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6 <title>Tutorial</title>
9 This chapter tries to answer the real-life questions of users and presents
10 the most common scenario use cases I could come up with.
11 The use cases are presented from most likely to less likely.
15 <chapter id="howto-gobject">
16 <title>How to define and implement a new GObject</title>
19 Clearly, this is one of the most common questions people ask: they just
20 want to crank code and implement a subclass of a GObject. Sometimes because
21 they want to create their own class hierarchy, sometimes because they want
22 to subclass one of GTK+'s widget. This chapter will focus on the
23 implementation of a subtype of GObject.
26 <sect1 id="howto-gobject-header">
27 <title>Boilerplate header code</title>
30 The first step before writing the code for your GObject is to write the
31 type's header which contains the needed type, function and macro
32 definitions. Each of these elements is nothing but a convention which
33 is followed not only by GTK+'s code but also by most users of GObject.
34 If you feel the need not to obey the rules stated below, think about it
37 <listitem><para>If your users are a bit accustomed to GTK+ code or any
38 GLib code, they will be a bit surprised and getting used to the
39 conventions you decided upon will take time (money) and will make them
40 grumpy (not a good thing)</para></listitem>
41 <listitem><para>You must assess the fact that these conventions might
42 have been designed by both smart and experienced people: maybe they
43 were at least partly right. Try to put your ego aside.</para></listitem>
48 Pick a name convention for your headers and source code and stick to it:
50 <listitem><para>use a dash to separate the prefix from the typename:
51 <filename>maman-bar.h</filename> and <filename>maman-bar.c</filename>
52 (this is the convention used by Nautilus and most GNOME libraries).</para></listitem>
53 <listitem><para>use an underscore to separate the prefix from the
54 typename: <filename>maman_bar.h</filename> and
55 <filename>maman_bar.c</filename>.</para></listitem>
56 <listitem><para>Do not separate the prefix from the typename:
57 <filename>mamanbar.h</filename> and <filename>mamanbar.c</filename>.
58 (this is the convention used by GTK+)</para></listitem>
60 Some people like the first two solutions better: it makes reading file
61 names easier for those with poor eyesight.
65 When you need some private (internal) declarations in several
66 (sub)classes, you can define them in a private header file which
67 is often named by appending the <emphasis>private</emphasis> keyword
68 to the public header name. For example, one could use
69 <filename>maman-bar-private.h</filename>,
70 <filename>maman_bar_private.h</filename> or
71 <filename>mamanbarprivate.h</filename>. Typically, such private header
72 files are not installed.
76 The basic conventions for any header which exposes a GType are described
77 in <xref linkend="gtype-conventions"/>. Most GObject-based code also
78 obeys one of of the following conventions: pick one and stick to it.
81 If you want to declare a type named bar with prefix maman, name the type instance
82 <function>MamanBar</function> and its class <function>MamanBarClass</function>
83 (name is case-sensitive). It is customary to declare them with code similar to the
87 * Copyright/Licensing information.
91 #ifndef __MAMAN_BAR_H__
92 #define __MAMAN_BAR_H__
94 #include <glib-object.h>
96 * Potentially, include other headers on which this header depends.
102 #define MAMAN_TYPE_BAR (maman_bar_get_type ())
103 #define MAMAN_BAR(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
104 #define MAMAN_IS_BAR(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
105 #define MAMAN_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
106 #define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
107 #define MAMAN_BAR_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))
109 typedef struct _MamanBar MamanBar;
110 typedef struct _MamanBarClass MamanBarClass;
114 GObject parent_instance;
116 /* instance members */
119 struct _MamanBarClass
121 GObjectClass parent_class;
126 /* used by MAMAN_TYPE_BAR */
127 GType maman_bar_get_type (void);
130 * Method definitions.
133 #endif /* __MAMAN_BAR_H__ */
137 Most GTK+ types declare their private fields in the public header
138 with a /* private */ comment, relying on their user's intelligence
139 not to try to play with these fields. Fields not marked private
140 are considered public by default. The /* protected */ comment
141 (same semantics as those of C++) is also used, mainly in the GType
142 library, in code written by Tim Janik.
146 GObject parent_instance;
148 /*< private >*/
154 All of Nautilus code and a lot of GNOME libraries use private
155 indirection members, as described by Herb Sutter in his Pimpl
156 articles(see <ulink url="http://www.gotw.ca/gotw/024.htm">Compilation Firewalls</ulink>
157 and <ulink url="http://www.gotw.ca/gotw/028.htm">The Fast Pimpl Idiom</ulink>:
158 he summarizes the different issues better than I will).
160 typedef struct _MamanBarPrivate MamanBarPrivate;
164 GObject parent_instance;
166 /*< private >*/
167 MamanBarPrivate *priv;
170 <note><simpara>Do not call this <varname>private</varname>, as
171 that is a registered c++ keyword.</simpara></note>
173 The private structure is then defined in the .c file, using the
174 g_type_class_add_private() function to notify the presence of
175 a private memory area for each instance and it can either
176 be retrieved using <function>G_TYPE_INSTANCE_GET_PRIVATE()</function>
177 each time is needed, or assigned to the <literal>priv</literal>
178 member of the instance structure inside the object's
179 <function>init</function> function.
181 #define MAMAN_BAR_GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MAMAN_TYPE_BAR, MamanBarPrivate))
183 struct _MamanBarPrivate
189 maman_bar_class_init (MamanBarClass *klass)
191 g_type_class_add_private (klass, sizeof (MamanBarPrivate));
195 maman_bar_init (MamanBar *self)
197 MamanBarPrivate *priv;
199 self->priv = priv = MAMAN_BAR_GET_PRIVATE (self);
207 You don't need to free or allocate the private structure, only the
208 objects or pointers that it may contain. Another advantage of this
209 to the previous version is that is lessens memory fragmentation,
210 as the public and private parts of the instance memory are
217 Finally, there are different header include conventions. Again, pick one
218 and stick to it. I personally use indifferently any of the two, depending
219 on the codebase I work on: the rule, as always, is consistency.
