2 <title>How To ?</title>
5 This chapter tries to answer the real-life questions of users and presents
6 the most common scenario use-cases I could come up with.
7 The use-cases are presented from most likely to less likely.
14 <sect1 id="howto-gobject">
15 <title>How To define and implement a new GObject ?</title>
18 Clearly, this is one of the most common question people ask: they just want to crank code and
19 implement a subclass of a GObject. Sometimes because they want to create their own class hierarchy,
20 sometimes because they want to subclass one of GTK+'s widget. This chapter will focus on the
21 implementation of a subtype of GObject. The sample source code
22 associated to this section can be found in the documentation's source tarball, in the
23 <filename>sample/gobject</filename> directory:
25 <listitem><para><filename>maman-bar.{h|c}</filename>: this is the source for a object which derives from
26 <type>GObject</type> and which shows how to declare different types of methods on the object.
28 <listitem><para><filename>maman-subbar.{h|c}</filename>: this is the source for a object which derives from
29 <type>MamanBar</type> and which shows how to override some of its parent's methods.
31 <listitem><para><filename>maman-foo.{h|c}</filename>: this is the source for an object which derives from
32 <type>GObject</type> and which declares a signal.
34 <listitem><para><filename>test.c</filename>: this is the main source which instantiates an instance of
35 type and exercises their API.
40 <sect2 id="howto-gobject-header">
41 <title>Boilerplate header code</title>
44 The first step before writing the code for your GObject is to write the type's header which contains
45 the needed type, function and macro definitions. Each of these elements is nothing but a convention
46 which is followed not only by GTK+'s code but also by most users of GObject. If you feel the need
47 not to obey the rules stated below, think about it twice:
49 <listitem><para>If your users are a bit accustomed to GTK+ code or any Glib code, they will
50 be a bit surprised and getting used to the conventions you decided upon will take time (money) and
51 will make them grumpy (not a good thing)
54 You must assess the fact that these conventions might have been designed by both smart
55 and experienced people: maybe they were at least partly right. Try to put your ego aside.
61 Pick a name convention for your headers and source code and stick to it:
64 use a dash to separate the prefix from the typename: <filename>maman-bar.h</filename> and
65 <filename>maman-bar.c</filename> (this is the convention used by Nautilus and most Gnome libraries).
68 use an underscore to separate the prefix from the typename: <filename>maman_bar.h</filename> and
69 <filename>maman_bar.c</filename>.
72 Do not separate the prefix from the typename: <filename>mamanbar.h</filename> and
73 <filename>mamanbar.c</filename>. (this is the convention used by GTK+)
76 I personally like the first solution better: it makes reading file names easier for those with poor
81 The basic conventions for any header which exposes a GType are described in
82 <xref linkend="gtype-conventions"/>. Most GObject-based code also obeys onf of the following
83 conventions: pick one and stick to it.
86 If you want to declare a type named bar with prefix maman, name the type instance
87 <function>MamanBar</function> and its class <function>MamanBarClass</function>
88 (name is case-sensitive). It is customary to declare them with code similar to the
92 * Copyright/Licensing information.
99 * Potentially, include other headers on which this header depends.
107 typedef struct _MamanBar MamanBar;
108 typedef struct _MamanBarClass MamanBarClass;
112 /* instance members */
115 struct _MamanBarClass {
120 /* used by MAMAN_BAR_TYPE */
121 GType maman_bar_get_type (void);
124 * Method definitions.
131 Most GTK+ types declare their private fields in the public header with a /* private */ comment,
132 relying on their user's intelligence not to try to play with these fields. Fields not marked private
133 are considered public by default. The /* protected */ comment (same semantics as those of C++)
134 is also used, mainly in the GType library, in code written by Tim Janik.
