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.
171 static void maman_bar_finalize(GObject *object) {
172 MamanBar *self = MAMAN_BAR (object);
177 static void maman_bar_init(GTypeInstance *instance, gpointer g_class) {
178 MamanBar *self = MAMAN_BAR (instance);
179 self->priv = g_new0(MamanBarPrivate,1);
186 A similar alternative, available since Glib version 2.4, is to define a private structure in the .c file,
187 declare it as a private structure in <function>class_init</function> using
188 <function><link linkend="g-type-class-add-private">g_type_class_add_private</link></function> and declare a macro to allow convenient access to this structure.
189 A private structure will then be attached to each newly created object by the GObject system.
190 You dont need to free or allocate the private structure, only the objects or pointers that it may contain.
192 typedef struct _MamanBarPrivate MamanBarPrivate;
194 struct _MamanBarPrivate {
198 #define MAMAN_BAR_GET_PRIVATE(o) (G_TYPE_INSTANCE_GET_PRIVATE ((o), MAMAN_BAR_TYPE, MamanBarPrivate))
201 maman_bar_class_init (MamanBarClass *klass)
204 g_type_class_add_private (klass, sizeof (MamanBarPrivate));
209 maman_bar_get_private_field (MamanBar *self)
211 MamanBarPrivate *priv = MAMAN_BAR_GET_PRIVATE (self);
213 return priv->private_field;
222 Finally, there are different header include conventions. Again, pick one and stick to it. I personally
223 use indifferently any of the two, depending on the codebase I work on: the rule is consistency.
226 Some people add at the top of their headers a number of #include directives to pull in
227 all the headers needed to compile client code. This allows client code to simply
228 #include "maman-bar.h".
231 Other do not #include anything and expect the client to #include themselves the headers
232 they need before including your header. This speeds up compilation because it minimizes the
233 amount of pre-processor work. This can be used in conjunction with the re-declaration of certain
234 unused types in the client code to minimize compile-time dependencies and thus speed up
242 <sect1 id="howto-gobject-code">
243 <title>Boilerplate code</title>
246 In your code, the first step is to #include the needed headers: depending on your header include strategy, this
247 can be as simple as #include "maman-bar.h" or as complicated as tens of #include lines ending with
248 #include "maman-bar.h":
251 * Copyright information
254 #include "maman-bar.h"
256 /* If you use Pimpls, include the private structure
257 * definition here. Some people create a maman-bar-private.h header
258 * which is included by the maman-bar.c file and which contains the
259 * definition for this private structure.
261 struct _MamanBarPrivate {
267 * forward definitions
273 Implement <function>maman_bar_get_type</function> and make sure the code compiles:
276 maman_bar_get_type (void)
278 static GType type = 0;
280 static const GTypeInfo info = {
281 sizeof (MamanBarClass),
282 NULL, /* base_init */
283 NULL, /* base_finalize */
284 NULL, /* class_init */
285 NULL, /* class_finalize */
286 NULL, /* class_data */
289 NULL /* instance_init */
291 type = g_type_register_static (G_TYPE_OBJECT,
301 <sect1 id="howto-gobject-construction">
302 <title>Object Construction</title>
305 People often get confused when trying to construct their GObjects because of the
306 sheer number of different ways to hook into the objects's construction process: it is
307 difficult to figure which is the <emphasis>correct</emphasis>, recommended way.
311 <xref linkend="gobject-construction-table"/> shows what user-provided functions
312 are invoked during object instanciation and in which order they are invoked.
313 A user looking for the equivalent of the simple C++ constructor function should use
314 the instance_init method. It will be invoked after all the parent's instance_init
315 functions have been invoked. It cannot take arbitrary construction parameters
316 (as in C++) but if your object needs arbitrary parameters to complete initialization,
317 you can use construction properties.
321 Construction properties will be set only after all instance_init functions have run.
322 No object reference will be returned to the client of <function><link linkend="g-object-new>">g_object_new></link></function>
323 until all the construction properties have been set.
327 As such, I would recommend writing the following code first:
330 maman_bar_init (GTypeInstance *instance,
333 MamanBar *self = (MamanBar *)instance;
334 self->private = g_new0 (MamanBarPrivate, 1);
336 /* initialize all public and private members to reasonable default values. */
337 /* If you need specific consruction properties to complete initialization,
338 * delay initialization completion until the property is set.
342 And make sure that you set <function>maman_bar_init</function> as the type's instance_init function
343 in <function>maman_bar_get_type</function>. Make sure the code builds and runs: create an instance
344 of the object and make sure <function>maman_bar_init</function> is called (add a
345 <function><link linkend="g-print">g_print</link></function> call in it).
349 Now, if you need special construction properties, install the properties in the class_init function,
350 override the set and get methods and implement the get and set methods as described in
351 <xref linkend="gobject-properties"/>. Make sure that these properties use a construct only
352 <type><link linkend="GParamSpec">GParamSpec</link></type> by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps
353 GType ensure that these properties are not set again later by malicious user code.
356 bar_class_init (MamanBarClass *klass)
358 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
359 GParamSpec *maman_param_spec;
361 gobject_class->set_property = bar_set_property;
362 gobject_class->get_property = bar_get_property;
364 maman_param_spec = g_param_spec_string ("maman",
365 "Maman construct prop",
367 "no-name-set" /* default value */,
368 G_PARAM_CONSTRUCT_ONLY |G_PARAM_READWRITE);
370 g_object_class_install_property (gobject_class,
375 If you need this, make sure you can build and run code similar to the code shown above. Make sure
376 your construct properties can set correctly during construction, make sure you cannot set them
377 afterwards and make sure that if your users do not call <function><link linkend="g-object-new">g_object_new</link></function>
378 with the required construction properties, these will be initialized with the default values.