222 Some people add at the top of their headers a number of #include
223 directives to pull in all the headers needed to compile client
224 code. This allows client code to simply #include "maman-bar.h".
227 Other do not #include anything and expect the client to #include
228 themselves the headers they need before including your header. This
229 speeds up compilation because it minimizes the amount of
230 pre-processor work. This can be used in conjunction with the
231 re-declaration of certain unused types in the client code to
232 minimize compile-time dependencies and thus speed up compilation.
239 <sect1 id="howto-gobject-code">
240 <title>Boilerplate code</title>
243 In your code, the first step is to #include the needed headers: depending
244 on your header include strategy, this can be as simple as
245 <literal>#include "maman-bar.h"</literal> or as complicated as tens
246 of #include lines ending with <literal>#include "maman-bar.h"</literal>:
249 * Copyright information
252 #include "maman-bar.h"
254 /* If you use Pimpls, include the private structure
255 * definition here. Some people create a maman-bar-private.h header
256 * which is included by the maman-bar.c file and which contains the
257 * definition for this private structure.
259 struct _MamanBarPrivate {
265 * forward definitions
271 Call the <function>G_DEFINE_TYPE</function> macro using the name
272 of the type, the prefix of the functions and the parent GType to
273 reduce the amount of boilerplate needed. This macro will:
276 <listitem><simpara>implement the <function>maman_bar_get_type</function>
277 function</simpara></listitem>
278 <listitem><simpara>define a parent class pointer accessible from
279 the whole .c file</simpara></listitem>
283 G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);
288 It is also possible to use the
289 <function>G_DEFINE_TYPE_WITH_CODE</function> macro to control the
290 get_type function implementation - for instance, to add a call to
291 <function>G_IMPLEMENT_INTERFACE</function> macro which will
292 call the <function>g_type_implement_interface</function> function.
296 <sect1 id="howto-gobject-construction">
297 <title>Object Construction</title>
300 People often get confused when trying to construct their GObjects because of the
301 sheer number of different ways to hook into the objects's construction process: it is
302 difficult to figure which is the <emphasis>correct</emphasis>, recommended way.
306 <xref linkend="gobject-construction-table"/> shows what user-provided functions
307 are invoked during object instantiation and in which order they are invoked.
308 A user looking for the equivalent of the simple C++ constructor function should use
309 the instance_init method. It will be invoked after all the parent's instance_init
310 functions have been invoked. It cannot take arbitrary construction parameters
311 (as in C++) but if your object needs arbitrary parameters to complete initialization,
312 you can use construction properties.
316 Construction properties will be set only after all instance_init functions have run.
317 No object reference will be returned to the client of <function><link linkend="g-object-new">g_object_new</link></function>
318 until all the construction properties have been set.
322 As such, I would recommend writing the following code first:
325 maman_bar_init (MamanBar *self)
327 self->priv = MAMAN_BAR_GET_PRIVATE (self);
329 /* initialize all public and private members to reasonable default values. */
331 /* If you need specific construction properties to complete initialization,
332 * delay initialization completion until the property is set.
339 Now, if you need special construction properties, install the properties in the class_init function,
340 override the set and get methods and implement the get and set methods as described in
341 <xref linkend="gobject-properties"/>. Make sure that these properties use a construct only
342 <link linkend="GParamSpec"><type>GParamSpec</type></link> by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps
343 GType ensure that these properties are not set again later by malicious user code.
346 bar_class_init (MamanBarClass *klass)
348 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
349 GParamSpec *maman_param_spec;
351 gobject_class->set_property = bar_set_property;
352 gobject_class->get_property = bar_get_property;
354 maman_param_spec = g_param_spec_string ("maman",
355 "Maman construct prop",
357 "no-name-set" /* default value */,
358 G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE);
359 g_object_class_install_property (gobject_class,
364 If you need this, make sure you can build and run code similar to the code shown above. Make sure
365 your construct properties can set correctly during construction, make sure you cannot set them
366 afterwards and make sure that if your users do not call <function><link linkend="g-object-new">g_object_new</link></function>
367 with the required construction properties, these will be initialized with the default values.
371 I consider good taste to halt program execution if a construction property is set its
372 default value. This allows you to catch client code which does not give a reasonable
373 value to the construction properties. Of course, you are free to disagree but you
374 should have a good reason to do so.
378 Some people sometimes need to construct their object but only after
379 the construction properties have been set. This is possible through
380 the use of the constructor class method as described in
381 <xref linkend="gobject-instantiation"/> or, more simply, using
382 the constructed class method available since GLib 2.12.
386 <sect1 id="howto-gobject-destruction">
387 <title>Object Destruction</title>
390 Again, it is often difficult to figure out which mechanism to use to
391 hook into the object's destruction process: when the last
392 <function><link linkend="g-object-unref">g_object_unref</link></function>
393 function call is made, a lot of things happen as described in
394 <xref linkend="gobject-destruction-table"/>.
398 The destruction process of your object might be split in two different
399 phases: dispose and the finalize.
401 #define MAMAN_BAR_GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MAMAN_TYPE_BAR, MamanBarPrivate))
403 struct _MamanBarPrivate
410 G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);
413 maman_bar_dispose (GObject *gobject)
415 MamanBar *self = MAMAN_BAR (gobject);
418 * In dispose, you are supposed to free all types referenced from this
419 * object which might themselves hold a reference to self. Generally,
420 * the most simple solution is to unref all members on which you own a
424 /* dispose might be called multiple times, so we must guard against
425 * calling g_object_unref() on an invalid GObject.