145 All of Nautilus code and a lot of Gnome libraries use private indirection members, as described
146 by Herb Sutter in his Pimpl articles (see <ulink></ulink>: Herb summarizes the different
147 issues better than I will):
149 typedef struct _MamanBarPrivate MamanBarPrivate;
154 MamanBarPrivate *priv;
157 The private structure is then defined in the .c file, instantiated in the object's
158 <function>init</function> function and destroyed in the object's <function>finalize</function> function.
160 static void maman_bar_finalize(GObject *object) {
161 MamanBar *self = MAMAN_BAR (object);
166 static void maman_bar_init(GTypeInstance *instance, gpointer g_class) {
167 MamanBar *self = MAMAN_BAR (instance);
168 self->priv = g_new0(MamanBarPrivate,1);
177 Finally, there are different header include conventions. Again, pick one and stick to it. I personally
178 use indifferently any of the two, depending on the codebase I work on: the rule is consistency.
181 Some people add at the top of their headers a number of #include directives to pull in
182 all the headers needed to compile client code. This allows client code to simply
183 #include "maman-bar.h".
186 Other do not #include anything and expect the client to #include themselves the headers
187 they need before including your header. This speeds up compilation because it minimizes the
188 amount of pre-processor work. This can be used in conjunction with the re-declaration of certain
189 unused types in the client code to minimize compile-time dependencies and thus speed up
197 <sect2 id="howto-gobject-code">
198 <title>Boilerplate code</title>
201 In your code, the first step is to #include the needed headers: depending on your header include strategy, this
202 can be as simple as #include "maman-bar.h" or as complicated as tens of #include lines ending with
203 #include "maman-bar.h":
206 * Copyright information
209 #include "maman-bar.h"
211 /* If you use Pimpls, include the private structure
212 * definition here. Some people create a maman-bar-private.h header
213 * which is included by the maman-bar.c file and which contains the
214 * definition for this private structure.
216 struct _MamanBarPrivate {
222 * forward definitions
228 Implement <function>maman_bar_get_type</function> and make sure the code compiles:
231 maman_bar_get_type (void)
233 static GType type = 0;
235 static const GTypeInfo info = {
236 sizeof (MamanBarClass),
237 NULL, /* base_init */
238 NULL, /* base_finalize */
239 NULL, /* class_init */
240 NULL, /* class_finalize */
241 NULL, /* class_data */
244 NULL /* instance_init */
246 type = g_type_register_static (G_TYPE_OBJECT,
256 <sect2 id="howto-gobject-construction">
257 <title>Object Construction</title>
260 People often get confused when trying to construct their GObjects because of the
261 sheer number of different ways to hook into the objects's construction process: it is
262 difficult to figure which is the <emphasis>correct</emphasis>, recommended way.
266 <xref linkend="gobject-construction-table"/> shows what user-provided functions
267 are invoked during object instanciation and in which order they are invoked.
268 A user looking for the equivalent of the simple C++ constructor function should use
269 the instance_init method. It will be invoked after all the parent's instance_init
270 functions have been invoked. It cannot take arbitrary construction parameters
271 (as in C++) but if your object needs arbitrary parameters to complete initialization,
272 you can use construction properties.
276 Construction properties will be set only after all instance_init functions have run.
277 No object reference will be returned to the client of <function>g_object_new></function>
278 until all the construction properties have been set.
282 As such, I would recommend writing the following code first:
285 maman_bar_init (GTypeInstance *instance,
288 MamanBar *self = (MamanBar *)instance;
289 self->private = g_new0 (MamanBarPrivate, 1);
291 /* initialize all public and private members to reasonable default values. */
292 /* If you need specific consruction properties to complete initialization,
293 * delay initialization completion until the property is set.
297 And make sure that you set <function>maman_bar_init</function> as the type's instance_init function
298 in <function>maman_bar_get_type</function>. Make sure the code builds and runs: create an instance
299 of the object and make sure <function>maman_bar_init</function> is called (add a
300 <function>g_print</function> call in it).
