3 This chapter tries to answer the real-life questions of users and presents
4 the most common scenario use-cases I could come up with.
5 The use-cases are presented from most likely to less likely.
13 <chapter id="howto-gobject">
14 <title>How To define and implement a new GObject?</title>
17 Clearly, this is one of the most common question people ask: they just want to crank code and
18 implement a subclass of a GObject. Sometimes because they want to create their own class hierarchy,
19 sometimes because they want to subclass one of GTK+'s widget. This chapter will focus on the
20 implementation of a subtype of GObject. The sample source code
21 associated to this section can be found in the documentation's source tarball, in the
22 <filename>sample/gobject</filename> directory:
24 <listitem><para><filename>maman-bar.{h|c}</filename>: this is the source for a object which derives from
25 <type><link linkend="GObject">GObject</link></type> and which shows how to declare different types of methods on the object.
27 <listitem><para><filename>maman-subbar.{h|c}</filename>: this is the source for a object which derives from
28 <type>MamanBar</type> and which shows how to override some of its parent's methods.
30 <listitem><para><filename>maman-foo.{h|c}</filename>: this is the source for an object which derives from
31 <type><link linkend="GObject">GObject</link></type> and which declares a signal.
33 <listitem><para><filename>test.c</filename>: this is the main source which instantiates an instance of
34 type and exercises their API.
39 <sect1 id="howto-gobject-header">
40 <title>Boilerplate header code</title>
43 The first step before writing the code for your GObject is to write the type's header which contains
44 the needed type, function and macro definitions. Each of these elements is nothing but a convention
45 which is followed not only by GTK+'s code but also by most users of GObject. If you feel the need
46 not to obey the rules stated below, think about it twice:
48 <listitem><para>If your users are a bit accustomed to GTK+ code or any Glib code, they will
49 be a bit surprised and getting used to the conventions you decided upon will take time (money) and
50 will make them grumpy (not a good thing)
53 You must assess the fact that these conventions might have been designed by both smart
54 and experienced people: maybe they were at least partly right. Try to put your ego aside.
60 Pick a name convention for your headers and source code and stick to it:
63 use a dash to separate the prefix from the typename: <filename>maman-bar.h</filename> and
64 <filename>maman-bar.c</filename> (this is the convention used by Nautilus and most GNOME libraries).
67 use an underscore to separate the prefix from the typename: <filename>maman_bar.h</filename> and
68 <filename>maman_bar.c</filename>.
71 Do not separate the prefix from the typename: <filename>mamanbar.h</filename> and
72 <filename>mamanbar.c</filename>. (this is the convention used by GTK+)
75 I personally like the first solution better: it makes reading file names easier for those with poor
80 When you need some private (internal) declarations in several (sub)classes,
81 you can define them in a private header file which is often named by
82 appending the <emphasis>private</emphasis> keyword to the public header name.
83 For example, one could use <filename>maman-bar-private.h</filename>,
84 <filename>maman_bar_private.h</filename> or <filename>mamanbarprivate.h</filename>.
85 Typically, such private header files are not installed.
89 The basic conventions for any header which exposes a GType are described in
90 <xref linkend="gtype-conventions"/>. Most GObject-based code also obeys onf of the following
91 conventions: pick one and stick to it.
94 If you want to declare a type named bar with prefix maman, name the type instance
95 <function>MamanBar</function> and its class <function>MamanBarClass</function>
96 (name is case-sensitive). It is customary to declare them with code similar to the
100 * Copyright/Licensing information.
107 * Potentially, include other headers on which this header depends.
115 typedef struct _MamanBar MamanBar;
116 typedef struct _MamanBarClass MamanBarClass;
120 /* instance members */
123 struct _MamanBarClass {
128 /* used by MAMAN_BAR_TYPE */
129 GType maman_bar_get_type (void);
132 * Method definitions.
139 Most GTK+ types declare their private fields in the public header with a /* private */ comment,
140 relying on their user's intelligence not to try to play with these fields. Fields not marked private
141 are considered public by default. The /* protected */ comment (same semantics as those of C++)
142 is also used, mainly in the GType library, in code written by Tim Janik.
153 All of Nautilus code and a lot of GNOME libraries use private indirection members, as described
154 by Herb Sutter in his Pimpl articles
155 (see <ulink url="http://www.gotw.ca/gotw/024.htm">Compilation Firewalls</ulink>
156 and <ulink url="http://www.gotw.ca/gotw/028.htm">The Fast Pimpl Idiom</ulink>
157 : he summarizes the different issues better than I will).
159 typedef struct _MamanBarPrivate MamanBarPrivate;
164 MamanBarPrivate *priv;
167 <note><simpara>Do not call this <varname>private</varname>, as that is a registered c++ keyword.</simpara></note>
168 The private structure is then defined in the .c file, instantiated in the object's
169 <function>init</function> function and destroyed in the object's <function>finalize</function> function.
172 maman_bar_finalize (GObject *object) {
173 MamanBar *self = MAMAN_BAR (object);
179 maman_bar_init (GTypeInstance *instance, gpointer g_class) {
180 MamanBar *self = MAMAN_BAR (instance);
181 self->priv = g_new0 (MamanBarPrivate,1);
188 A similar alternative, available since Glib version 2.4, is to define a private structure in the .c file,
189 declare it as a private structure in <function>maman_bar_class_init</function> using
190 <function><link linkend="g-type-class-add-private">g_type_class_add_private</link></function>.
191 Instead of allocating memory in <function>maman_bar_init</function> a pointer to the private memory area is
192 stored in the instance to allow convenient access to this structure.
193 A private structure will then be attached to each newly created object by the GObject system.
194 You dont need to free or allocate the private structure, only the objects or pointers that it may contain.
195 Another advantage of this to the previous version is that is lessens memory fragmentation,
196 as the public and private parts of the instance memory are allocated at once.
198 typedef struct _MamanBarPrivate MamanBarPrivate;
200 struct _MamanBarPrivate {
205 maman_bar_class_init (MamanBarClass *klass)
208 g_type_class_add_private (klass, sizeof (MamanBarPrivate));
213 maman_bar_init (GTypeInstance *instance, gpointer g_class) {
214 MamanBar *self = MAMAN_BAR (instance);
215 self->priv = G_TYPE_INSTANCE_GET_PRIVATE (self, MAMAN_BAR_TYPE, MamanBarPrivate);
225 Finally, there are different header include conventions. Again, pick one and stick to it. I personally
226 use indifferently any of the two, depending on the codebase I work on: the rule is consistency.