427 if (self->priv->an_object)
429 g_object_unref (self->priv->an_object);
431 self->priv->an_object = NULL;
434 /* Chain up to the parent class */
435 G_OBJECT_CLASS (maman_bar_parent_class)->dispose (gobject);
439 maman_bar_finalize (GObject *gobject)
441 MamanBar *self = MAMAN_BAR (gobject);
443 g_free (self->priv->a_string);
445 /* Chain up to the parent class */
446 G_OBJECT_CLASS (maman_bar_parent_class)->finalize (gobject);
450 maman_bar_class_init (MamanBarClass *klass)
452 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
454 gobject_class->dispose = maman_bar_dispose;
455 gobject_class->finalize = maman_bar_finalize;
457 g_type_class_add_private (klass, sizeof (MamanBarPrivate));
461 maman_bar_init (MamanBar *self);
463 self->priv = MAMAN_BAR_GET_PRIVATE (self);
465 self->priv->an_object = g_object_new (MAMAN_TYPE_BAZ, NULL);
466 self->priv->a_string = g_strdup ("Maman");
472 Add similar code to your GObject, make sure the code still builds
473 and runs: dispose and finalize must be called during the last unref.
477 It is possible that object methods might be invoked after dispose is
478 run and before finalize runs. GObject does not consider this to be a
479 program error: you must gracefully detect this and neither crash nor
484 <sect1 id="howto-gobject-methods">
485 <title>Object methods</title>
488 Just as with C++, there are many different ways to define object
489 methods and extend them: the following list and sections draw on
490 C++ vocabulary. (Readers are expected to know basic C++ buzzwords.
491 Those who have not had to write C++ code recently can refer to e.g.
492 <ulink url="http://www.cplusplus.com/doc/tutorial/"/> to refresh
496 non-virtual public methods,
499 virtual public methods and
502 virtual private methods
508 <title>Non-virtual public methods</title>
511 These are the simplest: you want to provide a simple method which
512 can act on your object. All you need to do is to provide a function
513 prototype in the header and an implementation of that prototype
516 /* declaration in the header. */
517 void maman_bar_do_action (MamanBar *self, /* parameters */);
519 /* implementation in the source file */
521 maman_bar_do_action (MamanBar *self, /* parameters */)
523 g_return_if_fail (MAMAN_IS_BAR (self));
530 <para>There is really nothing scary about this.</para>
534 <title>Virtual public methods</title>
537 This is the preferred way to create polymorphic GObjects. All you
538 need to do is to define the common method and its class function in
539 the public header, implement the common method in the source file
540 and re-implement the class function in each object which inherits
543 /* declaration in maman-bar.h. */
544 struct _MamanBarClass
546 GObjectClass parent_class;
549 void (*do_action) (MamanBar *self, /* parameters */);
552 void maman_bar_do_action (MamanBar *self, /* parameters */);
554 /* implementation in maman-bar.c */
556 maman_bar_do_action (MamanBar *self, /* parameters */)
558 g_return_if_fail (MAMAN_IS_BAR (self));
560 MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
563 The code above simply redirects the do_action call to the relevant
564 class function. Some users, concerned about performance, do not
565 provide the <function>maman_bar_do_action</function> wrapper function
566 and require users to dereference the class pointer themselves. This
567 is not such a great idea in terms of encapsulation and makes it
568 difficult to change the object's implementation afterwards, should
573 Other users, also concerned by performance issues, declare
574 the <function>maman_bar_do_action</function> function inline in the
575 header file. This, however, makes it difficult to change the
576 object's implementation later (although easier than requiring users
577 to directly dereference the class function) and is often difficult
578 to write in a portable way (the <emphasis>inline</emphasis> keyword
579 is part of the C99 standard but not every compiler supports it).
583 In doubt, unless a user shows you hard numbers about the performance
584 cost of the function call, just implement <function>maman_bar_do_action</function>
589 Please, note that it is possible for you to provide a default
590 implementation for this class method in the object's
591 <function>class_init</function> function: initialize the
592 klass->do_action field to a pointer to the actual implementation.
593 You can also make this class method pure virtual by initializing
594 the klass->do_action field to NULL:
597 maman_bar_real_do_action_two (MamanBar *self, /* parameters */)
599 /* Default implementation for the virtual method. */
603 maman_bar_class_init (BarClass *klass)
605 /* pure virtual method: mandates implementation in children. */
606 klass->do_action_one = NULL;
608 /* merely virtual method. */
609 klass->do_action_two = maman_bar_real_do_action_two;
613 maman_bar_do_action_one (MamanBar *self, /* parameters */)
615 g_return_if_fail (MAMAN_IS_BAR (self));
617 MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
621 maman_bar_do_action_two (MamanBar *self, /* parameters */)
623 g_return_if_fail (MAMAN_IS_BAR (self));
625 MAMAN_BAR_GET_CLASS (self)->do_action_two (self, /* parameters */);
632 <title>Virtual private Methods</title>
635 These are very similar to Virtual Public methods. They just don't
636 have a public function to call the function directly. The header
637 file contains only a declaration of the class function:
639 /* declaration in maman-bar.h. */
640 struct _MamanBarClass
645 void (* helper_do_specific_action) (MamanBar *self, /* parameters */);
648 void maman_bar_do_any_action (MamanBar *self, /* parameters */);
650 These class functions are often used to delegate part of the job
653 /* this accessor function is static: it is not exported outside of this file. */
655 maman_bar_do_specific_action (MamanBar *self, /* parameters */)
657 MAMAN_BAR_GET_CLASS (self)->do_specific_action (self, /* parameters */);
661 maman_bar_do_any_action (MamanBar *self, /* parameters */)
663 /* random code here */
666 * Try to execute the requested action. Maybe the requested action
667 * cannot be implemented here. So, we delegate its implementation
668 * to the child class:
670 maman_bar_do_specific_action (self, /* parameters */);
672 /* other random code here */
678 Again, it is possible to provide a default implementation for this
679 private virtual class function:
682 maman_bar_class_init (MamanBarClass *klass)
684 /* pure virtual method: mandates implementation in children. */
685 klass->do_specific_action_one = NULL;
687 /* merely virtual method. */
688 klass->do_specific_action_two = maman_bar_real_do_specific_action_two;
694 Children can then implement the subclass with code such as:
697 maman_bar_subtype_class_init (MamanBarSubTypeClass *klass)
699 MamanBarClass *bar_class = MAMAN_BAR_CLASS (klass);
701 /* implement pure virtual class function. */
702 bar_class->do_specific_action_one = maman_bar_subtype_do_specific_action_one;
709 <sect1 id="howto-gobject-chainup">
710 <title>Chaining up</title>
712 <para>Chaining up is often loosely defined by the following set of
715 <listitem><para>Parent class A defines a public virtual method named <function>foo</function> and
716 provides a default implementation.</para></listitem>
717 <listitem><para>Child class B re-implements method <function>foo</function>.</para></listitem>
718 <listitem><para>In the method B::foo, the child class B calls its parent class method A::foo.</para></listitem>
720 There are many uses to this idiom:
722 <listitem><para>You need to change the behaviour of a class without modifying its code. You create
723 a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
724 slightly and chain up to ensure that the previous behaviour is not really modified, just extended.