382 I consider good taste to halt program execution if a construction property is set its
383 default value. This allows you to catch client code which does not give a reasonable
384 value to the construction properties. Of course, you are free to disagree but you
385 should have a good reason to do so.
388 <para>Some people sometimes need to construct their object but only after the construction properties
389 have been set. This is possible through the use of the constructor class method as described in
390 <xref linkend="gobject-instanciation"/>. However, I have yet to see <emphasis>any</emphasis> reasonable
391 use of this feature. As such, to initialize your object instances, use by default the base_init function
392 and construction properties.
396 <sect1 id="howto-gobject-destruction">
397 <title>Object Destruction</title>
400 Again, it is often difficult to figure out which mechanism to use to hook into the object's
401 destruction process: when the last <function><link linkend="g-object-unref">g_object_unref</link></function> function call is made,
402 a lot of things happen as described in <xref linkend="gobject-destruction-table"/>.
406 The destruction process of your object must be split is two different phases: you must override
407 both the dispose and the finalize class methods.
409 struct _MamanBarPrivate {
410 gboolean dispose_has_run;
413 static GObjectClass parent_class = NULL;
416 bar_dispose (GObject *obj)
418 MamanBar *self = (MamanBar *)obj;
420 if (self->private->dispose_has_run) {
421 /* If dispose did already run, return. */
424 /* Make sure dispose does not run twice. */
425 object->private->dispose_has_run = TRUE;
428 * In dispose, you are supposed to free all types referenced from this
429 * object which might themselves hold a reference to self. Generally,
430 * the most simple solution is to unref all members on which you own a
434 /* Chain up to the parent class */
435 G_OBJECT_CLASS (parent_class)->dispose (obj);
439 bar_finalize (GObject *obj)
441 MamanBar *self = (MamanBar *)obj;
444 * Here, complete object destruction.
445 * You might not need to do much...
447 g_free (self->private);
449 /* Chain up to the parent class */
450 G_OBJECT_CLASS (parent_class)->finalize (obj);
454 bar_class_init (BarClass *klass)
456 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
458 gobject_class->dispose = bar_dispose;
459 gobject_class->finalize = bar_finalize;
463 maman_bar_init (GTypeInstance *instance,
466 MamanBar *self = (MamanBar *)instance;
467 self->private = g_new0 (MamanBarPrivate, 1);
468 self->private->dispose_has_run = FALSE;
470 parent_class = g_type_class_peek_parent (klass);
476 Add similar code to your GObject, make sure the code still builds and runs: dispose and finalize must be called
477 during the last unref.
478 It is possible that object methods might be invoked after dispose is run and before finalize runs. GObject
479 does not consider this to be a program error: you must gracefully detect this and neither crash nor warn
480 the user. To do this, you need something like the following code at the start of each object method, to make
481 sure the object's data is still valid before manipulating it:
483 if (self->private->dispose_has_run) {
484 /* Dispose has run. Data is not valid anymore. */
491 <sect1 id="howto-gobject-methods">
492 <title>Object methods</title>
495 Just as with C++, there are many different ways to define object
496 methods and extend them: the following list and sections draw on C++ vocabulary.
497 (Readers are expected to know basic C++ buzzwords. Those who have not had to
498 write C++ code recently can refer to e.g. <ulink url="http://www.cplusplus.com/doc/tutorial/"/> to refresh their
502 non-virtual public methods,
505 virtual public methods and
508 virtual private methods
514 <title>Non-virtual public methods</title>
517 These are the simplest: you want to provide a simple method which can act on your object. All you need
518 to do is to provide a function prototype in the header and an implementation of that prototype
521 /* declaration in the header. */
522 void maman_bar_do_action (MamanBar *self, /* parameters */);
523 /* implementation in the source file */
524 void maman_bar_do_action (MamanBar *self, /* parameters */)
531 <para>There is really nothing scary about this.</para>
535 <title>Virtual public methods</title>
538 This is the preferred way to create polymorphic GObjects. All you need to do is to
539 define the common method and its class function in the public header, implement the
540 common method in the source file and re-implement the class function in each object
541 which inherits from you.
543 /* declaration in maman-bar.h. */
544 struct _MamanBarClass {
548 void (*do_action) (MamanBar *self, /* parameters */);
550 void maman_bar_do_action (MamanBar *self, /* parameters */);
551 /* implementation in maman-bar.c */
552 void maman_bar_do_action (MamanBar *self, /* parameters */)
554 MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
557 The code above simply redirects the do_action call to the relevant class function. Some users,
558 concerned about performance, do not provide the <function>maman_bar_do_action</function>
559 wrapper function and require users to dereference the class pointer themselves. This is not such
560 a great idea in terms of encapsulation and makes it difficult to change the object's implementation
561 afterwards, should this be needed.
565 Other users, also concerned by performance issues, declare the <function>maman_bar_do_action</function>
566 function inline in the header file. This, however, makes it difficult to change the
567 object's implementation later (although easier than requiring users to directly dereference the class
568 function) and is often difficult to write in a portable way (the <emphasis>inline</emphasis> keyword
569 is not part of the C standard).
573 In doubt, unless a user shows you hard numbers about the performance cost of the function call,
574 just <function>maman_bar_do_action</function> in the source file.