229 Some people add at the top of their headers a number of #include directives to pull in
230 all the headers needed to compile client code. This allows client code to simply
231 #include "maman-bar.h".
234 Other do not #include anything and expect the client to #include themselves the headers
235 they need before including your header. This speeds up compilation because it minimizes the
236 amount of pre-processor work. This can be used in conjunction with the re-declaration of certain
237 unused types in the client code to minimize compile-time dependencies and thus speed up
245 <sect1 id="howto-gobject-code">
246 <title>Boilerplate code</title>
249 In your code, the first step is to #include the needed headers: depending on your header include strategy, this
250 can be as simple as #include "maman-bar.h" or as complicated as tens of #include lines ending with
251 #include "maman-bar.h":
254 * Copyright information
257 #include "maman-bar.h"
259 /* If you use Pimpls, include the private structure
260 * definition here. Some people create a maman-bar-private.h header
261 * which is included by the maman-bar.c file and which contains the
262 * definition for this private structure.
264 struct _MamanBarPrivate {
270 * forward definitions
276 Implement <function>maman_bar_get_type</function> and make sure the code compiles:
279 maman_bar_get_type (void)
281 static GType type = 0;
283 static const GTypeInfo info = {
284 sizeof (MamanBarClass),
285 NULL, /* base_init */
286 NULL, /* base_finalize */
287 NULL, /* class_init */
288 NULL, /* class_finalize */
289 NULL, /* class_data */
292 NULL /* instance_init */
294 type = g_type_register_static (G_TYPE_OBJECT,
304 <sect1 id="howto-gobject-construction">
305 <title>Object Construction</title>
308 People often get confused when trying to construct their GObjects because of the
309 sheer number of different ways to hook into the objects's construction process: it is
310 difficult to figure which is the <emphasis>correct</emphasis>, recommended way.
314 <xref linkend="gobject-construction-table"/> shows what user-provided functions
315 are invoked during object instanciation and in which order they are invoked.
316 A user looking for the equivalent of the simple C++ constructor function should use
317 the instance_init method. It will be invoked after all the parent's instance_init
318 functions have been invoked. It cannot take arbitrary construction parameters
319 (as in C++) but if your object needs arbitrary parameters to complete initialization,
320 you can use construction properties.
324 Construction properties will be set only after all instance_init functions have run.
325 No object reference will be returned to the client of <function><link linkend="g-object-new>">g_object_new></link></function>
326 until all the construction properties have been set.
330 As such, I would recommend writing the following code first:
333 maman_bar_init (GTypeInstance *instance,
336 MamanBar *self = (MamanBar *)instance;
337 self->private = g_new0 (MamanBarPrivate, 1);
339 /* initialize all public and private members to reasonable default values. */
340 /* If you need specific consruction properties to complete initialization,
341 * delay initialization completion until the property is set.
345 And make sure that you set <function>maman_bar_init</function> as the type's instance_init function
346 in <function>maman_bar_get_type</function>. Make sure the code builds and runs: create an instance
347 of the object and make sure <function>maman_bar_init</function> is called (add a
348 <function><link linkend="g-print">g_print</link></function> call in it).
352 Now, if you need special construction properties, install the properties in the class_init function,
353 override the set and get methods and implement the get and set methods as described in
354 <xref linkend="gobject-properties"/>. Make sure that these properties use a construct only
355 <type><link linkend="GParamSpec">GParamSpec</link></type> by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps
356 GType ensure that these properties are not set again later by malicious user code.
359 bar_class_init (MamanBarClass *klass)
361 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
362 GParamSpec *maman_param_spec;
364 gobject_class->set_property = bar_set_property;
365 gobject_class->get_property = bar_get_property;
367 maman_param_spec = g_param_spec_string ("maman",
368 "Maman construct prop",
370 "no-name-set" /* default value */,
371 G_PARAM_CONSTRUCT_ONLY |G_PARAM_READWRITE);
373 g_object_class_install_property (gobject_class,
378 If you need this, make sure you can build and run code similar to the code shown above. Make sure
379 your construct properties can set correctly during construction, make sure you cannot set them
380 afterwards and make sure that if your users do not call <function><link linkend="g-object-new">g_object_new</link></function>
381 with the required construction properties, these will be initialized with the default values.
385 I consider good taste to halt program execution if a construction property is set its
386 default value. This allows you to catch client code which does not give a reasonable
387 value to the construction properties. Of course, you are free to disagree but you
388 should have a good reason to do so.
391 <para>Some people sometimes need to construct their object but only after the construction properties
392 have been set. This is possible through the use of the constructor class method as described in
393 <xref linkend="gobject-instanciation"/>. However, I have yet to see <emphasis>any</emphasis> reasonable
394 use of this feature. As such, to initialize your object instances, use by default the base_init function
395 and construction properties.
399 <sect1 id="howto-gobject-destruction">
400 <title>Object Destruction</title>
403 Again, it is often difficult to figure out which mechanism to use to hook into the object's
404 destruction process: when the last <function><link linkend="g-object-unref">g_object_unref</link></function> function call is made,
405 a lot of things happen as described in <xref linkend="gobject-destruction-table"/>.
409 The destruction process of your object must be split is two different phases: you must override
410 both the dispose and the finalize class methods.
412 struct _MamanBarPrivate {
413 gboolean dispose_has_run;
416 static GObjectClass parent_class = NULL;
419 bar_dispose (GObject *obj)
421 MamanBar *self = (MamanBar *)obj;
423 if (self->private->dispose_has_run) {
424 /* If dispose did already run, return. */
427 /* Make sure dispose does not run twice. */
428 object->private->dispose_has_run = TRUE;
431 * In dispose, you are supposed to free all types referenced from this
432 * object which might themselves hold a reference to self. Generally,
433 * the most simple solution is to unref all members on which you own a
437 /* Chain up to the parent class */
438 G_OBJECT_CLASS (parent_class)->dispose (obj);
442 bar_finalize (GObject *obj)
444 MamanBar *self = (MamanBar *)obj;
447 * Here, complete object destruction.
