/ 
Glib-1.022
River stage three • 92 direct dependents • 195 total dependents
/ Glib::Object::Subclass

NAME

Glib::Object::Subclass - register a perl class as a gobjectclass

SYNOPSIS

use Glib::Object::Subclass
   Some::Base::Class::,   # parent class, derived from Glib::Object
   signals => {
          something_changed => {
             class_closure => sub { do_something_fun () },
             flags         => [qw(run-first)],
             return_type   => undef,
             param_types   => [],
          },
          some_existing_signal => \&class_closure_override,
   },
   properties => [
      Glib::ParamSpec->string (
         'some_string',
         'Some String Property',
         'This property is a string that is used as an example',
         'default value',
         [qw/readable writable/]
      ),
   ];

DESCRIPTION

This module allows you to create your own gobject classes, which is useful to e.g. implement your own Gtk2 widgets.

It doesn't "export" anything into your namespace, but acts more like a pragmatic module that modifies your class to make it work as a GObjectClass.

There are three different issues that this module tries to solve:

  • The gobject type and class system and the perl class system work slightly differently.

  • Classes must be registered before they can be used.

  • Some convenience methods to make GObjectClasses look more like normal perl objects.

You may be wondering why you can't just bless a Glib::Object into a different package and add some subs. Well, if you aren't interested in object parameters, signals, or having your new class interoperate transparently with other GObject-based modules (e.g., Gtk2 and friends), then you can just re-bless.

However, a GObject's signals, properties, and virtual functions are specific to its GObjectClass. If you want to create a new GObject which was a derivative of GtkDrawingArea, but added a new signal, you must create a new GObjectClass to which to add the new signal. If you don't, then all of the GtkDrawingAreas in your application will get that new signal!

Thus, the only way to create a new signal or object property in the perl bindings for Glib is to register a new subclass with Glib::Type. This module is a perl-developer-friendly interface to this bit of paradigm mismatch.

USAGE

This module works similar to the use base pragma in that it registers the current package as a subclass of some other class (which must be a GObjectClass implemented either in C or some other language).

TODO: document it.

OBJECT METHODS AND FUNCTIONS

The following methods are either added to your class on request (not yet implemented), or by default unless your own class implements them itself. This means that all these methods and functions will get sensible default implementations unless explicitly overwritten by you (by defining your own version).

Except for new, all of the following are functions and no methods. That means that you should not call the superclass method. Instead, the GObject system will call these functions per class as required, emulating normal inheritance.

$class->new (attr => value, ...)

The default constructor just calls Glib::Object::new, which allows you to set properties on the newly created object. This is done because many new methods inherited by Gtk2 or other libraries don't have new methods suitable for subclassing.

INIT_INSTANCE $self [not a method]

INIT_INSTANCE is called on each class in the hierarchy as the object is being created (i.e., from Glib::Object::new or our default new). Use this function to initialize any member data. The default implementation will leave the object untouched.

GET_PROPERTY $self, $pspec [not a method]

Get a property value, see SET_PROPERTY.

The default implementation looks like this:

my ($self, $pspec) = @_;
return $self->{$pspec->get_name};
SET_PROPERTY $self, $pspec, $newval [not a method]

GET_PROPERTY and SET_PROPERTY are called whenever somebody does $object->get ($propname) or $object->set ($propname => $newval) (from other languages, too). This is your hook that allows you to store/fetch properties in any way you need to (maybe you have to calculate something or read a file).

GET_PROPERTY is different from a C get_property method in that the perl method returns the retrieved value. For symmetry, the $newval and $pspec args on SET_PROPERTY are swapped from the C usage. The default get and set methods store property data in the object as hash values named for the parameter name.

The default SET_PROPERTY looks like this:

my ($self, $pspec, $newval) = @_;
$self->{$pspec->get_name} = $newval;
FINALIZE_INSTANCE $self [not a method]

FINALIZE_INSTANCE is called as the GObject is being finalized, that is, as it's being really destroyed. This is independent of the more common DESTROY on the perl object; in fact, you must NOT override DESTROY (it's not useful to you, in any case, as it is being called multiple times!).