578 Please, note that it is possible for you to provide a default implementation for this class method in
579 the object's class_init function: initialize the klass->do_action field to a pointer to the actual
580 implementation. You can also make this class method pure virtual by initializing the klass->do_action
584 maman_bar_real_do_action_two (MamanBar *self, /* parameters */)
586 /* Default implementation for the virtual method. */
590 maman_bar_class_init (BarClass *klass)
592 /* pure virtual method: mandates implementation in children. */
593 klass->do_action_one = NULL;
594 /* merely virtual method. */
595 klass->do_action_two = maman_bar_real_do_action_two;
598 void maman_bar_do_action_one (MamanBar *self, /* parameters */)
600 MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
602 void maman_bar_do_action_two (MamanBar *self, /* parameters */)
604 MAMAN_BAR_GET_CLASS (self)->do_action_two (self, /* parameters */);
611 <title>Virtual private Methods</title>
614 These are very similar to Virtual Public methods. They just don't have a public function to call the
615 function directly. The header file contains only a declaration of the class function:
617 /* declaration in maman-bar.h. */
618 struct _MamanBarClass {
622 void (*helper_do_specific_action) (MamanBar *self, /* parameters */);
624 void maman_bar_do_any_action (MamanBar *self, /* parameters */);
626 These class functions are often used to delegate part of the job to child classes:
628 /* this accessor function is static: it is not exported outside of this file. */
630 maman_bar_do_specific_action (MamanBar *self, /* parameters */)
632 MAMAN_BAR_GET_CLASS (self)->do_specific_action (self, /* parameters */);
635 void maman_bar_do_any_action (MamanBar *self, /* parameters */)
637 /* random code here */
640 * Try to execute the requested action. Maybe the requested action cannot be implemented
641 * here. So, we delegate its implementation to the child class:
643 maman_bar_do_specific_action (self, /* parameters */);
645 /* other random code here */
651 Again, it is possible to provide a default implementation for this private virtual class function:
654 maman_bar_class_init (MamanBarClass *klass)
656 /* pure virtual method: mandates implementation in children. */
657 klass->do_specific_action_one = NULL;
658 /* merely virtual method. */
659 klass->do_specific_action_two = maman_bar_real_do_specific_action_two;
665 Children can then implement the subclass with code such as:
668 maman_bar_subtype_class_init (MamanBarSubTypeClass *klass)
670 MamanBarClass *bar_class = MAMAN_BAR_CLASS (klass);
671 /* implement pure virtual class function. */
672 bar_class->do_specific_action_one = maman_bar_subtype_do_specific_action_one;
679 <sect1 id="howto-gobject-chainup">
680 <title>Chaining up</title>
682 <para>Chaining up is often loosely defined by the following set of conditions:
684 <listitem><para>Parent class A defines a public virtual method named <function>foo</function> and
685 provides a default implementation.</para></listitem>
686 <listitem><para>Child class B re-implements method <function>foo</function>.</para></listitem>
687 <listitem><para>In the method B::foo, the child class B calls its parent class method A::foo.</para></listitem>
689 There are many uses to this idiom:
691 <listitem><para>You need to change the behaviour of a class without modifying its code. You create
692 a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
693 slightly and chain up to ensure that the previous behaviour is not really modifed, just extended.
695 <listitem><para>You are lazy, you have access to the source code of the parent class but you don't want
696 to modify it to add method calls to new specialized method calls: it is faster to hack the child class
697 to chain up than to modify the parent to call down.</para></listitem>
698 <listitem><para>You need to implement the Chain Of Responsability pattern: each object of the inheritance
699 tree chains up to its parent (typically, at the begining or the end of the method) to ensure that
700 they each handler is run in turn.</para></listitem>
702 I am personally not really convinced any of the last two uses are really a good idea but since this
703 programming idiom is often used, this section attemps to explain how to implement it.
706 <para>To explicitely chain up to the implementation of the virtual method in the parent class,
707 you first need a handle to the original parent class structure. This pointer can then be used to
708 access the original class function pointer and invoke it directly.
710 <para>The <emphasis>original</emphasis> adjective used in this sentence is not innocuous. To fully
711 understand its meaning, you need to recall how class structures are initialized: for each object type,
712 the class structure associated to this object is created by first copying the class structure of its
713 parent type (a simple <function>memcpy</function>) and then by invoking the class_init callback on
714 the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
715 with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class
716 structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent
722 <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
723 class structure. Its input is a pointer to the class of the derived object and it returns a pointer
724 to the original parent class structure. The code below shows how you could use it:
727 b_method_to_call (B *obj, int a)
730 AClass *parent_class;
731 klass = B_GET_CLASS (obj);
732 parent_class = g_type_class_peek_parent (klass);
734 /* do stuff before chain up */
735 parent_class->method_to_call (obj, a);
736 /* do stuff after chain up */
739 A lot of people who use this idiom in GTK+ store the parent class structure pointer in a global static
740 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.
741 Typically, the class_init callback initializes the global static variable. <filename>gtk/gtkhscale.c</filename>
758 <chapter id="howto-interface">
759 <title>How To define and implement Interfaces?</title>
761 <sect1 id="howto-interface-define">
762 <title>How To define Interfaces?</title>
765 The bulk of interface definition has already been shown in <xref linkend="gtype-non-instantiable-classed"/>
766 but I feel it is needed to show exactly how to create an interface. The sample source code
767 associated to this section can be found in the documentation's source tarball, in the
768 <filename>sample/interface/maman-ibaz.{h|c}</filename> file.