448 * You might not need to do much...
450 g_free (self->private);
452 /* Chain up to the parent class */
453 G_OBJECT_CLASS (parent_class)->finalize (obj);
457 bar_class_init (BarClass *klass)
459 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
461 gobject_class->dispose = bar_dispose;
462 gobject_class->finalize = bar_finalize;
466 maman_bar_init (GTypeInstance *instance,
469 MamanBar *self = (MamanBar *)instance;
470 self->private = g_new0 (MamanBarPrivate, 1);
471 self->private->dispose_has_run = FALSE;
473 parent_class = g_type_class_peek_parent (klass);
479 Add similar code to your GObject, make sure the code still builds and runs: dispose and finalize must be called
480 during the last unref.
481 It is possible that object methods might be invoked after dispose is run and before finalize runs. GObject
482 does not consider this to be a program error: you must gracefully detect this and neither crash nor warn
483 the user. To do this, you need something like the following code at the start of each object method, to make
484 sure the object's data is still valid before manipulating it:
486 if (self->private->dispose_has_run) {
487 /* Dispose has run. Data is not valid anymore. */
494 <sect1 id="howto-gobject-methods">
495 <title>Object methods</title>
498 Just as with C++, there are many different ways to define object
499 methods and extend them: the following list and sections draw on C++ vocabulary.
500 (Readers are expected to know basic C++ buzzwords. Those who have not had to
501 write C++ code recently can refer to e.g. <ulink url="http://www.cplusplus.com/doc/tutorial/"/> to refresh their
505 non-virtual public methods,
508 virtual public methods and
511 virtual private methods
517 <title>Non-virtual public methods</title>
520 These are the simplest: you want to provide a simple method which can act on your object. All you need
521 to do is to provide a function prototype in the header and an implementation of that prototype
524 /* declaration in the header. */
525 void maman_bar_do_action (MamanBar *self, /* parameters */);
526 /* implementation in the source file */
527 void maman_bar_do_action (MamanBar *self, /* parameters */)
534 <para>There is really nothing scary about this.</para>
538 <title>Virtual public methods</title>
541 This is the preferred way to create polymorphic GObjects. All you need to do is to
542 define the common method and its class function in the public header, implement the
543 common method in the source file and re-implement the class function in each object
544 which inherits from you.
546 /* declaration in maman-bar.h. */
547 struct _MamanBarClass {
551 void (*do_action) (MamanBar *self, /* parameters */);
553 void maman_bar_do_action (MamanBar *self, /* parameters */);
554 /* implementation in maman-bar.c */
555 void maman_bar_do_action (MamanBar *self, /* parameters */)
557 MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
560 The code above simply redirects the do_action call to the relevant class function. Some users,
561 concerned about performance, do not provide the <function>maman_bar_do_action</function>
562 wrapper function and require users to dereference the class pointer themselves. This is not such
563 a great idea in terms of encapsulation and makes it difficult to change the object's implementation
564 afterwards, should this be needed.
568 Other users, also concerned by performance issues, declare the <function>maman_bar_do_action</function>
569 function inline in the header file. This, however, makes it difficult to change the
570 object's implementation later (although easier than requiring users to directly dereference the class
571 function) and is often difficult to write in a portable way (the <emphasis>inline</emphasis> keyword
572 is not part of the C standard).
576 In doubt, unless a user shows you hard numbers about the performance cost of the function call,
577 just <function>maman_bar_do_action</function> in the source file.
581 Please, note that it is possible for you to provide a default implementation for this class method in
582 the object's class_init function: initialize the klass->do_action field to a pointer to the actual
583 implementation. You can also make this class method pure virtual by initializing the klass->do_action
587 maman_bar_real_do_action_two (MamanBar *self, /* parameters */)
589 /* Default implementation for the virtual method. */
593 maman_bar_class_init (BarClass *klass)
595 /* pure virtual method: mandates implementation in children. */
596 klass->do_action_one = NULL;
597 /* merely virtual method. */
598 klass->do_action_two = maman_bar_real_do_action_two;
601 void maman_bar_do_action_one (MamanBar *self, /* parameters */)
603 MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
605 void maman_bar_do_action_two (MamanBar *self, /* parameters */)
607 MAMAN_BAR_GET_CLASS (self)->do_action_two (self, /* parameters */);
614 <title>Virtual private Methods</title>
617 These are very similar to Virtual Public methods. They just don't have a public function to call the
618 function directly. The header file contains only a declaration of the class function:
620 /* declaration in maman-bar.h. */
621 struct _MamanBarClass {
625 void (*helper_do_specific_action) (MamanBar *self, /* parameters */);
627 void maman_bar_do_any_action (MamanBar *self, /* parameters */);
629 These class functions are often used to delegate part of the job to child classes:
631 /* this accessor function is static: it is not exported outside of this file. */
633 maman_bar_do_specific_action (MamanBar *self, /* parameters */)
635 MAMAN_BAR_GET_CLASS (self)->do_specific_action (self, /* parameters */);
638 void maman_bar_do_any_action (MamanBar *self, /* parameters */)
640 /* random code here */
643 * Try to execute the requested action. Maybe the requested action cannot be implemented
644 * here. So, we delegate its implementation to the child class:
646 maman_bar_do_specific_action (self, /* parameters */);
648 /* other random code here */
654 Again, it is possible to provide a default implementation for this private virtual class function:
657 maman_bar_class_init (MamanBarClass *klass)
659 /* pure virtual method: mandates implementation in children. */
660 klass->do_specific_action_one = NULL;
661 /* merely virtual method. */
662 klass->do_specific_action_two = maman_bar_real_do_specific_action_two;
668 Children can then implement the subclass with code such as:
671 maman_bar_subtype_class_init (MamanBarSubTypeClass *klass)
673 MamanBarClass *bar_class = MAMAN_BAR_CLASS (klass);
674 /* implement pure virtual class function. */
675 bar_class->do_specific_action_one = maman_bar_subtype_do_specific_action_one;
682 <sect1 id="howto-gobject-chainup">
683 <title>Chaining up</title>
685 <para>Chaining up is often loosely defined by the following set of conditions:
687 <listitem><para>Parent class A defines a public virtual method named <function>foo</function> and
688 provides a default implementation.</para></listitem>
689 <listitem><para>Child class B re-implements method <function>foo</function>.</para></listitem>
690 <listitem><para>In the method B::foo, the child class B calls its parent class method A::foo.</para></listitem>
692 There are many uses to this idiom:
694 <listitem><para>You need to change the behaviour of a class without modifying its code. You create
695 a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
696 slightly and chain up to ensure that the previous behaviour is not really modifed, just extended.
