grab(n)                      Tk Built-In Commands                      grab(n)



NAME
       grab - Confine pointer and keyboard events to a window sub-tree

SYNOPSIS
       (grab window)

       (grab :global window)

       (grab option ?arg arg ...?)


DESCRIPTION
       This procedure implements simple pointer and keyboard grabs for Tk.
       Tk's grabs are different than the grabs described in the Xlib
       documentation.  When a grab is set for a particular window, Tk
       restricts all pointer events to the grab window and its descendants in
       Tk's window hierarchy.  Whenever the pointer is within the grab
       window's subtree, the pointer will behave exactly the same as if there
       had been no grab at all and all events will be reported in the normal
       fashion.  When the pointer is outside window's tree, button presses and
       releases and mouse motion events are reported to window, and window
       entry and window exit events are ignored.  The grab subtree ``owns''
       the pointer: windows outside the grab subtree will be visible on the
       screen but they will be insensitive until the grab is released.  The
       tree of windows underneath the grab window can include top-level
       windows, in which case all of those top-level windows and their
       descendants will continue to receive mouse events during the grab.

       Two forms of grabs are possible:  local and global.  A local grab
       affects only the grabbing application:  events will be reported to
       other applications as if the grab had never occurred.  Grabs are local
       by default.  A global grab locks out all applications on the screen, so
       that only the given subtree of the grabbing application will be
       sensitive to pointer events (mouse button presses, mouse button
       releases, pointer motions, window entries, and window exits).  During
       global grabs the window manager will not receive pointer events either.

       During local grabs, keyboard events (key presses and key releases) are
       delivered as usual:  the window manager controls which application
       receives keyboard events, and if they are sent to any window in the
       grabbing application then they are redirected to the focus window.
       During a global grab Tk grabs the keyboard so that all keyboard events
       are always sent to the grabbing application.  The focus procedure is
       still used to determine which window in the application receives the
       keyboard events.  The keyboard grab is released when the grab is
       released.

       Grabs apply to particular displays.  If an application has windows on
       multiple displays then it can establish a separate grab on each
       display.  The grab on a particular display affects only the windows on
       that display.  It is possible for different applications on a single
       display to have simultaneous local grabs, but only one application can
       have a global grab on a given display at once.

       The grab procedure can take any of the following forms:

       (grab  window)

       (grab :global window)
              Same as grab set, described below.

       (grab 'current )

       (grab 'current window)
              If window is specified, returns the name of the current grab
              window in this application for window's display, or an empty
              string if there is no such window.  If window is omitted, the
              procedure returns a list whose elements are all of the windows
              grabbed by this application for all displays, or an empty string
              if the application has no grabs.

       (grab 'release window)
              Releases the grab on window if there is one, otherwise does
              nothing.  Returns an empty string.

       (grab 'set  window)

       (grab 'set :global window)
              Sets a grab on window.  If :global is specified then the grab is
              global, otherwise it is local.  If a grab was already in effect
              for this application on window's display then it is
              automatically released.  If there is already a grab on window
              and it has the same global/local form as the requested grab,
              then the procedure does nothing.  Returns an empty string.

       (grab 'status window)
              Returns "none" if no grab is currently set on window, "local" if
              a local grab is set on window, and "global" if a global grab is
              set.


BUGS
       It took an incredibly complex and gross implementation to produce the
       simple grab effect described above.  Given the current implementation,
       it isn't safe for applications to use the Xlib grab facilities at all
       except through the Tk grab procedures.  If applications try to
       manipulate X's grab mechanisms directly, things will probably break.

       If a single process is managing several different Tk applications, only
       one of those applications can have a local grab for a given display at
       any given time.  If the applications are in different processes, this
       restriction doesn't exist.


SEE ALSO
       focus



STk                                   3.1                              grab(n)