726 <listitem><para>You are lazy, you have access to the source code of the parent class but you don't want
727 to modify it to add method calls to new specialized method calls: it is faster to hack the child class
728 to chain up than to modify the parent to call down.</para></listitem>
729 <listitem><para>You need to implement the Chain Of Responsibility pattern: each object of the inheritance
730 tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that
731 they each handler is run in turn.</para></listitem>
733 I am personally not really convinced any of the last two uses are really a good idea but since this
734 programming idiom is often used, this section attempts to explain how to implement it.
738 To explicitly chain up to the implementation of the virtual method in the parent class,
739 you first need a handle to the original parent class structure. This pointer can then be used to
740 access the original class function pointer and invoke it directly.
743 The <emphasis>original</emphasis> adjective used in this sentence is not innocuous. To fully
744 understand its meaning, you need to recall how class structures are initialized: for each object type,
745 the class structure associated to this object is created by first copying the class structure of its
746 parent type (a simple <function>memcpy</function>) and then by invoking the class_init callback on
747 the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
748 with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class
749 structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent
755 <para>The function <function><link linkend="g-type-class-peek-parent">g_type_class_peek_parent</link></function> is used to access the original parent
756 class structure. Its input is a pointer to the class of the derived object and it returns a pointer
757 to the original parent class structure. The code below shows how you could use it:
760 b_method_to_call (B *obj, int a)
763 AClass *parent_class;
765 klass = B_GET_CLASS (obj);
766 parent_class = g_type_class_peek_parent (klass);
768 /* do stuff before chain up */
770 parent_class->method_to_call (obj, a);
772 /* do stuff after chain up */
780 <!-- End Howto GObject -->
782 <chapter id="howto-interface">
783 <title>How to define and implement interfaces</title>
785 <sect1 id="howto-interface-define">
786 <title>How to define interfaces</title>
789 The bulk of interface definition has already been shown in <xref linkend="gtype-non-instantiable-classed"/>
790 but I feel it is needed to show exactly how to create an interface.
794 As above, the first step is to get the header right:
796 #ifndef __MAMAN_IBAZ_H__
797 #define __MAMAN_IBAZ_H__
799 #include <glib-object.h>
801 #define MAMAN_TYPE_IBAZ (maman_ibaz_get_type ())
802 #define MAMAN_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
803 #define MAMAN_IS_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
804 #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))
807 typedef struct _MamanIbaz MamanIbaz; /* dummy object */
808 typedef struct _MamanIbazInterface MamanIbazInterface;
810 struct _MamanIbazInterface
812 GTypeInterface parent_iface;
814 void (*do_action) (MamanIbaz *self);
817 GType maman_ibaz_get_type (void);
819 void maman_ibaz_do_action (MamanIbaz *self);
821 #endif /* __MAMAN_IBAZ_H__ */
823 This code is the same as the code for a normal <link linkend="GType"><type>GType</type></link>
824 which derives from a <link linkend="GObject"><type>GObject</type></link> except for a few details:
827 The <function>_GET_CLASS</function> macro is called <function>_GET_INTERFACE</function>
828 and not implemented with <function><link linkend="G_TYPE_INSTANCE_GET_CLASS">G_TYPE_INSTANCE_GET_CLASS</link></function>
829 but with <function><link linkend="G_TYPE_INSTANCE_GET_INTERFACE">G_TYPE_INSTANCE_GET_INTERFACE</link></function>.
832 The instance type, <type>MamanIbaz</type> is not fully defined: it is
833 used merely as an abstract type which represents an instance of
834 whatever object which implements the interface.
837 The parent of the <type>MamanIbazInterface</type> is not
838 <type>GObjectClass</type> but <type>GTypeInterface</type>.
844 The implementation of the <type>MamanIbaz</type> type itself is trivial:
846 <listitem><para><function>maman_ibaz_get_type</function> registers the
847 type in the type system.
849 <listitem><para><function>maman_ibaz_base_init</function> is expected
850 to register the interface's signals if there are any (we will see a bit
851 (later how to use them). Make sure to use a static local boolean variable
852 to make sure not to run the initialization code twice (as described in
853 <xref linkend="gtype-non-instantiable-classed-init"/>,
854 <function>base_init</function> is run once for each interface implementation
855 instantiation)</para></listitem>
856 <listitem><para><function>maman_ibaz_do_action</function> dereferences
857 the class structure to access its associated class function and calls it.