698 <listitem><para>You are lazy, you have access to the source code of the parent class but you don't want
699 to modify it to add method calls to new specialized method calls: it is faster to hack the child class
700 to chain up than to modify the parent to call down.</para></listitem>
701 <listitem><para>You need to implement the Chain Of Responsability pattern: each object of the inheritance
702 tree chains up to its parent (typically, at the begining or the end of the method) to ensure that
703 they each handler is run in turn.</para></listitem>
705 I am personally not really convinced any of the last two uses are really a good idea but since this
706 programming idiom is often used, this section attemps to explain how to implement it.
709 <para>To explicitely chain up to the implementation of the virtual method in the parent class,
710 you first need a handle to the original parent class structure. This pointer can then be used to
711 access the original class function pointer and invoke it directly.
713 <para>The <emphasis>original</emphasis> adjective used in this sentence is not innocuous. To fully
714 understand its meaning, you need to recall how class structures are initialized: for each object type,
715 the class structure associated to this object is created by first copying the class structure of its
716 parent type (a simple <function>memcpy</function>) and then by invoking the class_init callback on
717 the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
718 with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class
719 structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent
725 <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
726 class structure. Its input is a pointer to the class of the derived object and it returns a pointer
727 to the original parent class structure. The code below shows how you could use it:
730 b_method_to_call (B *obj, int a)
733 AClass *parent_class;
734 klass = B_GET_CLASS (obj);
735 parent_class = g_type_class_peek_parent (klass);
737 /* do stuff before chain up */
738 parent_class->method_to_call (obj, a);
739 /* do stuff after chain up */
742 A lot of people who use this idiom in GTK+ store the parent class structure pointer in a global static
743 variable to avoid the costly call to <function><link linkend="g-type-class-peek-parent">g_type_class_peek_parent</link></function> for each function call.
744 Typically, the class_init callback initializes the global static variable. <filename>gtk/gtkhscale.c</filename>
761 <chapter id="howto-interface">
762 <title>How To define and implement Interfaces?</title>
764 <sect1 id="howto-interface-define">
765 <title>How To define Interfaces?</title>
768 The bulk of interface definition has already been shown in <xref linkend="gtype-non-instantiable-classed"/>
769 but I feel it is needed to show exactly how to create an interface. The sample source code
770 associated to this section can be found in the documentation's source tarball, in the
771 <filename>sample/interface/maman-ibaz.{h|c}</filename> file.
775 As above, the first step is to get the header right:
780 #include <glib-object.h>
782 #define MAMAN_TYPE_IBAZ (maman_ibaz_get_type ())
783 #define MAMAN_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
784 #define MAMAN_IS_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
785 #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))
788 typedef struct _MamanIbaz MamanIbaz; /* dummy object */
789 typedef struct _MamanIbazInterface MamanIbazInterface;
791 struct _MamanIbazInterface {
792 GTypeInterface parent;
794 void (*do_action) (MamanIbaz *self);
797 GType maman_ibaz_get_type (void);
799 void maman_ibaz_do_action (MamanIbaz *self);
801 #endif /*MAMAN_IBAZ_H*/
803 This code is the same as the code for a normal <type><link linkend="GType">GType</link></type>
804 which derives from a <type><link linkend="GObject">GObject</link></type> except for a few details:
807 The <function>_GET_CLASS</function> macro is called <function>_GET_INTERFACE</function>
808 and not implemented with <function><link linkend="G_TYPE_INSTANCE_GET_CLASS">G_TYPE_INSTANCE_GET_CLASS</link></function>
809 but with <function><link linkend="G_TYPE_INSTANCE_GET_INTERFACE">G_TYPE_INSTANCE_GET_INTERFACE</link></function>.
812 The instance type, <type>MamanIbaz</type> is not fully defined: it is used merely as an abstract
813 type which represents an instance of whatever object which implements the interface.
819 The implementation of the <type>MamanIbaz</type> type itself is trivial:
821 <listitem><para><function>maman_ibaz_get_type</function> registers the
822 type in the type system.
824 <listitem><para><function>maman_ibaz_base_init</function> is expected
825 to register the interface's signals if there are any (we will see a bit
826 (later how to use them). Make sure to use a static local boolean variable
827 to make sure not to run the initialization code twice (as described in
828 <xref linkend="gtype-non-instantiable-classed-init"/>,
829 <function>base_init</function> is run once for each interface implementation
830 instanciation)</para></listitem>
831 <listitem><para><function>maman_ibaz_do_action</function> dereferences the class
832 structure to access its associated class function and calls it.
837 maman_ibaz_base_init (gpointer g_class)
839 static gboolean initialized = FALSE;
842 /* create interface signals here. */
848 maman_ibaz_get_type (void)
850 static GType type = 0;
852 static const GTypeInfo info = {
853 sizeof (MamanIbazInterface),
854 maman_ibaz_base_init, /* base_init */
855 NULL, /* base_finalize */
856 NULL, /* class_init */
857 NULL, /* class_finalize */
858 NULL, /* class_data */
861 NULL /* instance_init */
863 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz", &info, 0);
868 void maman_ibaz_do_action (MamanIbaz *self)
870 MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
876 <sect1 id="howto-interface-implement">
877 <title>How To define implement an Interface?</title>
880 Once the interface is defined, implementing it is rather trivial. Source code showing how to do this
881 for the <type>IBaz</type> interface defined in the previous section is located in
882 <filename>sample/interface/maman-baz.{h|c}</filename>.
