Maypole::Workflow(3)

NAME
Maypole::Workflow - Describes the progress of a request through Maypole

SYNOPSIS
                              config $h
                                  |
                            Maypole $r
    Apache::Request               |
         +---- $r->get_request ---+
        $ar                       |
                                  |
                          $r->parse_location
                                  |
                          $r->is_applicable
                                  |
    BeerDB::Beer        $r->call_authenticate
       ->authenticate ------------+------------ $r->authenticate
                                  |
                         $r->additional_data
                                  |
                    $r->model_class->process($r)
                                  |
                     $r->view_object->process($r)

DESCRIPTION
An application based on "Maypole" will provide an Apache handler, and eventually deliver a page. This document explains how that happens, and how to influence it. We'll use the "BeerDB" project as our example.

  Initialize class
When the first request comes in, the class will call its own "init" method. This creates a new view object, sets up inheritance relationships between the model classes and their parent, and so on.

  Construction
Once we have initialized, the handler obtains the configuration for your class, and puts it into a new object. We'll call this a request object for the purposes of this document; it will be a new "BeerDB" object.

  Getting the request
Next, the handler calls "get_request" on the new object to have it store a copy of the "Apache::Request". Of course, if you're not using Apache, you might want to subclass this method to return something that looks like an "Apache::Request" object, and possibly also subclass the next stage too to get more control over what methods are called on your "A::R"-lookalike. "get_request" is expected to put the object in the "ar" slot of the request object.

  Handling the URL
Typically, the details of the request will be passed in the URL. This is done with the "parse_location" method, which is expected to populate several slots of the request object. First, "table" and "action" should be populated with the name of the table and the action parts of the URL. Any other arguments should be placed in a listref in the "args" slot, and GET and POST parameters should be arranged into a hash and placed in the "query" and "params" slots, respectively.

Some people may not like the idea of passing everything around in the URL; this is the method to override for you. Of course, you'll also need to provide your own default templates to construct links using your preferred format.

  Is this an applicable URL?
Next, the "is_applicable" method works out if this is actually something that "Maypole" should care about - whether the class exists in the application, whether it supports the given action, and so on. The action is "supported" if it exists in the model class (or its ancestors) and is marked with the ":Exported" attribute; this stops web users from firing off random subroutines in your code.

This should return an Apache status code; "OK" if the request should proceed, "DECLINED" if it should be passed on to the default handlers, or whatever other codes for permissions problems.

  Are we allowed to do this?
We then look for an appropriate "authenticate" method to call; first it will try calling the "authenticate" method of the model class, or, if that does not exist, the "authenticate" method on itself. By default, this allows access to everyone for everything. Similarly, this should return an Apache status code.

  Add any additional data to the request
The open-ended "additional_data" method allows any additional fiddling with the request object before it is despatched. Specifically, it allows you to add to the "template_args" slot, which is a hash of arguments to be added to the template.

  Ask model for widget set
Asking the model class to "process" the current request allows it to do any work it needs for the given command, and populate the "objects" and "template" slots of the request.

  Ask view to process template
Now the view class has its "process" method called, finds the appropriate templates, passes the "objects" and any additional data to the template, and pushes the output to the web server.

We will go into more detail about these last two phases.

Model class processing
The model's "process" method is usually a thin wrapper around the action that we have selected. It sets the template name to the name of the action, fills "objects" with an object of that class whose ID comes from the URL arguments if there is one. For instance, "/beer/foo/12" will do the moral equivalent of

    $r->objects([ BeerDB::Beer->retrieve(12) ]);

Then it calls the right method: in this case, the "foo" method with the request object. This method will usually do any actions which are required, including modifying the list of objects to be passed to the template, or the name of the template to be called.

Template class processing
Finally, the template processor is handed the objects, the template name, and various other bits and pieces, and tries to find the right template. It does this by looking first for "/beer/foo": that is, a specific template appropriate to the class. Next, it looks at "/custom/foo", a local modification, before looking for "/factory/foo", one of the default templates that came with "Maypole".

  Default template arguments
The following things are passed to the Template Toolkit template by default:

request
    The whole "Maypole" request object, for people getting really dirty with the templates.

objects
    The objects handed to us by the model.

base
    The base URL of the application.

config
    The whole configuration hash for the application.

classmetadata
    A hash consisting of:

    "name" - The name of the model class for the request: e.g. "BeerDB::Beer".

    "columns" - The names of the columns in this class.

    "colnames" - A hash mapping between the database's idea of a column name and a human-readable equivalent. ("abv" should be mapped to "A.B.V.", perhaps.)

    "related_accessors" - A list of accessors which are not exactly fields in the table but are related by a has-many relationship. For instance, breweries have many beers, so "beers" would appear in the list.

    "moniker" - The human-readable name for the class: "beer".

    "plural" - The same, only plural: "beers".

    "cgi" - A hash mapping columns and "HTML::Element" objects representing a form field for editing that column.

    "description" - (Perhaps) a user-supplied description of the class.

Additionally, depending on the number of objects, there will be an alias for the "objects" slot with the name of the moniker or plural moniker.

That sounds a bit tricky, but what it means is that if you look at "/beer/view/4" then "beer" will be populated with a "BeerDB::Beer" object with ID 4. On the other hand, if you look at "/beer/list" you can get all the beers in "beers" as well as in "objects".