304 Now, if you need special construction properties, install the properties in the class_init function,
305 override the set and get methods and implement the get and set methods as described in
306 <xref linkend="gobject-properties"/>. Make sure that these properties use a construct only
307 <type>GParamSpec</type> by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps
308 GType ensure that these properties are not set again later by malicious user code.
311 bar_class_init (MamanBarClass *klass)
313 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
314 GParamSpec *maman_param_spec;
316 gobject_class->set_property = bar_set_property;
317 gobject_class->get_property = bar_get_property;
319 maman_param_spec = g_param_spec_string ("maman",
320 "Maman construct prop",
322 "no-name-set" /* default value */,
323 G_PARAM_CONSTRUCT_ONLY |G_PARAM_READWRITE);
325 g_object_class_install_property (gobject_class,
330 If you need this, make sure you can build and run code similar to the code shown above. Make sure
331 your construct properties can set correctly during construction, make sure you cannot set them
332 afterwards and make sure that if your users do not call <function>g_object_new</function>
333 with the required construction properties, these will be initialized with the default values.
337 I consider good taste to halt program execution if a construction property is set its
338 default value. This allows you to catch client code which does not give a reasonable
339 value to the construction properties. Of course, you are free to disagree but you
340 should have a good reason to do so.
343 <para>Some people sometimes need to construct their object but only after the construction properties
344 have been set. This is possible through the use of the constructor class method as described in
345 <xref linkend="gobject-instanciation"/>. However, I have yet to see <emphasis>any</emphasis> reasonable
346 use of this feature. As such, to initialize your object instances, use by default the base_init function
347 and construction properties.
351 <sect2 id="howto-gobject-destruction">
352 <title>Object Destruction</title>
355 Again, it is often difficult to figure out which mechanism to use to hook into the object's
356 destruction process: when the last <function>g_object_unref</function> function call is made,
357 a lot of things happen as described in <xref linkend="gobject-destruction-table"/>.
361 The destruction process of your object must be split is two different phases: you must override
362 both the dispose and the finalize class methods.
364 struct _MamanBarPrivate {
365 gboolean dispose_has_run;
369 bar_dispose (MamanBar *self)
371 if (self->private->dispose_has_run) {
372 /* If dispose did already run, return. */
375 /* Make sure dispose does not run twice. */
376 object->private->dispose_has_run = TRUE;
379 * In dispose, you are supposed to free all types referenced from this
380 * object which might themselves hold a reference to self. Generally,
381 * the most simple solution is to unref all members on which you own a
387 bar_finalize (MamanBar *self)
390 * Here, complete object destruction.
391 * You might not need to do much...
393 g_free (self->private);
397 bar_class_init (BarClass *klass)
399 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
401 gobject_class->dispose = bar_dispose;
402 gobject_class->finalize = bar_finalize;
406 maman_bar_init (GTypeInstance *instance,
409 MamanBar *self = (MamanBar *)instance;
410 self->private = g_new0 (MamanBarPrivate, 1);
411 self->private->dispose_has_run = FALSE;
417 Add similar code to your GObject, make sure the code still builds and runs: dispose and finalize must be called
418 during the last unref.
419 It is possible that object methods might be invoked after dispose is run and before finalize runs. GObject
420 does not consider this to be a program error: you must gracefully detect this and neither crash nor warn
421 the user. To do this, you need something like the following code at the start of each object method, to make
422 sure the object's data is still valid before manipulating it:
424 if (self->private->dispose_has_run) {
425 /* Dispose has run. Data is not valid anymore. */
432 <sect2 id="howto-gobject-methods">
433 <title>Object methods</title>
436 Just as with C++, there are many different ways to define object
437 methods and extend them: the following list and sections draw on C++ vocabulary.