886 The first step is to define a normal GType. Here, we have decided to use a GType which derives from
887 GObject. Its name is <type>MamanBaz</type>:
892 #include <glib-object.h>
894 #define MAMAN_TYPE_BAZ (maman_baz_get_type ())
895 #define MAMAN_BAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
896 #define MAMAN_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_BAZ, MamanbazClass))
897 #define MAMAN_IS_BAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
898 #define MAMAN_IS_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_TYPE ((vtable), MAMAN_TYPE_BAZ))
899 #define MAMAN_BAZ_GET_CLASS(inst) (G_TYPE_INSTANCE_GET_CLASS ((inst), MAMAN_TYPE_BAZ, MamanbazClass))
902 typedef struct _MamanBaz MamanBaz;
903 typedef struct _MamanBazClass MamanBazClass;
910 struct _MamanBazClass {
914 GType maman_baz_get_type (void);
919 There is clearly nothing specifically weird or scary about this header: it does not define any weird API
920 or derives from a weird type.
924 The second step is to implement <function>maman_baz_get_type</function>:
927 maman_baz_get_type (void)
929 static GType type = 0;
931 static const GTypeInfo info = {
932 sizeof (MamanBazClass),
933 NULL, /* base_init */
934 NULL, /* base_finalize */
935 NULL, /* class_init */
936 NULL, /* class_finalize */
937 NULL, /* class_data */
940 baz_instance_init /* instance_init */
942 static const GInterfaceInfo ibaz_info = {
943 (GInterfaceInitFunc) baz_interface_init, /* interface_init */
944 NULL, /* interface_finalize */
945 NULL /* interface_data */
947 type = g_type_register_static (G_TYPE_OBJECT,
950 g_type_add_interface_static (type,
957 This function is very much like all the similar functions we looked at previously. The only interface-specific
958 code present here is the call to <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function> which is used to inform
959 the type system that this just-registered <type><link linkend="GType">GType</link></type> also implements the interface
960 <function>MAMAN_TYPE_IBAZ</function>.
964 <function>baz_interface_init</function>, the interface initialization function, is also pretty simple:
966 static void baz_do_action (MamanBaz *self)
968 g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
971 baz_interface_init (gpointer g_iface,
974 MamanIbazInteface *iface = (MamanIbazInteface *)g_iface;
975 iface->do_action = (void (*) (MamanIbaz *self))baz_do_action;
978 baz_instance_init (GTypeInstance *instance,
981 MamanBaz *self = MAMAN_BAZ(instance);
982 self->instance_member = 0xdeadbeaf;
985 <function>baz_interface_init</function> merely initializes the interface methods to the implementations
986 defined by <type>MamanBaz</type>: <function>maman_baz_do_action</function> does nothing very useful
993 <title>Interface definition prerequisites</title>
995 <para>To specify that an interface requires the presence of other interfaces when implemented,
996 GObject introduces the concept of <emphasis>prerequisites</emphasis>: it is possible to associate
997 a list of prerequisite interfaces to an interface. For example, if object A wishes to implement interface
998 I1, and if interface I1 has a prerequisite on interface I2, A has to implement both I1 and I2.
1001 <para>The mechanism described above is, in practice, very similar to Java's interface I1 extends
1002 interface I2. The example below shows the GObject equivalent:
1005 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &info, 0);
1006 /* Make the MamanIbar interface require MamanIbaz interface. */
1007 g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
1009 The code shown above adds the MamanIbaz interface to the list of prerequisites of MamanIbar while the
1010 code below shows how an implementation can implement both interfaces and register their implementations:
1012 static void ibar_do_another_action (MamanBar *self)
1014 g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n", self->instance_member);
1018 ibar_interface_init (gpointer g_iface,
1019 gpointer iface_data)
1021 MamanIbarInterface *iface = (MamanIbarInterface *)g_iface;
1022 iface->do_another_action = (void (*) (MamanIbar *self))ibar_do_another_action;
1026 static void ibaz_do_action (MamanBar *self)
1028 g_print ("Bar implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
1032 ibaz_interface_init (gpointer g_iface,
1033 gpointer iface_data)
1035 MamanIbazInterface *iface = (MamanIbazInterface *)g_iface;
1036 iface->do_action = (void (*) (MamanIbaz *self))ibaz_do_action;
1041 bar_instance_init (GTypeInstance *instance,
1044 MamanBar *self = (MamanBar *)instance;
1045 self->instance_member = 0x666;
1050 maman_bar_get_type (void)
1052 static GType type = 0;
1054 static const GTypeInfo info = {
1055 sizeof (MamanBarClass),
1056 NULL, /* base_init */
1057 NULL, /* base_finalize */
1058 NULL, /* class_init */
1059 NULL, /* class_finalize */
1060 NULL, /* class_data */
1062 0, /* n_preallocs */
1063 bar_instance_init /* instance_init */
1065 static const GInterfaceInfo ibar_info = {
1066 (GInterfaceInitFunc) ibar_interface_init, /* interface_init */
1067 NULL, /* interface_finalize */
1068 NULL /* interface_data */
1070 static const GInterfaceInfo ibaz_info = {
1071 (GInterfaceInitFunc) ibaz_interface_init, /* interface_init */
1072 NULL, /* interface_finalize */
1073 NULL /* interface_data */
1075 type = g_type_register_static (G_TYPE_OBJECT,
1078 g_type_add_interface_static (type,
1081 g_type_add_interface_static (type,
1088 It is very important to notice that the order in which interface implementations are added to the main object
1089 is not random: <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function> must be invoked first on the interfaces which have
1090 no prerequisites and then on the others.
1094 Complete source code showing how to define the MamanIbar interface which requires MamanIbaz and how to
1095 implement the MamanIbar interface is located in <filename>sample/interface/maman-ibar.{h|c}</filename>
1096 and <filename>sample/interface/maman-bar.{h|c}</filename>.
1101 <sect1 id="howto-interface-properties">
1102 <title>Interface Properties</title>
1104 <para>Starting from version 2.4 of glib, gobject interfaces can also have properties.
1105 Declaration of the interface properties is similar to declaring the properties of
1106 ordinary gobject types as explained in <xref linkend="gobject-properties"/>,
1107 except that <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> is used to
1108 declare the properties instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
1111 <para>To include a property named 'name' of type <type>string</type> in the
1112 <type>maman_ibaz</type> interface example code above, we only need to add one
1115 That really is one line extended to six for the sake of clarity
1118 line in the <function>maman_ibaz_base_init</function>
1121 The <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> can also be called from
1122 <function>class_init</function> but it must not be called after that point.