772 As above, the first step is to get the header right:
777 #include <glib-object.h>
779 #define MAMAN_TYPE_IBAZ (maman_ibaz_get_type ())
780 #define MAMAN_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
781 #define MAMAN_IS_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
782 #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))
785 typedef struct _MamanIbaz MamanIbaz; /* dummy object */
786 typedef struct _MamanIbazInterface MamanIbazInterface;
788 struct _MamanIbazInterface {
789 GTypeInterface parent;
791 void (*do_action) (MamanIbaz *self);
794 GType maman_ibaz_get_type (void);
796 void maman_ibaz_do_action (MamanIbaz *self);
798 #endif /*MAMAN_IBAZ_H*/
800 This code is almost exactly similar to the code for a normal <type><link linkend="GType">GType</link></type>
801 which derives from a <type><link linkend="GObject">GObject</link></type> except for a few details:
804 The <function>_GET_CLASS</function> macro is called <function>_GET_INTERFACE</function>
805 and not implemented with <function><link linkend="G_TYPE_INSTANCE_GET_CLASS">G_TYPE_INSTANCE_GET_CLASS</link></function>
806 but with <function><link linkend="G_TYPE_INSTANCE_GET_INTERFACE">G_TYPE_INSTANCE_GET_INTERFACE</link></function>.
809 The instance type, <type>MamanIbaz</type> is not fully defined: it is used merely as an abstract
810 type which represents an instance of whatever object which implements the interface.
816 The implementation of the <type>MamanIbaz</type> type itself is trivial:
818 <listitem><para><function>maman_ibaz_get_type</function> registers the
819 type in the type system.
821 <listitem><para><function>maman_ibaz_base_init</function> is expected
822 to register the interface's signals if there are any (we will see a bit
823 (later how to use them). Make sure to use a static local boolean variable
824 to make sure not to run the initialization code twice (as described in
825 <xref linkend="gtype-non-instantiable-classed-init"/>,
826 <function>base_init</function> is run once for each interface implementation
827 instanciation)</para></listitem>
828 <listitem><para><function>maman_ibaz_do_action</function> dereferences the class
829 structure to access its associated class function and calls it.
834 maman_ibaz_base_init (gpointer g_class)
836 static gboolean initialized = FALSE;
839 /* create interface signals here. */
845 maman_ibaz_get_type (void)
847 static GType type = 0;
849 static const GTypeInfo info = {
850 sizeof (MamanIbazInterface),
851 maman_ibaz_base_init, /* base_init */
852 NULL, /* base_finalize */
853 NULL, /* class_init */
854 NULL, /* class_finalize */
855 NULL, /* class_data */
858 NULL /* instance_init */
860 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz", &info, 0);
865 void maman_ibaz_do_action (MamanIbaz *self)
867 MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
873 <sect1 id="howto-interface-implement">
874 <title>How To define and implement an implementation of an Interface?</title>
877 Once the interface is defined, implementing it is rather trivial. Source code showing how to do this
878 for the <type>IBaz</type> interface defined in the previous section is located in
879 <filename>sample/interface/maman-baz.{h|c}</filename>.
883 The first step is to define a normal GType. Here, we have decided to use a GType which derives from
884 GObject. Its name is <type>MamanBaz</type>:
889 #include <glib-object.h>
891 #define MAMAN_TYPE_BAZ (maman_baz_get_type ())
892 #define MAMAN_BAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
893 #define MAMAN_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_BAZ, MamanbazClass))
894 #define MAMAN_IS_BAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
895 #define MAMAN_IS_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_TYPE ((vtable), MAMAN_TYPE_BAZ))
896 #define MAMAN_BAZ_GET_CLASS(inst) (G_TYPE_INSTANCE_GET_CLASS ((inst), MAMAN_TYPE_BAZ, MamanbazClass))
899 typedef struct _MamanBaz MamanBaz;
900 typedef struct _MamanBazClass MamanBazClass;
907 struct _MamanBazClass {
911 GType maman_baz_get_type (void);
916 There is clearly nothing specifically weird or scary about this header: it does not define any weird API
917 or derives from a weird type.
921 The second step is to implement <function>maman_baz_get_type</function>:
924 maman_baz_get_type (void)
926 static GType type = 0;
928 static const GTypeInfo info = {
929 sizeof (MamanBazClass),
930 NULL, /* base_init */
931 NULL, /* base_finalize */
932 NULL, /* class_init */
933 NULL, /* class_finalize */
934 NULL, /* class_data */
937 baz_instance_init /* instance_init */
939 static const GInterfaceInfo ibaz_info = {
940 (GInterfaceInitFunc) baz_interface_init, /* interface_init */
941 NULL, /* interface_finalize */
942 NULL /* interface_data */
944 type = g_type_register_static (G_TYPE_OBJECT,
947 g_type_add_interface_static (type,
954 This function is very much like all the similar functions we looked at previously. The only interface-specific
955 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
956 the type system that this just-registered <type><link linkend="GType">GType</link></type> also implements the interface
957 <function>MAMAN_TYPE_IBAZ</function>.
