man Tk::composite (3): Defining a new composite widget class

SYNOPSIS

package Tk::MyNewWidget;
use Tk::widgets qw/ list of Tk widgets /;
use base qw/ Tk::Frame /; # or Tk::Toplevel
Construct Tk::Widget 'MyNewWidget';
sub ClassInit {
my( $class, $mw ) = @_;
#... e.g., class bindings here ...
$class->SUPER::ClassInit( $mw );
}
sub Populate {
my( $self, $args ) = @_;
my $flag = delete $args->{-flag};
if( defined $flag ) {
# handle -flag => xxx which can only be done at create
# time the delete above ensures that new() does not try
# and do $self->configure( -flag => xxx );
}
$self->SUPER::Populate( $args );
$self = $self->Component( ... );
$self->Delegates( ... );
$self->ConfigSpecs(
'-cursor' => [ SELF, 'cursor', 'Cursor', undef ],
'-something' => [ METHOD, dbName, dbClass, default ],
'-text' => [ $label, dbName, dbClass, default ],
'-heading' => [ {-text => $head},
heading, Heading, 'My Heading' ],
);
}
sub something {
my( $self, $value) = @_;
if ( @_ > 1 ) {
# set it
}
return # current value
}
1;
__END__
=head1 NAME
Tk::Whatever - a whatever widget
=head1 SYNOPSIS
use Tk::Whatever;
$widget = $parent->Whatever(...);
=head1 DESCRIPTION
...

DESCRIPTION

The intention behind a composite is to create a higher-level widget, sometimes called a ``super-widget'' or ``mega-widget''. Most often, a composite will be built upon other widgets by using them, as opposed to specializing on them. For example, the supplied composite widget LabEntry is made of an Entry and a Label; it is neither a kind-of Label nor is it a kind-of Entry.

Most of the work of a composite widget consistd in creating subwidgets, arranging to dispatch configure options to the proper subwidgets and manage composite-specific configure options.

GLORY DETAILS

Depending on your Perl/Tk knowledge this section may be enlighting or confusing.

Composite Widget

Since Perl/Tk is heavilly using an object-oriented approach, it is no suprise that creating a composite goes through a new() method. However, the composite does not normally define a new() method itself: it is usually sufficient to simply inherit it from Tk::Widget.

This is what happens when the composite uses

    use base qw/ Tk::Frame /;  # or Tk::Toplevel

to specify its inheritance chain. To complete the initialisation of the widget, it must call the Construct method from class Widget. That method accepts the name of the new class to create, i.e. the package name of your composite widget:

    Construct Tk::Widget 'MyNewWidget';

Here, MyNewWidget is the package name (aka the widget's class). This will define a constructor method for MyNewWidget, normally named after the widget's class. Instanciating that composite in client code would the look like:

    $mw = MainWindow->new;       # creates a top-level MainWindow
    $self = $mw->MyNewWidget();  # creates an instance of the
                                 # composite widget MyNewWidget

Whenever a composite is instanciated in client code, "Tk::Widget::new()" will be invoked via the widget's class constructor. That new method will call

    $self->Populate(\%args);

where %args is the arguments passed to the widget's constructor. Note that Populate receives a reference to the hash array containing all arguments.

Populate is typically defined in the composite class (package), which creates the characteristic subwidgets of the class.

Creating Subwidgets

Subwidget creation happens usually in Populate(). The composite usually calls the subwidget's constructor method either directly, for ``private'' subwidgets, or indirectly through the Component method for subwidgets that should be advertised to clients.

Populate may call Delegates to direct calls to methods of chosen subwidgets. For simple composites, typically most if not all methods are directed to a single subwidget - e.g. ScrListbox directs all methods to the core Listbox so that $composite->get(...) calls $listbox->get(...).

Defining mega-widget options

Populate should also call ConfigSpecs() to specify the way that configure-like options should be handled in the composite. Once Populate returns, method Tk::Frame::ConfigDefault walks through the ConfigSpecs entries and populates %$args hash with defaults for options from X resources (.Xdefaults, etc).