callbacks(3)

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 call
back 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('UpDown
Line',1)]);

KEYWORDS

callback, closure, anonymous subroutine, bind

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