961 <function>baz_interface_init</function>, the interface initialization function, is also pretty simple:
963 static void baz_do_action (MamanBaz *self)
965 g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
968 baz_interface_init (gpointer g_iface,
971 MamanIbazInteface *iface = (MamanIbazInteface *)g_iface;
972 iface->do_action = (void (*) (MamanIbaz *self))baz_do_action;
975 baz_instance_init (GTypeInstance *instance,
978 MamanBaz *self = MAMAN_BAZ(instance);
979 self->instance_member = 0xdeadbeaf;
982 <function>baz_interface_init</function> merely initializes the interface methods to the implementations
983 defined by <type>MamanBaz</type>: <function>maman_baz_do_action</function> does nothing very useful
990 <title>Interface definition prerequisites</title>
992 <para>To specify that an interface requires the presence of other interfaces when implemented,
993 GObject introduces the concept of <emphasis>prerequisites</emphasis>: it is possible to associate
994 a list of prerequisite interfaces to an interface. For example, if object A wishes to implement interface
995 I1, and if interface I1 has a prerequisite on interface I2, A has to implement both I1 and I2.
998 <para>The mechanism described above is, in practice, very similar to Java's interface I1 extends
999 interface I2. The example below shows the GObject equivalent:
1002 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &info, 0);
1003 /* Make the MamanIbar interface require MamanIbaz interface. */
1004 g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
1006 The code shown above adds the MamanIbaz interface to the list of prerequisites of MamanIbar while the
1007 code below shows how an implementation can implement both interfaces and register their implementations:
1009 static void ibar_do_another_action (MamanBar *self)
1011 g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n", self->instance_member);
1015 ibar_interface_init (gpointer g_iface,
1016 gpointer iface_data)
1018 MamanIbarInterface *iface = (MamanIbarInterface *)g_iface;
1019 iface->do_another_action = (void (*) (MamanIbar *self))ibar_do_another_action;
1023 static void ibaz_do_action (MamanBar *self)
1025 g_print ("Bar implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
1029 ibaz_interface_init (gpointer g_iface,
1030 gpointer iface_data)
1032 MamanIbazInterface *iface = (MamanIbazInterface *)g_iface;
1033 iface->do_action = (void (*) (MamanIbaz *self))ibaz_do_action;
1038 bar_instance_init (GTypeInstance *instance,
1041 MamanBar *self = (MamanBar *)instance;
1042 self->instance_member = 0x666;
1047 maman_bar_get_type (void)
1049 static GType type = 0;
1051 static const GTypeInfo info = {
1052 sizeof (MamanBarClass),
1053 NULL, /* base_init */
1054 NULL, /* base_finalize */
1055 NULL, /* class_init */
1056 NULL, /* class_finalize */
1057 NULL, /* class_data */
1059 0, /* n_preallocs */
1060 bar_instance_init /* instance_init */
1062 static const GInterfaceInfo ibar_info = {
1063 (GInterfaceInitFunc) ibar_interface_init, /* interface_init */
1064 NULL, /* interface_finalize */
1065 NULL /* interface_data */
1067 static const GInterfaceInfo ibaz_info = {
1068 (GInterfaceInitFunc) ibaz_interface_init, /* interface_init */
1069 NULL, /* interface_finalize */
1070 NULL /* interface_data */
1072 type = g_type_register_static (G_TYPE_OBJECT,
1075 g_type_add_interface_static (type,
1078 g_type_add_interface_static (type,
1085 It is very important to notice that the order in which interface implementations are added to the main object
1086 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
1087 no prerequisites and then on the others.
1091 Complete source code showing how to define the MamanIbar interface which requires MamanIbaz and how to
1092 implement the MamanIbar interface is located in <filename>sample/interface/maman-ibar.{h|c}</filename>
1093 and <filename>sample/interface/maman-bar.{h|c}</filename>.
1098 <sect1 id="howto-interface-properties">
1099 <title>Interface Properties</title>
1101 <para>Starting from version 2.4 of glib, gobject interfaces can also have properties.
1102 Declaration of the interface properties is similar to declaring the properties of
1103 ordinary gobject types as explained in <xref linkend="gobject-properties"/>,
1104 except that <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> is used to
1105 declare the properties instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
1108 <para>To include a property named 'name' of type <type>string</type> in the
1109 <type>maman_ibaz</type> interface example code above, we only need to add one
1112 That really is one line extended to six for the sake of clarity
1115 line in the <function>maman_ibaz_base_init</function>
1118 The <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> can also be called from
1119 <function>class_init</function> but it must not be called after that point.
1125 maman_ibaz_base_init (gpointer g_iface)
1127 static gboolean initialized = FALSE;
1130 /* create interface signals here. */
1132 g_object_interface_install_property (g_iface,
1133 g_param_spec_string ("name",
1135 "Name of the MamanIbaz",
1137 G_PARAM_READWRITE));
1144 <para>One point worth noting is that the declared property wasn't assigned an
1145 integer ID. The reason being that integer IDs of properities are utilized only
1146 inside the get and set methods and since interfaces do not implement properties,
1147 there is no need to assign integer IDs to interface properties.
1150 <para>The story for the implementers of the interface is also quite trivial.