862 maman_ibaz_base_init (gpointer g_class)
864 static gboolean is_initialized = FALSE;
868 /* add properties and signals to the interface here */
870 is_initialized = TRUE;
875 maman_ibaz_get_type (void)
877 static GType iface_type = 0;
880 static const GTypeInfo info = {
881 sizeof (MamanIbazInterface),
882 maman_ibaz_base_init, /* base_init */
883 NULL, /* base_finalize */
886 iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
894 maman_ibaz_do_action (MamanIbaz *self)
896 g_return_if_fail (MAMAN_IS_IBAZ (self));
898 MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
904 <sect1 id="howto-interface-implement">
905 <title>How To define implement an Interface?</title>
908 Once the interface is defined, implementing it is rather trivial.
912 The first step is to define a normal GObject class, like:
914 #ifndef __MAMAN_BAZ_H__
915 #define __MAMAN_BAZ_H__
917 #include <glib-object.h>
919 #define MAMAN_TYPE_BAZ (maman_baz_get_type ())
920 #define MAMAN_BAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
921 #define MAMAN_IS_BAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
922 #define MAMAN_BAZ_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAZ, MamanbazClass))
923 #define MAMAN_IS_BAZ_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAZ))
924 #define MAMAN_BAZ_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAZ, MamanbazClass))
927 typedef struct _MamanBaz MamanBaz;
928 typedef struct _MamanBazClass MamanBazClass;
932 GObject parent_instance;
937 struct _MamanBazClass
939 GObjectClass parent_class;
942 GType maman_baz_get_type (void);
944 #endif /* __MAMAN_BAZ_H__ */
946 There is clearly nothing specifically weird or scary about this header:
947 it does not define any weird API or derives from a weird type.
951 The second step is to implement <type>MamanBaz</type> by defining
952 its GType. Instead of using <function>G_DEFINE_TYPE</function> we
953 use <function>G_DEFINE_TYPE_WITH_CODE</function> and the
954 <function>G_IMPLEMENT_INTERFACE</function> macros.
956 static void maman_ibaz_interface_init (MamanIbazInterface *iface);
958 G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
959 G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
960 maman_ibaz_interface_init));
962 This definition is very much like all the similar functions we looked
963 at previously. The only interface-specific code present here is the call
964 to <function>G_IMPLEMENT_INTERFACE</function>.
967 <note><para>Classes can implement multiple interfaces by using multiple
968 calls to <function>G_IMPLEMENT_INTERFACE</function> inside the call
969 to <function>G_DEFINE_TYPE_WITH_CODE</function>.</para></note>
972 <function>maman_baz_interface_init</function>, the interface
973 initialization function: inside it every virtual method of the interface
974 must be assigned to its implementation:
977 maman_baz_do_action (MamanBaz *self)
979 g_print ("Baz implementation of IBaz interface Action: 0x%x.\n",
980 self->instance_member);
984 maman_ibaz_interface_init (MamanIbazInterface *iface)
986 iface->do_action = baz_do_action;
990 maman_baz_init (MamanBaz *self)
992 MamanBaz *self = MAMAN_BAZ (instance);
993 self->instance_member = 0xdeadbeaf;
1001 <title>Interface definition prerequisites</title>
1004 To specify that an interface requires the presence of other interfaces
1005 when implemented, GObject introduces the concept of
1006 <emphasis>prerequisites</emphasis>: it is possible to associate
1007 a list of prerequisite interfaces to an interface. For example, if
1008 object A wishes to implement interface I1, and if interface I1 has a
1009 prerequisite on interface I2, A has to implement both I1 and I2.
1013 The mechanism described above is, in practice, very similar to
1014 Java's interface I1 extends interface I2. The example below shows
1015 the GObject equivalent:
1017 /* inside the GType function of the MamanIbar interface */
1018 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &info, 0);
1020 /* Make the MamanIbar interface require MamanIbaz interface. */
1021 g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
1023 The code shown above adds the MamanIbaz interface to the list of
1024 prerequisites of MamanIbar while the code below shows how an
1025 implementation can implement both interfaces and register their
1029 maman_ibar_do_another_action (MamanIbar *ibar)
1031 MamanBar *self = MAMAN_BAR (ibar);
1033 g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n",
1034 self->instance_member);
1038 maman_ibar_interface_init (MamanIbarInterface *iface)
1040 iface->do_another_action = maman_ibar_do_another_action;
1044 maman_ibaz_do_action (MamanIbaz *ibaz)
1046 MamanBar *self = MAMAN_BAR (ibaz);
1048 g_print ("Bar implementation of IBaz interface Action: 0x%x.\n",
1049 self->instance_member);
1053 maman_ibaz_interface_init (MamanIbazInterface *iface)
1055 iface->do_action = maman_ibaz_do_action;
1059 maman_bar_class_init (MamanBarClass *klass)
1065 maman_bar_init (MamanBar *self)
1067 self->instance_member = 0x666;
1070 G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
1071 G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
1072 maman_ibaz_interface_init)
1073 G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAR,
1074 maman_ibar_interface_init));
1076 It is very important to notice that the order in which interface
1077 implementations are added to the main object is not random:
1078 <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function>,
1079 which is called by <function>G_IMPLEMENT_INTERFACE</function>, must be
1080 invoked first on the interfaces which have no prerequisites and then on
1085 <sect1 id="howto-interface-properties">
1086 <title>Interface Properties</title>
1089 Starting from version 2.4 of GLib, GObject interfaces can also have
1090 properties. Declaration of the interface properties is similar to
1091 declaring the properties of ordinary GObject types as explained in
1092 <xref linkend="gobject-properties"/>,
1093 except that <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> is used to
1094 declare the properties instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
1098 To include a property named 'name' of type <type>string</type> in the
1099 <type>maman_ibaz</type> interface example code above, we only need to
1103 That really is one line extended to six for the sake of clarity
1106 line in the <function>maman_ibaz_base_init</function>
1109 The <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function>
1110 can also be called from <function>class_init</function> but it must
1111 not be called after that point.
1117 maman_ibaz_base_init (gpointer g_iface)
1119 static gboolean is_initialized = FALSE;
1121 if (!is_initialized)
1123 g_object_interface_install_property (g_iface,
1124 g_param_spec_string ("name",
1126 "Name of the MamanIbaz",
1128 G_PARAM_READWRITE));
1129 is_initialized = TRUE;
1136 One point worth noting is that the declared property wasn't assigned an
1137 integer ID. The reason being that integer IDs of properties are used
1138 only inside the get and set methods and since interfaces do not
1139 implement properties, there is no need to assign integer IDs to
1140 interface properties.