438 (Readers are expected to know basic C++ buzzwords. Those who have not had to
439 write C++ code recently can refer to <ulink>XXXX</ulink> to refresh their
443 non-virtual public methods,
446 virtual public methods and
449 virtual private methods
455 <title>Non-virtual public methods</title>
458 These are the simplest: you want to provide a simple method which can act on your object. All you need
459 to do is to provide a function prototype in the header and an implementation of that prototype
462 /* declaration in the header. */
463 void maman_bar_do_action (MamanBar *self, /* parameters */);
464 /* implementation in the source file */
465 void maman_bar_do_action (MamanBar *self, /* parameters */)
472 <para>There is really nothing scary about this.</para>
476 <title>Virtual public methods</title>
479 This is the preferred way to create polymorphic GObjects. All you need to do is to
480 define the common method and its class function in the public header, implement the
481 common method in the source file and re-implement the class function in each object
482 which inherits from you.
484 /* declaration in maman-bar.h. */
485 struct _MamanBarClass {
489 void (*do_action) (MamanBar *self, /* parameters */);
491 void maman_bar_do_action (MamanBar *self, /* parameters */);
492 /* implementation in maman-bar.c */
493 void maman_bar_do_action (MamanBar *self, /* parameters */)
495 MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
498 The code above simply redirects the do_action call to the relevant class function. Some users,
499 concerned about performance, do not provide the <function>maman_bar_do_action</function>
500 wrapper function and require users to de-reference the class pointer themselves. This is not such
501 a great idea in terms of encapsulation and makes it difficult to change the object's implementation
502 afterwards, should this be needed.
506 Other users, also concerned by performance issues, declare the <function>maman_bar_do_action</function>
507 function inline in the header file. This, however, makes it difficult to change the
508 object's implementation later (although easier than requiring users to directly de-reference the class
509 function) and is often difficult to write in a portable way (the <emphasis>inline</emphasis> keyword
510 is not part of the C standard).
514 In doubt, unless a user shows you hard numbers about the performance cost of the function call,
515 just <function>maman_bar_do_action</function> in the source file.
519 Please, note that it is possible for you to provide a default implementation for this class method in
520 the object's class_init function: initialize the klass->do_action field to a pointer to the actual
521 implementation. You can also make this class method pure virtual by initializing the klass->do_action
525 maman_bar_real_do_action_two (MamanBar *self, /* parameters */)
527 /* Default implementation for the virtual method. */
531 maman_bar_class_init (BarClass *klass)
533 /* pure virtual method: mandates implementation in children. */
534 klass->do_action_one = NULL;
535 /* merely virtual method. */
536 klass->do_action_two = maman_bar_real_do_action_two;
539 void maman_bar_do_action_one (MamanBar *self, /* parameters */)
541 MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
543 void maman_bar_do_action_two (MamanBar *self, /* parameters */)
545 MAMAN_BAR_GET_CLASS (self)->do_action_two (self, /* parameters */);
552 <title>Virtual private Methods</title>
555 These are very similar to Virtual Public methods. They just don't have a public function to call the
556 function directly. The header file contains only a declaration of the class function:
558 /* declaration in maman-bar.h. */
559 struct _MamanBarClass {
563 void (*helper_do_specific_action) (MamanBar *self, /* parameters */);
565 void maman_bar_do_any_action (MamanBar *self, /* parameters */);
567 These class functions are often used to delegate part of the job to child classes:
569 /* this accessor function is static: it is not exported outside of this file. */
571 maman_bar_do_specific_action (MamanBar *self, /* parameters */)
573 MAMAN_BAR_GET_CLASS (self)->do_specific_action (self, /* parameters */);
576 void maman_bar_do_any_action (MamanBar *self, /* parameters */)
578 /* random code here */
581 * Try to execute the requested action. Maybe the requested action cannot be implemented
582 * here. So, we delegate its implementation to the child class:
584 maman_bar_do_specific_action (self, /* parameters */);
586 /* other random code here */
592 Again, it is possible to provide a default implementation for this private virtual class function:
