Tk::callbacks(3pm) - Man pages

callbacks(3pm)        User Contributed Perl Documentation        callbacks(3pm)

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 v5.40.1                       2025-04-13                    callbacks(3pm)

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