1151 An implementer shall declare and define it's properties in the usual way as
1152 explained in <xref linkend="gobject-properties"/>, except for one small
1153 change: it shall declare the properties of the interface it implements using
1154 <function><link linkend="g-object-class-override-property">g_object_class_override_property</link></function> instead of
1155 <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>. The following code snipet
1156 shows the modifications needed in the <type>MamanBaz</type> declaration and
1157 implementation above:
1162 gint instance_member;
1163 gchar *name; /* placeholder for property */
1173 maman_baz_get_type (void)
1175 static GType type = 0;
1177 static const GTypeInfo info = {
1178 sizeof (MamanBazClass),
1179 NULL, /* base_init */
1180 NULL, /* base_finalize */
1181 baz_class_init, /* class_init */
1182 NULL, /* class_finalize */
1183 NULL, /* class_data */
1185 0, /* n_preallocs */
1186 baz_instance_init /* instance_init */
1188 static const GInterfaceInfo ibaz_info = {
1189 (GInterfaceInitFunc) baz_interface_init, /* interface_init */
1190 NULL, /* interface_finalize */
1191 NULL /* interface_data */
1193 type = g_type_register_static (G_TYPE_OBJECT,
1196 g_type_add_interface_static (type,
1204 maman_baz_class_init (MamanBazClass * klass)
1206 GObjectClass *gobject_class;
1208 gobject_class = (GObjectClass *) klass;
1210 parent_class = g_type_class_ref (G_TYPE_OBJECT);
1212 gobject_class->set_property = maman_baz_set_property;
1213 gobject_class->get_property = maman_baz_get_property;
1215 g_object_class_override_property (gobject_class, ARG_NAME, "name");
1219 maman_baz_set_property (GObject * object, guint prop_id,
1220 const GValue * value, GParamSpec * pspec)
1225 /* it's not null if we got it, but it might not be ours */
1226 g_return_if_fail (G_IS_MAMAN_BAZ (object));
1228 baz = MAMAN_BAZ (object);
1232 baz->name = g_value_get_string (value);
1235 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1241 maman_baz_get_property (GObject * object, guint prop_id,
1242 GValue * value, GParamSpec * pspec)
1246 /* it's not null if we got it, but it might not be ours */
1247 g_return_if_fail (G_IS_TEXT_PLUGIN (object));
1249 baz = MAMAN_BAZ (object);
1253 g_value_set_string (value, baz->name);
1256 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
1270 End Howto Interfaces
1279 <chapter id="howto-signals">
1280 <title>Howto create and use signals</title>
1284 The signal system which was built in GType is pretty complex and flexible: it is possible for its users
1285 to connect at runtime any number of callbacks (implemented in any language for which a binding exists)
1287 <para>A python callback can be connected to any signal on any C-based GObject.
1291 to any signal and to stop the emission of any signal at any
1292 state of the signal emission process. This flexibility makes it possible to use GSignal for much more than
1293 just emit events which can be received by numerous clients.
1296 <sect1 id="howto-simple-signals">
1297 <title>Simple use of signals</title>
1299 <para>The most basic use of signals is to implement simple event notification: for example, if we have a
1300 MamanFile object, and if this object has a write method, we might wish to be notified whenever someone
1301 uses this method. The code below shows how the user can connect a callback to the write signal. Full code
1302 for this simple example is located in <filename>sample/signal/maman-file.{h|c}</filename> and
1303 in <filename>sample/signal/test.c</filename>
1305 file = g_object_new (MAMAN_FILE_TYPE, NULL);
1307 g_signal_connect (G_OBJECT (file), "write",
1308 (GCallback)write_event,
1311 maman_file_write (file, buffer, 50);
1316 The <type>MamanFile</type> signal is registered in the class_init function:
1318 klass->write_signal_id =
1319 g_signal_newv ("write",
1320 G_TYPE_FROM_CLASS (g_class),
1321 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1322 NULL /* class closure */,
1323 NULL /* accumulator */,
1324 NULL /* accu_data */,
1325 g_cclosure_marshal_VOID__VOID,
1326 G_TYPE_NONE /* return_type */,
1328 NULL /* param_types */);
1330 and the signal is emited in <function>maman_file_write</function>:
1332 void maman_file_write (MamanFile *self, guint8 *buffer, guint32 size)
1334 /* First write data. */
1335 /* Then, notify user of data written. */
1336 g_signal_emit (self, MAMAN_FILE_GET_CLASS (self)->write_signal_id,
1341 As shown above, you can safely set the details parameter to zero if you do not know what it can be used for.
1342 For a discussion of what you could used it for, see <xref linkend="signal-detail"/>
1346 The signature of the signal handler in the above example is defined as
1347 <function>g_cclosure_marshal_VOID__VOID</function>. Its name follows
1348 a simple convention which encodes the function parameter and return value
1349 types in the function name. Specifically, the value infront of the double
1350 underscore is the type of the return value, while the value(s) after the
1351 double underscore denote the parameter types.
1352 The header <filename>gobject/gmarshal.h</filename> defines a set of commonly
1353 needed closures that one can use.
1360 <title>How to provide more flexibility to users?</title>
1362 <para>The previous implementation does the job but the signal facility of GObject can be used to provide
1363 even more flexibility to this file change notification mechanism. One of the key ideas is to make the process
1364 of writing data to the file part of the signal emission process to allow users to be notified either
1365 before or after the data is written to the file.
1368 <para>To integrate the process of writing the data to the file into the signal emission mechanism, we can
1369 register a default class closure for this signal which will be invoked during the signal emission, just like
1370 any other user-connected signal handler.
1373 <para>The first step to implement this idea is to change the signature of the signal: we need to pass
1374 around the buffer to write and its size. To do this, we use our own marshaller which will be generated
1375 through glib's genmarshall tool. We thus create a file named <filename>marshall.list</filename> which contains
1376 the following single line:
1380 and use the Makefile provided in <filename>sample/signal/Makefile</filename> to generate the file named
1381 <filename>maman-file-complex-marshall.c</filename>. This C file is finally included in
1382 <filename>maman-file-complex.c</filename>.
1385 <para>Once the marshaller is present, we register the signal and its marshaller in the class_init function
1386 of the object <type>MamanFileComplex</type> (full source for this object is included in
1387 <filename>sample/signal/maman-file-complex.{h|c}</filename>):
1389 GClosure *default_closure;
1390 GType param_types[2];
1392 default_closure = g_cclosure_new (G_CALLBACK (default_write_signal_handler),
1393 (gpointer)0xdeadbeaf /* user_data */,
1394 NULL /* destroy_data */);
1396 param_types[0] = G_TYPE_POINTER;
1397 param_types[1] = G_TYPE_UINT;
1398 klass->write_signal_id =
1399 g_signal_newv ("write",
1400 G_TYPE_FROM_CLASS (g_class),
1401 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1402 default_closure /* class closure */,
1403 NULL /* accumulator */,
1404 NULL /* accu_data */,
1405 maman_file_complex_VOID__POINTER_UINT,
1406 G_TYPE_NONE /* return_type */,
1408 param_types /* param_types */);
1410 The code shown above first creates the closure which contains the code to complete the file write. This
1411 closure is registered as the default class_closure of the newly created signal.