1144 An implementation shall declare and define it's properties in the usual
1145 way as explained in <xref linkend="gobject-properties"/>, except for one
1146 small change: it must declare the properties of the interface it
1147 implements using <function><link linkend="g-object-class-override-property">g_object_class_override_property</link></function>
1148 instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
1149 The following code snippet shows the modifications needed in the
1150 <type>MamanBaz</type> declaration and implementation above:
1155 GObject parent_instance;
1157 gint instance_member;
1169 maman_baz_set_property (GObject *object,
1171 const GValue *value,
1174 MamanBaz *baz = MAMAN_BAZ (object);
1181 baz->name = g_value_dup_string (value);
1185 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1191 maman_baz_get_property (GObject *object,
1196 MamanBaz *baz = MAMAN_BAZ (object);
1201 g_value_set_string (value, baz->name);
1205 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1211 maman_baz_class_init (MamanBazClass *klass)
1213 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
1215 gobject_class->set_property = maman_baz_set_property;
1216 gobject_class->get_property = maman_baz_get_property;
1218 g_object_class_override_property (gobject_class, PROP_NAME, "name");
1226 <!-- End Howto Interfaces -->
1228 <chapter id="howto-signals">
1229 <title>How to create and use signals</title>
1232 The signal system which was built in GType is pretty complex and
1233 flexible: it is possible for its users to connect at runtime any
1234 number of callbacks (implemented in any language for which a binding
1237 <para>A Python callback can be connected to any signal on any
1241 to any signal and to stop the emission of any signal at any
1242 state of the signal emission process. This flexibility makes it
1243 possible to use GSignal for much more than just emit signals which
1244 can be received by numerous clients.
1247 <sect1 id="howto-simple-signals">
1248 <title>Simple use of signals</title>
1251 The most basic use of signals is to implement simple event
1252 notification: for example, if we have a MamanFile object, and
1253 if this object has a write method, we might wish to be notified
1254 whenever someone has changed something via our MamanFile instance.
1255 The code below shows how the user can connect a callback to the
1258 file = g_object_new (MAMAN_FILE_TYPE, NULL);
1260 g_signal_connect (file, "changed", G_CALLBACK (changed_event), NULL);
1262 maman_file_write (file, buffer, strlen (buffer));
1267 The <type>MamanFile</type> signal is registered in the class_init
1270 file_signals[CHANGED] =
1271 g_signal_newv ("changed",
1272 G_TYPE_FROM_CLASS (gobject_class),
1273 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1275 NULL /* accumulator */,
1276 NULL /* accumulator data */,
1277 g_cclosure_marshal_VOID__VOID,
1278 G_TYPE_NONE /* return_type */,
1280 NULL /* param_types */);
1282 and the signal is emitted in <function>maman_file_write</function>:
1285 maman_file_write (MamanFile *self,
1286 const guchar *buffer,
1289 /* First write data. */
1291 /* Then, notify user of data written. */
1292 g_signal_emit (self, file_signals[CHANGED], 0 /* details */);
1295 As shown above, you can safely set the details parameter to zero if
1296 you do not know what it can be used for. For a discussion of what you
1297 could used it for, see <xref linkend="signal-detail"/>
1301 The signature of the signal handler in the above example is defined as
1302 <function>g_cclosure_marshal_VOID__VOID</function>. Its name follows
1303 a simple convention which encodes the function parameter and return value
1304 types in the function name. Specifically, the value in front of the
1305 double underscore is the type of the return value, while the value(s)
1306 after the double underscore denote the parameter types.
1310 The header <filename>gobject/gmarshal.h</filename> defines a set of
1311 commonly needed closures that one can use. If you want to have complex
1312 marshallers for your signals you should probably use glib-genmarshal
1313 to autogenerate them from a file containing their return and
1319 this is utterly wrong and should be completely removed - or rewritten
1320 with a better example than writing a buffer using synchronous signals.
1323 <title>How to provide more flexibility to users?</title>
1326 The previous implementation does the job but the signal facility of
1327 GObject can be used to provide even more flexibility to this file
1328 change notification mechanism. One of the key ideas is to make the
1329 process of writing data to the file part of the signal emission
1330 process to allow users to be notified either before or after the
1331 data is written to the file.
1335 To integrate the process of writing the data to the file into the
1336 signal emission mechanism, we can register a default class closure
1337 for this signal which will be invoked during the signal emission,
1338 just like any other user-connected signal handler.
1342 The first step to implement this idea is to change the signature of
1343 the signal: we need to pass around the buffer to write and its size.
1344 To do this, we use our own marshaller which will be generated
1345 through GLib's glib-genmarshal tool. We thus create a file named <filename>marshall.list</filename> which contains
1346 the following single line:
1350 and use the Makefile provided in <filename>sample/signal/Makefile</filename> to generate the file named
1351 <filename>maman-file-complex-marshall.c</filename>. This C file is finally included in
1352 <filename>maman-file-complex.c</filename>.
1356 Once the marshaller is present, we register the signal and its marshaller in the class_init function
1357 of the object <type>MamanFileComplex</type> (full source for this object is included in
1358 <filename>sample/signal/maman-file-complex.{h|c}</filename>):
1360 GClosure *default_closure;
1361 GType param_types[2];
1363 default_closure = g_cclosure_new (G_CALLBACK (default_write_signal_handler),
1364 (gpointer)0xdeadbeaf /* user_data */,
1365 NULL /* destroy_data */);
1367 param_types[0] = G_TYPE_POINTER;
1368 param_types[1] = G_TYPE_UINT;
1369 klass->write_signal_id =
1370 g_signal_newv ("write",
1371 G_TYPE_FROM_CLASS (g_class),
1372 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1373 default_closure /* class closure */,
1374 NULL /* accumulator */,
1375 NULL /* accu_data */,
1376 maman_file_complex_VOID__POINTER_UINT,
1377 G_TYPE_NONE /* return_type */,
1379 param_types /* param_types */);
1381 The code shown above first creates the closure which contains the code to complete the file write. This
1382 closure is registered as the default class_closure of the newly created signal.
