Tk::callbacks - Specifying code for Tk to call.
One can specify a callback in one of the following ways:
Without arguments:
... => \&subname, ... ... => sub { ... }, ... ... => methodname, ...
or with arguments:
... => [ \&subname, args ... ], ... ... => [ sub { ... }, args... ], ... ... => [ methodname, args... ], ...
Perl/Tk has a callback, where Tcl/Tk has a command string (i.e. a fragment of Tcl to be executed). A perl/Tk callback can take one of the following basic forms:
Reference to a subroutine \&subname
Anonymous subroutine (closure) sub { ... }
A method name methodname
Any of these can be provided with arguments by enclosing them and the arguments in []. Here are some examples:
$mw->bind($class,
<Delete> => 'Delete');
This will call
$widget->Delete, the
$widget being provided (by bind) as the one where
the Delete key was pressed.
While having bind provide a widget object for you is ideal in many cases it can be irritating in others. Using the list form this behaviour can be modified:
$a->bind(<Delete>,[$b
=> 'Delete']);
because the first element $b is an object bind
will call $b->Delete.
Note that method/object ordering only matters for bind
callbacks, the auto-quoting in perl5.001 makes the first of these a
little more readable:
$w->configure(-yscrollcommand => [ set => $ysb]); $w->configure(-yscrollcommand => [ $ysb => set ]);
but both will call $ysb->set(args provided by Tk)
Another use of arguments allows you to write generalized methods which are easier to re-use:
$a->bind("<Next>",[Next,Page]); $a->bind("<Down>",[Next,Line]);
This will call $a->Next('Page') or
$a->Next('Line') respectively.
Note that the contents of the [] are evaluated by perl
when the callback is created. It is often desirable for the arguments
provided to the callback to depend on the details of the event which
caused it to be executed. To allow for this callbacks can be nested
using the Ev(...) constructor. Ev(...) inserts
callback objects into the argument list. When perl/Tk glue code is
preparing the argument list for the callback it is about to call it
spots these special objects and recursively applies the callback process
to them.
$entry->bind(<Return> => [$w , validate, Ev([get])]); $toplevel->bind(all, <Visibility>, [\&unobscure, Ev(s)]); $mw->bind($class, <Down>, [SetCursor, Ev(UpDownLine,1)]);
Tk::bind Tk::after Tk::options Tk::fileevent
callback, closure, anonymous subroutine, bind