NAME

       Tk::callbacks - Specifying code for Tk to call.

SYNOPSIS

       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... ], ...

DESCRIPTION

       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.

EXAMPLES

           $entry->bind('<Return>' => [$w , 'validate', Ev(['get'])]);

           $toplevel->bind('all', '<Visibility>', [\&unobscure, Ev('s')]);

           $mw->bind($class, '<Down>', ['SetCursor', Ev('UpDownLine',1)]);

SEE ALSO

       Tk::bind Tk::after Tk::options Tk::fileevent

KEYWORDS

       callback, closure, anonymous subroutine, bind

perl v5.40.0                                       2024-07-26                                     callbacks(3pm)