagate -background to {comp1 comp2 comp3}
                  propagate -foreground to {comp1 comp2 comp3}
              }


   ARE THERE ARE RESTRICTIONS ON MACRO NAMES?
       Yes, there are.  You can't redefine any standard Tcl commands or Snit
       type definition statements.  You can use any other command name,
       including the name of a previously defined macro.

       If you're using Snit macros in your application, go ahead and name them
       in the global namespace, as shown above.  But if you're using them to
       define types or widgets for use by others, you should define your
       macros in the same namespace as your types or widgets.  That way, they
       won't conflict with other people's macros.

       If my fancy snit::widget is called ::mylib::mywidget, for example, then
       I should define my propagate macro as ::mylib::propagate:

              snit::macro mylib::propagate {option "to" components} { ... }

              snit::widget ::mylib::mywidget {
                  option -background default -white
                  option -foreground default -black

                  mylib::propagate -background to {comp1 comp2 comp3}
                  mylib::propagate -foreground to {comp1 comp2 comp3}
              }


BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category snit of
       the Tcllib SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].  Please also report
       any ideas for enhancements you may have for either package and/or
       documentation.

KEYWORDS
       BWidget, C++, Incr Tcl, adaptors, class, mega widget, object, object
       oriented, widget, widget adaptors

CATEGORY
       Programming tools

COPYRIGHT
       Copyright (c) 2003-2006, by William H. Duquette




snit                                  2.2                           snitfaq(n)