1128 maman_ibaz_base_init (gpointer g_iface)
1130 static gboolean initialized = FALSE;
1133 /* create interface signals here. */
1135 g_object_interface_install_property (g_iface,
1136 g_param_spec_string ("name",
1138 "Name of the MamanIbaz",
1140 G_PARAM_READWRITE));
1147 <para>One point worth noting is that the declared property wasn't assigned an
1148 integer ID. The reason being that integer IDs of properities are utilized only
1149 inside the get and set methods and since interfaces do not implement properties,
1150 there is no need to assign integer IDs to interface properties.
1153 <para>The story for the implementers of the interface is also quite trivial.
1154 An implementer shall declare and define it's properties in the usual way as
1155 explained in <xref linkend="gobject-properties"/>, except for one small
1156 change: it shall declare the properties of the interface it implements using
1157 <function><link linkend="g-object-class-override-property">g_object_class_override_property</link></function> instead of
1158 <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>. The following code snipet
1159 shows the modifications needed in the <type>MamanBaz</type> declaration and
1160 implementation above:
1165 gint instance_member;
1166 gchar *name; /* placeholder for property */
1176 maman_baz_get_type (void)
1178 static GType type = 0;
1180 static const GTypeInfo info = {
1181 sizeof (MamanBazClass),
1182 NULL, /* base_init */
1183 NULL, /* base_finalize */
1184 baz_class_init, /* class_init */
1185 NULL, /* class_finalize */
1186 NULL, /* class_data */
1188 0, /* n_preallocs */
1189 baz_instance_init /* instance_init */
1191 static const GInterfaceInfo ibaz_info = {
1192 (GInterfaceInitFunc) baz_interface_init, /* interface_init */
1193 NULL, /* interface_finalize */
1194 NULL /* interface_data */
1196 type = g_type_register_static (G_TYPE_OBJECT,
1199 g_type_add_interface_static (type,
1207 maman_baz_class_init (MamanBazClass * klass)
1209 GObjectClass *gobject_class;
1211 gobject_class = (GObjectClass *) klass;
1213 parent_class = g_type_class_ref (G_TYPE_OBJECT);
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, ARG_NAME, "name");
1222 maman_baz_set_property (GObject * object, guint prop_id,
1223 const GValue * value, GParamSpec * pspec)
1228 /* it's not null if we got it, but it might not be ours */
1229 g_return_if_fail (G_IS_MAMAN_BAZ (object));
1231 baz = MAMAN_BAZ (object);
1235 baz->name = g_value_get_string (value);
1238 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1244 maman_baz_get_property (GObject * object, guint prop_id,
1245 GValue * value, GParamSpec * pspec)
1249 /* it's not null if we got it, but it might not be ours */
1250 g_return_if_fail (G_IS_TEXT_PLUGIN (object));
1252 baz = MAMAN_BAZ (object);
1256 g_value_set_string (value, baz->name);
1259 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1273 End Howto Interfaces
1282 <chapter id="howto-signals">
1283 <title>Howto create and use signals</title>
1287 The signal system which was built in GType is pretty complex and flexible: it is possible for its users
1288 to connect at runtime any number of callbacks (implemented in any language for which a binding exists)
1290 <para>A python callback can be connected to any signal on any C-based GObject.
1294 to any signal and to stop the emission of any signal at any
1295 state of the signal emission process. This flexibility makes it possible to use GSignal for much more than
1296 just emit events which can be received by numerous clients.
1299 <sect1 id="howto-simple-signals">
1300 <title>Simple use of signals</title>
1302 <para>The most basic use of signals is to implement simple event notification: for example, if we have a
1303 MamanFile object, and if this object has a write method, we might wish to be notified whenever someone
1304 uses this method. The code below shows how the user can connect a callback to the write signal. Full code
1305 for this simple example is located in <filename>sample/signal/maman-file.{h|c}</filename> and
1306 in <filename>sample/signal/test.c</filename>
1308 file = g_object_new (MAMAN_FILE_TYPE, NULL);
1310 g_signal_connect (G_OBJECT (file), "write",
1311 (GCallback)write_event,
1314 maman_file_write (file, buffer, 50);
1319 The <type>MamanFile</type> signal is registered in the class_init function:
1321 klass->write_signal_id =
1322 g_signal_newv ("write",
1323 G_TYPE_FROM_CLASS (g_class),
1324 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1325 NULL /* class closure */,
1326 NULL /* accumulator */,
1327 NULL /* accu_data */,
1328 g_cclosure_marshal_VOID__VOID,
1329 G_TYPE_NONE /* return_type */,
1331 NULL /* param_types */);
1333 and the signal is emited in <function>maman_file_write</function>:
1335 void maman_file_write (MamanFile *self, guint8 *buffer, guint32 size)
1337 /* First write data. */
1338 /* Then, notify user of data written. */
1339 g_signal_emit (self, MAMAN_FILE_GET_CLASS (self)->write_signal_id,
1344 As shown above, you can safely set the details parameter to zero if you do not know what it can be used for.
1345 For a discussion of what you could used it for, see <xref linkend="signal-detail"/>
1349 The signature of the signal handler in the above example is defined as
1350 <function>g_cclosure_marshal_VOID__VOID</function>. Its name follows
1351 a simple convention which encodes the function parameter and return value
1352 types in the function name. Specifically, the value infront of the double
1353 underscore is the type of the return value, while the value(s) after the
1354 double underscore denote the parameter types.
1355 The header <filename>gobject/gmarshal.h</filename> defines a set of commonly
1356 needed closures that one can use.
1363 <title>How to provide more flexibility to users?</title>
1365 <para>The previous implementation does the job but the signal facility of GObject can be used to provide
1366 even more flexibility to this file change notification mechanism. One of the key ideas is to make the process
1367 of writing data to the file part of the signal emission process to allow users to be notified either
1368 before or after the data is written to the file.
