callbacks(3) User Contributed Perl Documentation 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 callback can take one of
the following basic forms:
o Reference to a subroutine "\&subname"
o Anonymous subroutine (closure) "sub { ... }"
o 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 v5.18.0 2012-01-16 callbacks(3)
perl-Tk 804.030_502 - Generated Sat Aug 17 07:59:29 CDT 2013