1415 Of course, you need to implement completely the code for the default closure since I just provided
1419 default_write_signal_handler (GObject *obj, guint8 *buffer, guint size, gpointer user_data)
1421 g_assert (user_data == (gpointer)0xdeadbeaf);
1422 /* Here, we trigger the real file write. */
1423 g_print ("default signal handler: 0x%x %u\n", buffer, size);
1428 <para>Finally, the client code must invoke the <function>maman_file_complex_write</function> function which
1429 triggers the signal emission:
1431 void maman_file_complex_write (MamanFileComplex *self, guint8 *buffer, guint size)
1434 g_signal_emit (self,
1435 MAMAN_FILE_COMPLEX_GET_CLASS (self)->write_signal_id,
1442 <para>The client code (as shown in <filename>sample/signal/test.c</filename> and below) can now connect signal handlers before
1443 and after the file write is completed: since the default signal handler which does the write itself runs during the
1444 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>
1445 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
1446 which emits signals, I would thus urge you to create all your signals with the G_SIGNAL_RUN_LAST such that your users
1447 have a maximum of flexibility as to when to get the event. Here, we combined it with G_SIGNAL_NO_RECURSE and
1448 G_SIGNAL_NO_HOOKS to ensure our users will not try to do really weird things with our GObject. I strongly advise you
1449 to do the same unless you really know why (in which case you really know the inner workings of GSignal by heart and
1450 you are not reading this).
1455 static void complex_write_event_before (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1457 g_assert (user_data == NULL);
1458 g_print ("Complex Write event before: 0x%x, %u\n", buffer, size);
1461 static void complex_write_event_after (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1463 g_assert (user_data == NULL);
1464 g_print ("Complex Write event after: 0x%x, %u\n", buffer, size);
1467 static void test_file_complex (void)
1472 file = g_object_new (MAMAN_FILE_COMPLEX_TYPE, NULL);
1474 g_signal_connect (G_OBJECT (file), "write",
1475 (GCallback)complex_write_event_before,
1478 g_signal_connect_after (G_OBJECT (file), "write",
1479 (GCallback)complex_write_event_after,
1482 maman_file_complex_write (MAMAN_FILE_COMPLEX (file), buffer, 50);
1484 g_object_unref (G_OBJECT (file));
1487 The code above generates the following output on my machine:
1489 Complex Write event before: 0xbfffe280, 50
1490 default signal handler: 0xbfffe280 50
1491 Complex Write event after: 0xbfffe280, 50
1497 <title>How most people do the same thing with less code</title>
1499 <para>For many historic reasons related to how the ancestor of GObject used to work in GTK+ 1.x versions,
1500 there is a much <emphasis>simpler</emphasis>
1502 <para>I personally think that this method is horribly mind-twisting: it adds a new indirection
1503 which unecessarily complicates the overall code path. However, because this method is widely used
1504 by all of GTK+ and GObject code, readers need to understand it. The reason why this is done that way
1505 in most of GTK+ is related to the fact that the ancestor of GObject did not provide any other way to
1506 create a signal with a default handler than this one. Some people have tried to justify that it is done
1507 that way because it is better, faster (I am extremly doubtfull about the faster bit. As a matter of fact,
1508 the better bit also mystifies me ;-). I have the feeling no one really knows and everyone does it
1509 because they copy/pasted code from code which did the same. It is probably better to leave this
1510 specific trivia to hacker legends domain...
1513 way to create a signal with a default handler than to create
1514 a closure by hand and to use the <function><link linkend="g-signal-newv">g_signal_newv</link></function>.
1517 <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
1518 handler which is stored in the class structure of the object. More specifically, the class structure
1519 contains a function pointer which is accessed during signal emission to invoke the default handler and
1520 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
1521 class structure to the function pointer.
1523 <para>I would like to point out here that the reason why the default handler of a signal is named everywhere
1524 a class_closure is probably related to the fact that it used to be really a function pointer stored in
1525 the class structure.
1530 <para>The following code shows the declaration of the <type>MamanFileSimple</type> class structure which contains
1531 the <function>write</function> function pointer.
1533 struct _MamanFileSimpleClass {
1534 GObjectClass parent;
1536 guint write_signal_id;
1538 /* signal default handlers */
1539 void (*write) (MamanFileSimple *self, guint8 *buffer, guint size);
1542 The <function>write</function> function pointer is initialied in the class_init function of the object
1543 to <function>default_write_signal_handler</function>:
1546 maman_file_simple_class_init (gpointer g_class,
1547 gpointer g_class_data)
1549 GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
1550 MamanFileSimpleClass *klass = MAMAN_FILE_SIMPLE_CLASS (g_class);
1552 klass->write = default_write_signal_handler;
1554 Finally, the signal is created with <function><link linkend="g-signal-new">g_signal_new</link></function> in the same class_init function:
1556 klass->write_signal_id =
1557 g_signal_new ("write",
1558 G_TYPE_FROM_CLASS (g_class),
1559 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1560 G_STRUCT_OFFSET (MamanFileSimpleClass, write),
1561 NULL /* accumulator */,
1562 NULL /* accu_data */,
1563 maman_file_complex_VOID__POINTER_UINT,
1564 G_TYPE_NONE /* return_type */,
1569 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>
1570 macro which indicates the offset of the member <emphasis>write</emphasis> from the start of the
1571 <type>MamanFileSimpleClass</type> class structure.
