mega(3pm) User Contributed Perl Documentation mega(3pm)
NAME
Tk::mega - Perl/Tk support for writing widgets in pure Perl
SYNOPSIS
Define the widget's new class name:
package Tk::MyNewWidget;
For composite widget classes:
use base qw/ Tk::container /; # where container is Frame or Toplevel
For derived widget classes:
use base qw/ Tk::Derived Tk::DerivedWidget /;
Install the new widget in Tk's namespace and establish class and
instance constructors.
Construct Tk::Widget 'MyNewWidget';
sub ClassInit { my ($self, $args) = @_; ... }
sub Populate { my ($self, $args) = @_; ... }
DESCRIPTION
The goal of the mega-widget support of Perl/Tk is to make it easy to
write mega-widgets that obey the same protocol and interface that the Tk
core widgets support. For mega-widget sample code please run the widget
demonstration program and go to the section Sample Perl Mega-Widgets.
There are two kinds of mega-widgets:
• Composite Widgets
A composite widget is composed with one or more existing widgets.
The composite widget looks to the user like a simple single widget.
A well known example is the file selection box.
• Derived Widgets
A derived widget adds/modifies/removes properties and methods from a
single widget (this widget may itself be a mega-widget).
MEGA-WIDGET SUPPORT
Advertise
Give a subwidget a symbolic name.
Usage:
$self->Advertise(name=>$widget);
Gives a subwidget $widget of the mega-widget $self the name name. One
can retrieve the reference of an advertised subwidget with the Subwidget
method.
Comment: Mega-Widget Writers: Please make sure to document the
advertised widgets that are intended for public use. If there are none,
document this fact, e.g.:
=head1 ADVERTISED WIDGETS
None.
Callback
Invoke a callback specified with an option.
Usage:
$self->Callback(-option ?,args ...?);
Callback executes the callback defined with $self->ConfigSpecs(-option,
[CALLBACK, ...]); If args are given they are passed to the callback. If
-option is not defined it does nothing.
ClassInit
Initialization of the mega-widget class.
Usage:
sub ClassInit { my ($class, $mw) = @_; ... }
ClassInit is called once for each MainWindow just before the first
widget instance of a class is created in the widget tree of MainWindow.
ClassInit is often used to define bindings and/or other resources shared
by all instances, e.g., images.
Examples:
$mw->bind($class,"<Tab>", sub { my $w = shift; $w->Insert("\t"); $w->focus; $w->break});
$mw->bind($class,"<Return>", ['Insert',"\n"]);
$mw->bind($class,"<Delete>",'Delete');
Notice that $class is the class name (e.g. Tk::MyText) and $mw is the
mainwindow.
Don't forget to call $class->SUPER::ClassInit($mw) in ClassInit.
Component
Convenience function to create subwidgets.
Usage:
$cw->Component('Whatever', 'AdvertisedName',
-delegate => ['method1', 'method2', ...],
... more widget options ...,
);
Component does several things for you with one call:
o Creates the widget
o Advertises it with a given name (overridden by 'Name' option)
o Delegates a set of methods to this widget (optional)
Example:
$cw->Component('Button', 'quitButton', -command => sub{$mw->'destroy'});
ConfigSpecs
Defines options and their treatment
Usage:
$cw->ConfigSpecs(
-option => [ where, dbname, dbclass, default],
...,
DEFAULT => [where],
);
Defines the options of a mega-widget and what actions are triggered by
configure/cget of an option (see Tk::ConfigSpecs and Tk::Derived for
details).
Construct
Make the new mega-widget known to Tk.
Usage:
Construct baseclass 'Name';
Construct declares the new widget class so that your mega-widget works
like normal Perl/Tk widgets.
Examples:
Construct Tk::Widget 'Whatever'; Construct Tk::Menu 'MyItem';
First example lets one use $widget->Whatever to create new Whatever
widget.
The second example restricts the usage of the MyItem constructor method
to widgets that are derived from Menu: $isamenu->MyItem.
CreateArgs
Process options before any widget is created:
sub CreateArgs { my ($package, $parent, $args) = @_; ...; return
@newargs; }
$package is the package of the mega-widget (e.g., Tk::MyText, $parent
the parent of the widget to be created and $args the hash reference to
the options specified in the widget constructor call.
Don't forget to call $package->SUPER::CreateArgs($parent, $args) in
CreateArgs.