Use this hook to release anything you have to clean up manually. FINALIZE_INSTANCE will be called for each perl instance, in reverse order of construction.

The default finalizer does nothing.

$object->DESTROY [DO NOT OVERWRITE]

Don't ever overwrite DESTROY, use FINALIZE_INSTANCE instead.

The DESTROY method of all perl classes derived from GTypes is implemented in the Glib module and (ab-)used for it's own internal purposes. Overwriting it is not useful as it will be called multiple times, and often long before the object actually gets destroyed. Overwriting might be very harmful to your program, so never do that. Especially watch out for other classes in your ISA tree.

PROPERTIES

To create gobject properties, supply a list of Glib::ParamSpec objects as the value for the key 'properties'. There are lots of different paramspec constructors, documented in the C API reference's Parameters and Values page.

TODO: put a list here with the proper perl syntax for each

SIGNALS

Creating new signals for your new object is easy. Just provide a hash of signal names and signal descriptions under the key 'signals'. Each signal description is also a hash, with a few expected keys. All the keys are allowed to default.

flags => GSignalFlags

If not present, assumed to be run-first

param_types => reference to a list of package names

If not present, assumed to be empty (no parameters)

class_closure => reference to a subroutine to call as the class closure.

may also be a string interpreted as the name of a subroutine to call, but you should be very very very careful about that.

If not present, the library will attempt to call the method named "do_signal_name" for the signal "signal_name" (uses underscores).

You'll want to be careful not to let this handler method be a publically callable method, or one that has the name name as something that emits the signal. Due to the funky ways in which Glib is different from Perl, the class closures should not inherit through normal perl inheritance.

return_type => package name for return value.

If undefined or not present, the signal expects no return value. if defined, the signal is expected to return a value; flags must be set such that the signal does not run only first (at least use 'run-last').

accumulator => signal return value accumulator

quoting the Glib manual: "The signal accumulator is a special callback function that can be used to collect return values of the various callbacks that are called during a signal emission."

If not specified, the default accumulator is used, and you just get the return value of the last handler to run.

Accumulators are not really documented very much in the C reference, and the perl interface here is slightly different, so here's an inordinate amount of detail for this arcane feature:

The accumulator function is called for every handler. It is given three arguments: the signal invocation hint as an anonymous hash (containing the signal name, notably); the current accumulated return value; and the value returned by the most recent handler. The accumulator must return two values: a boolean value determining whether signal emission should continue (false stops the emission), and the new value for the accumulated return value. (This is different from the C version, which writes through the return_accu.)

OVERRIDING BASE METHODS

Glib pulls some fancy tricks with function pointers to implement methods in C. This is not very language-binding-friendly, as you might guess.

However, as described above, every signal allows a "class closure"; you may override thie class closure with your own function, and you can chain from the overridden method to the original. This serves to implement virtual overrides for language bindings.

So, to override a method, you supply a subroutine reference instead of a signal description hash as the value for the name of the existing signal in the "signals" hash described in the SIGNALs section.

  # override some important widget methods:
  use Glib::Object::Subclass
        Gtk2::Widget::,
	signals => {
		expose_event => \&expose_event,
		configure_event => \&configure_event,
		button_press_event => \&button_press_event,
		button_release_event => \&button_release_event,
		motion_notify_event => \&motion_notify_event,
		# note the choice of names here... see the discussion.
		size_request => \&do_size_request,
	}

It's important to note that the handlers you supply for these are class-specific, and that normal perl method inheritance rules are not followed to invoke them from within the library. However, perl code can still find them! Therefore it's rather important that you choose your handlers' names carefully, avoiding any public interfaces that you might call from perl. Case in point, since size_request is a widget method, i chose do_size_request as the override handler.

SEE ALSO

GObject - http://developer.gnome.org/doc/API/2.0/gobject/

AUTHORS

Marc Lehmann <pcg@goof.com>, muppet <scott at asofyet dot org>

COPYRIGHT AND LICENSE

Copyright 2003 by muppet and the gtk2-perl team

This library is free software; you can redistribute it and/or modify it under the terms of the Lesser General Public License (LGPL). For more information, see http://www.fsf.org/licenses/lgpl.txt