1386 Of course, you need to implement completely the code for the default closure since I just provided
1390 default_write_signal_handler (GObject *obj, guint8 *buffer, guint size, gpointer user_data)
1392 g_assert (user_data == (gpointer)0xdeadbeaf);
1393 /* Here, we trigger the real file write. */
1394 g_print ("default signal handler: 0x%x %u\n", buffer, size);
1400 Finally, the client code must invoke the <function>maman_file_complex_write</function> function which
1401 triggers the signal emission:
1403 void maman_file_complex_write (MamanFileComplex *self, guint8 *buffer, guint size)
1406 g_signal_emit (self,
1407 MAMAN_FILE_COMPLEX_GET_CLASS (self)->write_signal_id,
1415 The client code (as shown in <filename>sample/signal/test.c</filename> and below) can now connect signal handlers before
1416 and after the file write is completed: since the default signal handler which does the write itself runs during the
1417 RUN_LAST phase of the signal emission, it will run after all handlers connected with <function><link linkend="g-signal-connect">g_signal_connect</link></function>
1418 and before all handlers connected with <function><link linkend="g-signal-connect-after">g_signal_connect_after</link></function>. If you intent to write a GObject
1419 which emits signals, I would thus urge you to create all your signals with the G_SIGNAL_RUN_LAST such that your users
1420 have a maximum of flexibility as to when to get the event. Here, we combined it with G_SIGNAL_NO_RECURSE and
1421 G_SIGNAL_NO_HOOKS to ensure our users will not try to do really weird things with our GObject. I strongly advise you
1422 to do the same unless you really know why (in which case you really know the inner workings of GSignal by heart and
1423 you are not reading this).
1428 static void complex_write_event_before (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1430 g_assert (user_data == NULL);
1431 g_print ("Complex Write event before: 0x%x, %u\n", buffer, size);
1434 static void complex_write_event_after (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1436 g_assert (user_data == NULL);
1437 g_print ("Complex Write event after: 0x%x, %u\n", buffer, size);
1440 static void test_file_complex (void)
1445 file = g_object_new (MAMAN_FILE_COMPLEX_TYPE, NULL);
1447 g_signal_connect (G_OBJECT (file), "write",
1448 (GCallback)complex_write_event_before,
1451 g_signal_connect_after (G_OBJECT (file), "write",
1452 (GCallback)complex_write_event_after,
1455 maman_file_complex_write (MAMAN_FILE_COMPLEX (file), buffer, 50);
1457 g_object_unref (G_OBJECT (file));
1460 The code above generates the following output on my machine:
1462 Complex Write event before: 0xbfffe280, 50
1463 default signal handler: 0xbfffe280 50
1464 Complex Write event after: 0xbfffe280, 50
1471 this is also utterly wrong on so many levels that I don't even want
1472 to enumerate them. it's also full of completely irrelevant footnotes
1473 about personal preferences demonstrating a severe lack of whatsoever
1474 clue. the whole idea of storing the signal ids inside the Class
1475 structure is so fundamentally flawed that I'll require a frontal
1476 lobotomy just to forget I've ever seen it.
1479 <title>How most people do the same thing with less code</title>
1481 <para>For many historic reasons related to how the ancestor of GObject used to work in GTK+ 1.x versions,
1482 there is a much <emphasis>simpler</emphasis>
1484 <para>I personally think that this method is horribly mind-twisting: it adds a new indirection
1485 which unnecessarily complicates the overall code path. However, because this method is widely used
1486 by all of GTK+ and GObject code, readers need to understand it. The reason why this is done that way
1487 in most of GTK+ is related to the fact that the ancestor of GObject did not provide any other way to
1488 create a signal with a default handler than this one. Some people have tried to justify that it is done
1489 that way because it is better, faster (I am extremely doubtful about the faster bit. As a matter of fact,
1490 the better bit also mystifies me ;-). I have the feeling no one really knows and everyone does it
1491 because they copy/pasted code from code which did the same. It is probably better to leave this
1492 specific trivia to hacker legends domain...
1495 way to create a signal with a default handler than to create
1496 a closure by hand and to use the <function><link linkend="g-signal-newv">g_signal_newv</link></function>.
1499 <para>For example, <function><link linkend="g-signal-new">g_signal_new</link></function> can be used to create a signal which uses a default
1500 handler which is stored in the class structure of the object. More specifically, the class structure
1501 contains a function pointer which is accessed during signal emission to invoke the default handler and
1502 the user is expected to provide to <function><link linkend="g-signal-new">g_signal_new</link></function> the offset from the start of the
1503 class structure to the function pointer.
1505 <para>I would like to point out here that the reason why the default handler of a signal is named everywhere
1506 a class_closure is probably related to the fact that it used to be really a function pointer stored in
1507 the class structure.
1512 <para>The following code shows the declaration of the <type>MamanFileSimple</type> class structure which contains
1513 the <function>write</function> function pointer.
1515 struct _MamanFileSimpleClass {
1516 GObjectClass parent;
1518 guint write_signal_id;
1520 /* signal default handlers */
1521 void (*write) (MamanFileSimple *self, guint8 *buffer, guint size);
1524 The <function>write</function> function pointer is initialized in the class_init function of the object
1525 to <function>default_write_signal_handler</function>:
1528 maman_file_simple_class_init (gpointer g_class,
1529 gpointer g_class_data)
1531 GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
1532 MamanFileSimpleClass *klass = MAMAN_FILE_SIMPLE_CLASS (g_class);
1534 klass->write = default_write_signal_handler;
1536 Finally, the signal is created with <function><link linkend="g-signal-new">g_signal_new</link></function> in the same class_init function:
1538 klass->write_signal_id =
1539 g_signal_new ("write",
1540 G_TYPE_FROM_CLASS (g_class),
1541 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1542 G_STRUCT_OFFSET (MamanFileSimpleClass, write),
1543 NULL /* accumulator */,
1544 NULL /* accu_data */,
1545 maman_file_complex_VOID__POINTER_UINT,
1546 G_TYPE_NONE /* return_type */,
1551 Of note, here, is the 4th argument to the function: it is an integer calculated by the <function><link linkend="G-STRUCT-OFFSET">G_STRUCT_OFFSET</link></function>
1552 macro which indicates the offset of the member <emphasis>write</emphasis> from the start of the
1553 <type>MamanFileSimpleClass</type> class structure.