Delegates
Redirect a method of the mega-widget to a subwidget of the composite
widget
Usage:
$cw->Delegates(
'method1' => $subwidget1,
'method2' => 'advertived_name',
...,
'Construct' => $subwidget2,
'DEFAULT' => $subwidget3,
);
The 'Construct' delegation has a special meaning. After 'Construct' is
delegated all Widget constructors are redirected. E.g. after
$self->Delegates('Construct'=>$subframe);
a $self->Button does really a $subframe->Button so the created button is
a child of $subframe and not $self.
Comment: Delegates works only with methods that $cw does not have
itself.
InitObject
Note: this method should not, in general, be used, as it has been
superceeded by Populate and specifying Tk::Derived as one of the base
classes.
Defines construction and interface of derived widgets.
Usage:
sub InitObject {
my ($derived, $args) = @_;
...
}
where $derived is the widget reference of the already created baseclass
widget and $args is the reference to a hash of -option-value pairs.
InitObject is almost identical to Populate method. Populate does some
more 'magic' things useful for mega-widgets with several widgets.
Don't forget to call $derived->SUPER::InitObject($args) in InitObject.
OnDestroy
Define a callback invoked when the mega-widget is destroyed.
Usage:
$widget->OnDestroy(callback);
OnDestroy installs a callback that's called when a widget is going to to
be destroyed. Useful for special cleanup actions. It differs from a
normal destroy in that all the widget's data structures are still
intact.
Comment: This method could be used with any widgets not just for mega-
widgets. It's listed here because of its usefulness.
Populate
Defines construction and interface of the composite widget.
Usage:
sub Populate {
my ($self, $args) = @_;
...
}
where $self is the widget reference of the already created baseclass
widget and $args is the reference to a hash of -option-value pairs.
Most the other support function are normally used inside the Populate
subroutine.
Don't forget to call $cw->SUPER::Populate($args) in Populate.
privateData
Set/get a private hash of a widget to storage composite internal data
Usage:
$hashref = $self->privateData();
$another = $self->privateData(unique_key|package);
Subwidget
Get the widget reference of an advertised subwidget.
@subwidget = $cw->Subwidget();
$subwidget = $cw->Subwidget(name);
@subwidget = $cw->Subwidget(name ?,...?);
Returns the widget reference(s) of the subwidget known under the given
name(s). Without arguments, return all known subwidgets of $cw. See
Advertise method how to define name for a subwidget.
Comment: Mega-Widget Users: Use Subwidget to get only documented
subwidgets.
PITFALLS
• Resource DB class name
Some of the standard options use a resource date base class that is
not equal to the resource database name. E.g.,
Switch: Name: Class:
-padx padX Pad
-activerelief activeRelief Relief
-activebackground activeBackground Foreground
-status undef undef
One should do the same when one defines one of these options via
ConfigSpecs.
• Method delegation
Redirecting methods to a subwidget with Delegate can only work if
the base widget itself does have a method with this name. Therefore
one can't ``delegate'' any of the methods listed in Tk::Widget. A
common problematic method is bind. In this case one as to
explicitly redirect the method.
sub bind {
my $self = shift;
my $to = $self->privateData->{'my_bind_target'};
$to->bind(@_);
}
• privateData
Graham Barr wrote: ... It is probably more private than most people
think. Not all calls to privateData will return that same HASH
reference. The HASH reference that is returned depends on the
package it was called from, a different HASH is returned for each
package. This allows a widget to hold private data, but then if it
is sub-classed the sub-class will get a different HASH and so not
cause duplicate name clashes.
But privateData does take an optional argument if you want to force
which HASH is returned.
• Scrolled and Composite
Scrolled(Kind,...) constructor can not be used with Composite. One
has to use $cw->Composite(ScrlKind => 'name', ...);
MISSING
Of course Perl/Tk does not define support function for all necessities.
Here's a short list of things you have to handle yourself:
• No support to define construction-time only options.
• No support to remove an option that is known to the base widget.
• It's hard to define undef as fallback for an widget option that is
not already undef.
• Frame in Perl/Tk carries magic and overhead not needed for composite
widget class definition.
• No support methods for bindings that are shared between all widgets
of a composite widget (makes sense at all?)
KEYWORDS
mega, composite, derived, widget
SEE ALSO
Tk::composite Tk::ConfigSpecs Tk::option Tk::callbacks Tk::bind
perl v5.40.1 2025-04-13 mega(3pm)
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