bindtags(3pm)

NAME
Tk::bindtags - Determine which bindings apply to a window, and order of evaluation

SYNOPSIS
$widget->bindtags([tagList]);

@tags = $widget->bindtags;

DESCRIPTION
When a binding is created with the bind command, it is associated either with a particular window such as $widget, a class name such as Tk::Button, the keyword all, or any other string. All of these forms are called binding tags. Each window has a list of binding tags that determine how events are processed for the window. When an event occurs in a window, it is applied to each of the window's tags in order: for each tag, the most specific binding that matches the given tag and event is executed. See the Tk::bind documentation for more information on the matching process.

By default, each window has four binding tags consisting of the the window's class name, name of the window, the name of the window's nearest toplevel ancestor, and all, in that order. Toplevel windows have only three tags by default, since the toplevel name is the same as that of the window.

Note that this order is different from order used by Tcl/Tk. Tcl/Tk has the window ahead of the class name in the binding order. This is because Tcl is procedural rather than object oriented and the normal way for Tcl/Tk applications to override class bindings is with an instance binding. However, with perl/Tk the normal way to override a class binding is to derive a class. The perl/Tk order causes instance bindings to execute after the class binding, and so instance bind callbacks can make use of state changes (e.g. changes to the selection) than the class bindings have made.

The bindtags command allows the binding tags for a window to be read and modified.

If $widget->bindtags is invoked without an argument, then the current set of binding tags for $widget is returned as a list. If the tagList argument is specified to bindtags, then it must be a reference to and array; the tags for $widget are changed to the elements of the array. (A reference to an anonymous array can be created by enclosin the elements in [ ].) The elements of tagList may be arbitrary strings or widget objects, if no window exists for an object at the time an event is processed, then the tag is ignored for that event. The order of the elements in tagList determines the order in which binding callbacks are executed in response to events. For example, the command

 $b->bindtags([$b,ref($b),$b->toplevel,'all'])

applies the Tcl/Tk binding order which binding callbacks will be evaluated for a button (say) $b so that $b's instance bindings are invoked first, following by bindings for $b's class, followed by bindings for $b's toplevel, followed by 'all' bindings.

If tagList is an empty list i.e. [], then the binding tags for $widget are returned to the perl/Tk default state described above.

The bindtags command may be used to introduce arbitrary additional binding tags for a window, or to remove standard tags. For example, the command

 $b->bindtags(['TrickyButton',$b->toplevel,'all'])

replaces the (say) Tk::Button tag for $b with TrickyButton. This means that the default widget bindings for buttons, which are associated with the Tk::Button tag, will no longer apply to $b, but any bindings associated with TrickyButton (perhaps some new button behavior) will apply.

BUGS
The current mapping of the 'native' Tk behaviour of this method i.e. returning a list but only accepting a reference to an array is counter intuitive. The perl/Tk interface may be tidied up, returning a list is sensible so, most likely fix will be to allow a list to be passed to set the bindtags.

SEE ALSO
Tk::bind Tk::callbacks

KEYWORDS
binding, event, tag