Provided by: libglib-perl_1.329.3-3build3_amd64 
NAME
Glib::Object::Subclass - register a perl class as a GObject class
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 GObject class.
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, virtual functions, and GInterface implementations are specific
to its GObjectClass. If you want to create a new GObject which was a derivative of GtkDrawingArea, but
adds 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 the GLib type system via Glib::Type::register_object(). The Glib::Object::Subclass
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).
The pragma requires at least one argument, the parent class name. The remaining arguments are key/value
pairs, in any order, all optional:
- properties => []
Add object properties; see "PROPERTIES".
- signals => {}
Add or override signals; see "SIGNALS" and "OVERRIDING BASE METHODS".
- interfaces => []
Add GInterfaces to your class; see "INTERFACES".
(Actually, these parameters are all passed straight through to Glib::Type::register_object(), adding
__PACKAGE__ (the current package name) as the name of the new child class.)
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]
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 => $newval)" (from other languages, too). The default implementations hold
property values in the object hash, equivalent to
sub GET_PROPERTY {
my ($self, $pspec) = @_;
my $pname = $pspec->get_name;
return (exists $self->{$pname} ? $self->{$pname}
: $pspec->get_default_value); # until set
}
sub SET_PROPERTY {
my ($self, $pspec, $newval) = @_;
$self->{$pspec->get_name} = $newval;
}
Because "$pspec->get_name" converts hyphens to underscores, a property "line-style" is in the hash as
"line_style".
These methods let you store/fetch properties in any way you need to. They don't have to be in the
hash, you can calculate something, read a file, whatever.
Most often you'll write your own "SET_PROPERTY" so you can take action when a property changes, like
redraw or resize a widget. Eg.
sub SET_PROPERTY {
my ($self, $pspec, $newval) = @_;
my $pname = $pspec->get_name
$self->{$pname} = $newval; # ready for default GET_PROPERTY
if ($pname eq 'line_style') {
$self->queue_draw; # redraw with new lines
}
}
Care must be taken with boxed non-reference-counted types such as "Gtk2::Gdk::Color". In
"SET_PROPERTY" the $newval is generally good only for the duration of the call. Use "copy" or
similar if keeping it longer (see Glib::Boxed). In "GET_PROPERTY" the returned memory must last long
enough to reach the caller, which generally means returning a field, not a newly created object
(which is destroyed with the scalar holding it).
"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.
FINALIZE_INSTANCE $self [not a method]
"FINALIZE_INSTANCE" is called as the GObject is being finalized, that is, as it is 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 its 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, as well as Glib::ParamSpec.
As of Glib 1.060, you can also specify explicit getters and setters for your properties at creation time.
The default values in your properties are also honored if you don't set anything else. See
Glib::Type::register_object in Glib::Type for an example.
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 as
($cont, $acc) = &$func ($invocation_hint, $acc, $ret)
$invocation_hint is an anonymous hash (including the signal name); $acc is the current accumulated
return value; $ret is the value from the most recent handler.
The two return values are a boolean $cont for whether signal emission should continue (false to
stop); and a new $acc accumulated return value. (This is different from the C version, which writes
through a 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 the 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 "SIGNALS".
# 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.
INTERFACES
GObject supports only single inheritance; in place of multiple inheritance, GObject uses GInterfaces. In
the Perl bindings we have mostly masqueraded this with multiple inheritance (that is, simply adding the
GInterface class to the @ISA of the implementing class), but in deriving new objects the facade breaks
and the magic leaks out.
In order to derive an object that implements a GInterface, you have to tell the GLib type system you want
your class to include a GInterface. To do this, simply pass a list of package names through the
"interfaces" key; this will add these packages to your @ISA, and cause perl to invoke methods that you
must provide.
package Mup::MultilineEntry;
use Glib::Object::Subclass
'Gtk2::TextView',
interfaces => [ 'Gtk2::CellEditable' ],
;
# perl will now invoke these methods, which are part of the
# GtkCellEditable GInterface, when somebody invokes the
# corresponding lower-case methods on your objects.
sub START_EDITING { warn "start editing\n"; }
sub EDITING_DONE { warn "editing done\n"; }
sub REMOVE_WIDGET { warn "remove widget\n"; }
SEE ALSO
GObject - http://developer.gnome.org/doc/API/2.0/gobject/
AUTHORS
Marc Lehmann <schmorp@schmorp.de>, muppet <scott at asofyet dot org>
COPYRIGHT AND LICENSE
Copyright 2003-2004, 2010 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
perl v5.38.2 2024-03-31 Glib::Object::Subclass(3pm)