1371 <para>To integrate the process of writing the data to the file into the signal emission mechanism, we can
1372 register a default class closure for this signal which will be invoked during the signal emission, just like
1373 any other user-connected signal handler.
1376 <para>The first step to implement this idea is to change the signature of the signal: we need to pass
1377 around the buffer to write and its size. To do this, we use our own marshaller which will be generated
1378 through glib's genmarshall tool. We thus create a file named <filename>marshall.list</filename> which contains
1379 the following single line:
1383 and use the Makefile provided in <filename>sample/signal/Makefile</filename> to generate the file named
1384 <filename>maman-file-complex-marshall.c</filename>. This C file is finally included in
1385 <filename>maman-file-complex.c</filename>.
1388 <para>Once the marshaller is present, we register the signal and its marshaller in the class_init function
1389 of the object <type>MamanFileComplex</type> (full source for this object is included in
1390 <filename>sample/signal/maman-file-complex.{h|c}</filename>):
1392 GClosure *default_closure;
1393 GType param_types[2];
1395 default_closure = g_cclosure_new (G_CALLBACK (default_write_signal_handler),
1396 (gpointer)0xdeadbeaf /* user_data */,
1397 NULL /* destroy_data */);
1399 param_types[0] = G_TYPE_POINTER;
1400 param_types[1] = G_TYPE_UINT;
1401 klass->write_signal_id =
1402 g_signal_newv ("write",
1403 G_TYPE_FROM_CLASS (g_class),
1404 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1405 default_closure /* class closure */,
1406 NULL /* accumulator */,
1407 NULL /* accu_data */,
1408 maman_file_complex_VOID__POINTER_UINT,
1409 G_TYPE_NONE /* return_type */,
1411 param_types /* param_types */);
1413 The code shown above first creates the closure which contains the code to complete the file write. This
1414 closure is registered as the default class_closure of the newly created signal.
1418 Of course, you need to implement completely the code for the default closure since I just provided
1422 default_write_signal_handler (GObject *obj, guint8 *buffer, guint size, gpointer user_data)
1424 g_assert (user_data == (gpointer)0xdeadbeaf);
1425 /* Here, we trigger the real file write. */
1426 g_print ("default signal handler: 0x%x %u\n", buffer, size);
1431 <para>Finally, the client code must invoke the <function>maman_file_complex_write</function> function which
1432 triggers the signal emission:
1434 void maman_file_complex_write (MamanFileComplex *self, guint8 *buffer, guint size)
1437 g_signal_emit (self,
1438 MAMAN_FILE_COMPLEX_GET_CLASS (self)->write_signal_id,
1445 <para>The client code (as shown in <filename>sample/signal/test.c</filename> and below) can now connect signal handlers before
1446 and after the file write is completed: since the default signal handler which does the write itself runs during the
1447 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>
1448 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
1449 which emits signals, I would thus urge you to create all your signals with the G_SIGNAL_RUN_LAST such that your users
1450 have a maximum of flexibility as to when to get the event. Here, we combined it with G_SIGNAL_NO_RECURSE and
1451 G_SIGNAL_NO_HOOKS to ensure our users will not try to do really weird things with our GObject. I strongly advise you
1452 to do the same unless you really know why (in which case you really know the inner workings of GSignal by heart and
1453 you are not reading this).
1458 static void complex_write_event_before (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1460 g_assert (user_data == NULL);
1461 g_print ("Complex Write event before: 0x%x, %u\n", buffer, size);
1464 static void complex_write_event_after (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1466 g_assert (user_data == NULL);
1467 g_print ("Complex Write event after: 0x%x, %u\n", buffer, size);
1470 static void test_file_complex (void)
1475 file = g_object_new (MAMAN_FILE_COMPLEX_TYPE, NULL);
1477 g_signal_connect (G_OBJECT (file), "write",
1478 (GCallback)complex_write_event_before,
1481 g_signal_connect_after (G_OBJECT (file), "write",
1482 (GCallback)complex_write_event_after,
1485 maman_file_complex_write (MAMAN_FILE_COMPLEX (file), buffer, 50);
1487 g_object_unref (G_OBJECT (file));
1490 The code above generates the following output on my machine:
1492 Complex Write event before: 0xbfffe280, 50
1493 default signal handler: 0xbfffe280 50
1494 Complex Write event after: 0xbfffe280, 50
1500 <title>How most people do the same thing with less code</title>
1502 <para>For many historic reasons related to how the ancestor of GObject used to work in GTK+ 1.x versions,
1503 there is a much <emphasis>simpler</emphasis>
1505 <para>I personally think that this method is horribly mind-twisting: it adds a new indirection
1506 which unecessarily complicates the overall code path. However, because this method is widely used
1507 by all of GTK+ and GObject code, readers need to understand it. The reason why this is done that way
1508 in most of GTK+ is related to the fact that the ancestor of GObject did not provide any other way to
1509 create a signal with a default handler than this one. Some people have tried to justify that it is done
1510 that way because it is better, faster (I am extremly doubtfull about the faster bit. As a matter of fact,
1511 the better bit also mystifies me ;-). I have the feeling no one really knows and everyone does it
1512 because they copy/pasted code from code which did the same. It is probably better to leave this
1513 specific trivia to hacker legends domain...
1516 way to create a signal with a default handler than to create
1517 a closure by hand and to use the <function><link linkend="g-signal-newv">g_signal_newv</link></function>.
1520 <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
1521 handler which is stored in the class structure of the object. More specifically, the class structure
1522 contains a function pointer which is accessed during signal emission to invoke the default handler and
1523 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
1524 class structure to the function pointer.
1526 <para>I would like to point out here that the reason why the default handler of a signal is named everywhere
1527 a class_closure is probably related to the fact that it used to be really a function pointer stored in
1528 the class structure.
1533 <para>The following code shows the declaration of the <type>MamanFileSimple</type> class structure which contains
1534 the <function>write</function> function pointer.