1555 <para>GSignal uses this offset to create a special wrapper closure
1556 which first retrieves the target function pointer before calling it.
1562 While the complete code for this type of default handler looks less cluttered as shown in
1563 <filename>sample/signal/maman-file-simple.{h|c}</filename>, it contains numerous subtleties.
1564 The main subtle point which everyone must be aware of is that the signature of the default
1565 handler created that way does not have a user_data argument:
1566 <function>default_write_signal_handler</function> is different in
1567 <filename>sample/signal/maman-file-complex.c</filename> and in
1568 <filename>sample/signal/maman-file-simple.c</filename>.
1571 <para>If you have doubts about which method to use, I would advise you to use the second one which
1572 involves <function><link linkend="g-signal-new">g_signal_new</link></function> rather than <function><link linkend="g-signal-newv">g_signal_newv</link></function>:
1573 it is better to write code which looks like the vast majority of other GTK+/GObject code than to
1574 do it your own way. However, now, you know why.
1583 yet another pointless section. if we are scared of possible abuses
1584 from the users then we should not be mentioning it inside a tutorial
1585 for beginners. but, obviously, there's nothing to be afraid of - it's
1586 just that this section must be completely reworded.
1589 <title>How users can abuse signals (and why some think it is good)</title>
1591 <para>Now that you know how to create signals to which the users can connect easily and at any point in
1592 the signal emission process thanks to <function><link linkend="g-signal-connect">g_signal_connect</link></function>,
1593 <function><link linkend="g-signal-connect-after">g_signal_connect_after</link></function> and G_SIGNAL_RUN_LAST, it is time to look into how your
1594 users can and will screw you. This is also interesting to know how you too, can screw other people.
1595 This will make you feel good and eleet.
1601 <listitem><para>stop the emission of the signal at anytime</para></listitem>
1602 <listitem><para>override the default handler of the signal if it is stored as a function
1603 pointer in the class structure (which is the preferred way to create a default signal handler,
1604 as discussed in the previous section).</para></listitem>
1609 In both cases, the original programmer should be as careful as possible to write code which is
1610 resistant to the fact that the default handler of the signal might not able to run. This is obviously
1611 not the case in the example used in the previous sections since the write to the file depends on whether
1612 or not the default handler runs (however, this might be your goal: to allow the user to prevent the file
1613 write if he wishes to).
1617 If all you want to do is to stop the signal emission from one of the callbacks you connected yourself,
1618 you can call <function><link linkend="g-signal-stop-by-name">g_signal_stop_by_name</link></function>. Its use is very simple which is why I won't detail
1623 If the signal's default handler is just a class function pointer, it is also possible to override
1624 it yourself from the class_init function of a type which derives from the parent. That way, when the signal
1625 is emitted, the parent class will use the function provided by the child as a signal default handler.
1626 Of course, it is also possible (and recommended) to chain up from the child to the parent's default signal
1627 handler to ensure the integrity of the parent object.
1631 Overriding a class method and chaining up was demonstrated in <xref linkend="howto-gobject-methods"/>
1632 which is why I won't bother to show exactly how to do it here again.
1643 <title>Warning on signal creation and default closure</title>
1646 Most of the existing code I have seen up to now (in both GTK+, GNOME libraries and
1647 many GTK+ and GNOME applications) using signals uses a small
1648 variation of the default handler pattern I have shown in the previous section.
1652 Usually, the <function><link linkend="g-signal-new">g_signal_new</link></function> function is preferred over
1653 <function><link linkend="g-signal-newv">g_signal_newv</link></function>. When <function><link linkend="g-signal-new">g_signal_new</link></function>
1654 is used, the default closure is exported as a class function. For example,
1655 <filename>gobject.h</filename> contains the declaration of <link linkend="GObjectClass"><type>GObjectClass</type></link>
1656 whose notify class function is the default handler for the <emphasis>notify</emphasis>
1659 struct _GObjectClass
1661 GTypeClass g_type_class;
1663 /* class methods and other stuff. */
1666 void (*notify) (GObject *object,
1673 <filename>gobject.c</filename>'s <function><link linkend="g-object-do-class-init">g_object_do_class_init</link></function> function
1674 registers the <emphasis>notify</emphasis> signal and initializes this class function
1678 g_object_do_class_init (GObjectClass *class)
1683 class->notify = NULL;
1685 gobject_signals[NOTIFY] =
1686 g_signal_new ("notify",
1687 G_TYPE_FROM_CLASS (class),
1688 G_SIGNAL_RUN_FIRST | G_SIGNAL_NO_RECURSE | G_SIGNAL_DETAILED | G_SIGNAL_NO_HOOKS,
1689 G_STRUCT_OFFSET (GObjectClass, notify),
1691 g_cclosure_marshal_VOID__PARAM,
1696 <function><link linkend="g-signal-new">g_signal_new</link></function> creates a <link linkend="GClosure"><type>GClosure</type></link> which dereferences the
1697 type's class structure to access the class function pointer and invoke it if it not NULL. The
1698 class function is ignored it is set to NULL.
1702 To understand the reason for such a complex scheme to access the signal's default handler,
1703 you must remember the whole reason for the use of these signals. The goal here is to delegate
1704 a part of the process to the user without requiring the user to subclass the object to override
1705 one of the class functions. The alternative to subclassing, that is, the use of signals
1706 to delegate processing to the user, is, however, a bit less optimal in terms of speed: rather
1707 than just dereferencing a function pointer in a class structure, you must start the whole
1708 process of signal emission which is a bit heavyweight.
1712 This is why some people decided to use class functions for some signal's default handlers:
1713 rather than having users connect a handler to the signal and stop the signal emission
1714 from within that handler, you just need to override the default class function which is
1715 supposedly more efficient.