1573 <para>GSignal uses this offset to create a special wrapper closure
1574 which first retrieves the target function pointer before calling it.
1580 While the complete code for this type of default handler looks less clutered as shown in
1581 <filename>sample/signal/maman-file-simple.{h|c}</filename>, it contains numerous subtleties.
1582 The main subtle point which everyone must be aware of is that the signature of the default
1583 handler created that way does not have a user_data argument:
1584 <function>default_write_signal_handler</function> is different in
1585 <filename>sample/signal/maman-file-complex.c</filename> and in
1586 <filename>sample/signal/maman-file-simple.c</filename>.
1589 <para>If you have doubts about which method to use, I would advise you to use the second one which
1590 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>:
1591 it is better to write code which looks like the vast majority of other GTK+/Gobject code than to
1592 do it your own way. However, now, you know why.
1603 <title>How users can abuse signals (and why some think it is good)</title>
1605 <para>Now that you know how to create signals to which the users can connect easily and at any point in
1606 the signal emission process thanks to <function><link linkend="g-signal-connect">g_signal_connect</link></function>,
1607 <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
1608 users can and will screw you. This is also interesting to know how you too, can screw other people.
1609 This will make you feel good and eleet.
1612 <para>The users can:
1614 <listitem><para>stop the emission of the signal at anytime</para></listitem>
1615 <listitem><para>override the default handler of the signal if it is stored as a function
1616 pointer in the class structure (which is the prefered way to create a default signal handler,
1617 as discussed in the previous section).</para></listitem>
1621 <para>In both cases, the original programmer should be as careful as possible to write code which is
1622 resistant to the fact that the default handler of the signal might not able to run. This is obviously
1623 not the case in the example used in the previous sections since the write to the file depends on whether
1624 or not the default handler runs (however, this might be your goal: to allow the user to prevent the file
1625 write if he wishes to).
1628 <para>If all you want to do is to stop the signal emission from one of the callbacks you connected yourself,
1629 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
1633 <para>If the signal's default handler is just a class function pointer, it is also possible to override
1634 it yourself from the class_init function of a type which derives from the parent. That way, when the signal
1635 is emitted, the parent class will use the function provided by the child as a signal default handler.
1636 Of course, it is also possible (and recommended) to chain up from the child to the parent's default signal
1637 handler to ensure the integrity of the parent object.
1640 <para>Overriding a class method and chaining up was demonstrated in <xref linkend="howto-gobject-methods"/>
1641 which is why I won't bother to show exactly how to do it here again.</para>
1650 <title>Warning on signal creation and default closure</title>
1653 Most of the existing code I have seen up to now (in both GTK+, Gnome libraries and
1654 many GTK+ and Gnome applications) using signals uses a small
1655 variation of the default handler pattern I have shown in the previous section.
1659 Usually, the <function><link linkend="g-signal-new">g_signal_new</link></function> function is preferred over
1660 <function><link linkend="g-signal-newv">g_signal_newv</link></function>. When <function><link linkend="g-signal-new">g_signal_new</link></function>
1661 is used, the default closure is exported as a class function. For example,
1662 <filename>gobject.h</filename> contains the declaration of <type><link linkend="GObjectClass">GObjectClass</link></type>
1663 whose notify class function is the default handler for the <emphasis>notify</emphasis>
1666 struct _GObjectClass
1668 GTypeClass g_type_class;
1670 /* class methods and other stuff. */
1673 void (*notify) (GObject *object,
1680 <filename>gobject.c</filename>'s <function><link linkend="g-object-do-class-init">g_object_do_class_init</link></function> function
1681 registers the <emphasis>notify</emphasis> signal and initializes this class function
1685 g_object_do_class_init (GObjectClass *class)
1690 class->notify = NULL;
1692 gobject_signals[NOTIFY] =
1693 g_signal_new ("notify",
1694 G_TYPE_FROM_CLASS (class),
1695 G_SIGNAL_RUN_FIRST | G_SIGNAL_NO_RECURSE | G_SIGNAL_DETAILED | G_SIGNAL_NO_HOOKS,
1696 G_STRUCT_OFFSET (GObjectClass, notify),
1698 g_cclosure_marshal_VOID__PARAM,
1703 <function><link linkend="g-signal-new">g_signal_new</link></function> creates a <type><link linkend="GClosure">GClosure</link></type> which dereferences the
1704 type's class structure to access the class function pointer and invoke it if it not NULL. The
1705 class function is ignored it is set to NULL.
1709 To understand the reason for such a complex scheme to access the signal's default handler,
1710 you must remember the whole reason for the use of these signals. The goal here is to delegate
1711 a part of the process to the user without requiring the user to subclass the object to override
1712 one of the class functions. The alternative to subclassing, that is, the use of signals
1713 to delegate processing to the user, is, however, a bit less optimal in terms of speed: rather
1714 than just dereferencing a function pointer in a class structure, you must start the whole
1715 process of signal emission which is a bit heavyweight.
1719 This is why some people decided to use class functions for some signal's default handlers:
1720 rather than having users connect a handler to the signal and stop the signal emission
1721 from within that handler, you just need to override the default class function which is
1722 supposedly more efficient.
1730 <capter1 id="howto-doc">
1731 <title>How to generate API documentation for your type?</title>