1536 struct _MamanFileSimpleClass {
1537 GObjectClass parent;
1539 guint write_signal_id;
1541 /* signal default handlers */
1542 void (*write) (MamanFileSimple *self, guint8 *buffer, guint size);
1545 The <function>write</function> function pointer is initialied in the class_init function of the object
1546 to <function>default_write_signal_handler</function>:
1549 maman_file_simple_class_init (gpointer g_class,
1550 gpointer g_class_data)
1552 GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
1553 MamanFileSimpleClass *klass = MAMAN_FILE_SIMPLE_CLASS (g_class);
1555 klass->write = default_write_signal_handler;
1557 Finally, the signal is created with <function><link linkend="g-signal-new">g_signal_new</link></function> in the same class_init function:
1559 klass->write_signal_id =
1560 g_signal_new ("write",
1561 G_TYPE_FROM_CLASS (g_class),
1562 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1563 G_STRUCT_OFFSET (MamanFileSimpleClass, write),
1564 NULL /* accumulator */,
1565 NULL /* accu_data */,
1566 maman_file_complex_VOID__POINTER_UINT,
1567 G_TYPE_NONE /* return_type */,
1572 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>
1573 macro which indicates the offset of the member <emphasis>write</emphasis> from the start of the
1574 <type>MamanFileSimpleClass</type> class structure.
1576 <para>GSignal uses this offset to create a special wrapper closure
1577 which first retrieves the target function pointer before calling it.
1583 While the complete code for this type of default handler looks less clutered as shown in
1584 <filename>sample/signal/maman-file-simple.{h|c}</filename>, it contains numerous subtleties.
1585 The main subtle point which everyone must be aware of is that the signature of the default
1586 handler created that way does not have a user_data argument:
1587 <function>default_write_signal_handler</function> is different in
1588 <filename>sample/signal/maman-file-complex.c</filename> and in
1589 <filename>sample/signal/maman-file-simple.c</filename>.
1592 <para>If you have doubts about which method to use, I would advise you to use the second one which
1593 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>:
1594 it is better to write code which looks like the vast majority of other GTK+/Gobject code than to
1595 do it your own way. However, now, you know why.
1606 <title>How users can abuse signals (and why some think it is good)</title>
1608 <para>Now that you know how to create signals to which the users can connect easily and at any point in
1609 the signal emission process thanks to <function><link linkend="g-signal-connect">g_signal_connect</link></function>,
1610 <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
1611 users can and will screw you. This is also interesting to know how you too, can screw other people.
1612 This will make you feel good and eleet.
1615 <para>The users can:
1617 <listitem><para>stop the emission of the signal at anytime</para></listitem>
1618 <listitem><para>override the default handler of the signal if it is stored as a function
1619 pointer in the class structure (which is the prefered way to create a default signal handler,
1620 as discussed in the previous section).</para></listitem>
1624 <para>In both cases, the original programmer should be as careful as possible to write code which is
1625 resistant to the fact that the default handler of the signal might not able to run. This is obviously
1626 not the case in the example used in the previous sections since the write to the file depends on whether
1627 or not the default handler runs (however, this might be your goal: to allow the user to prevent the file
1628 write if he wishes to).
1631 <para>If all you want to do is to stop the signal emission from one of the callbacks you connected yourself,
1632 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
1636 <para>If the signal's default handler is just a class function pointer, it is also possible to override
1637 it yourself from the class_init function of a type which derives from the parent. That way, when the signal
1638 is emitted, the parent class will use the function provided by the child as a signal default handler.
1639 Of course, it is also possible (and recommended) to chain up from the child to the parent's default signal
1640 handler to ensure the integrity of the parent object.
1643 <para>Overriding a class method and chaining up was demonstrated in <xref linkend="howto-gobject-methods"/>
1644 which is why I won't bother to show exactly how to do it here again.</para>
1653 <title>Warning on signal creation and default closure</title>
1656 Most of the existing code I have seen up to now (in both GTK+, GNOME libraries and
1657 many GTK+ and GNOME applications) using signals uses a small
1658 variation of the default handler pattern I have shown in the previous section.
1662 Usually, the <function><link linkend="g-signal-new">g_signal_new</link></function> function is preferred over
1663 <function><link linkend="g-signal-newv">g_signal_newv</link></function>. When <function><link linkend="g-signal-new">g_signal_new</link></function>
1664 is used, the default closure is exported as a class function. For example,
1665 <filename>gobject.h</filename> contains the declaration of <type><link linkend="GObjectClass">GObjectClass</link></type>
1666 whose notify class function is the default handler for the <emphasis>notify</emphasis>
1669 struct _GObjectClass
1671 GTypeClass g_type_class;
1673 /* class methods and other stuff. */
1676 void (*notify) (GObject *object,
1683 <filename>gobject.c</filename>'s <function><link linkend="g-object-do-class-init">g_object_do_class_init</link></function> function
1684 registers the <emphasis>notify</emphasis> signal and initializes this class function
1688 g_object_do_class_init (GObjectClass *class)
1693 class->notify = NULL;
1695 gobject_signals[NOTIFY] =
1696 g_signal_new ("notify",
1697 G_TYPE_FROM_CLASS (class),
1698 G_SIGNAL_RUN_FIRST | G_SIGNAL_NO_RECURSE | G_SIGNAL_DETAILED | G_SIGNAL_NO_HOOKS,
1699 G_STRUCT_OFFSET (GObjectClass, notify),
1701 g_cclosure_marshal_VOID__PARAM,
1706 <function><link linkend="g-signal-new">g_signal_new</link></function> creates a <type><link linkend="GClosure">GClosure</link></type> which dereferences the
1707 type's class structure to access the class function pointer and invoke it if it not NULL. The
1708 class function is ignored it is set to NULL.
1712 To understand the reason for such a complex scheme to access the signal's default handler,
1713 you must remember the whole reason for the use of these signals. The goal here is to delegate
1714 a part of the process to the user without requiring the user to subclass the object to override
1715 one of the class functions. The alternative to subclassing, that is, the use of signals
1716 to delegate processing to the user, is, however, a bit less optimal in terms of speed: rather
1717 than just dereferencing a function pointer in a class structure, you must start the whole
1718 process of signal emission which is a bit heavyweight.
1722 This is why some people decided to use class functions for some signal's default handlers:
1723 rather than having users connect a handler to the signal and stop the signal emission
1724 from within that handler, you just need to override the default class function which is
1725 supposedly more efficient.
1733 <capter1 id="howto-doc">
1734 <title>How to generate API documentation for your type?</title>