595 maman_bar_class_init (MamanBarClass *klass)
597 /* pure virtual method: mandates implementation in children. */
598 klass->do_specific_action_one = NULL;
599 /* merely virtual method. */
600 klass->do_specific_action_two = maman_bar_real_do_specific_action_two;
606 Children can then implement the subclass with code such as:
609 maman_bar_subtype_class_init (MamanBarSubTypeClass *klass)
611 MamanBarClass *bar_class = MAMAN_BAR_CLASS (klass);
612 /* implement pure virtual class function. */
613 bar_class->do_specific_action_one = maman_bar_subtype_do_specific_action_one;
620 <sect2 id="howto-gobject-chainup">
621 <title>Chaining up</title>
623 <para>Chaining up is often loosely defined by the following set of conditions:
625 <listitem><para>Parent class A defines a public virtual method named <function>foo</function> and
626 provides a default implementation.</para></listitem>
627 <listitem><para>Child class B re-implements method <function>foo</function>.</para></listitem>
628 <listitem><para>In the method B::foo, the child class B calls its parent class method A::foo.</para></listitem>
630 There are many uses to this idiom:
632 <listitem><para>You need to change the behaviour of a class without modifying its code. You create
633 a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
634 slightly and chain up to ensure that the previous behaviour is not really modifed, just extended.
636 <listitem><para>You are lazy, you have access to the source code of the parent class but you don't want
637 to modify it to add method calls to new specialized method calls: it is faster to hack the child class
638 to chain up than to modify the parent to call down.</para></listitem>
639 <listitem><para>You need to implement the Chain Of Responsability pattern: each object of the inheritance
640 tree chains up to its parent (typically, at the begining or the end of the method) to ensure that
641 they each handler is run in turn.</para></listitem>
643 I am personally not really convinced any of the last two uses are really a good idea but since this
644 programming idiom is often used, this section attemps to explain how to implement it.
647 <para>To explicitely chain up to the implementation of the virtual method in the parent class,
648 you first need a handle to the original parent class structure. This pointer can then be used to
649 access the original class function pointer and invoke it directly.
651 <para>The <emphasis>original</emphasis> adjective used in this sentence is not innocuous. To fully
652 understand its meaning, you need to recall how class structures are initialized: for each object type,
653 the class structure associated to this object is created by first copying the class structure of its
654 parent type (a simple <function>memcpy</function>) and then by invoking the class_init callback on
655 the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
656 with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class
657 structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent
663 <para>The function <function>g_type_class_peek_parent</function> is used to access the original parent
664 class structure. Its input is a pointer to the class of the derived object and it returns a pointer
665 to the original parent class structure. The code below shows how you could use it:
668 b_method_to_call (B *obj, int a)
671 AClass *parent_class;
672 klass = B_GET_CLASS (obj);
673 parent_class = g_type_class_peek_parent (klass);
675 /* do stuff before chain up */
676 parent_class->method_to_call (obj, a);
677 /* do stuff after chain up */
680 A lot of people who use this idiom in GTK+ store the parent class structure pointer in a global static
681 variable to avoid the costly call to <function>g_type_class_peek_parent</function> for each function call.
682 Typically, the class_init callback initializes the global static variable. <filename>gtk/gtkhscale.c</filename>
699 <sect1 id="howto-interface">
700 <title>How To define and implement Interfaces ?</title>
702 <sect2 id="howto-interface-define">
703 <title>How To define Interfaces ?</title>
706 The bulk of interface definition has already been shown in <xref linkend="gtype-non-instantiable-classed"/>
707 but I feel it is needed to show exactly how to create an interface. The sample source code
708 associated to this section can be found in the documentation's source tarball, in the
709 <filename>sample/interface/maman-ibaz.{h|c}</filename> file.
713 As above, the first step is to get the header right:
718 #include <glib-object.h>
720 #define MAMAN_TYPE_IBAZ (maman_ibaz_get_type ())
721 #define MAMAN_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
722 #define MAMAN_IBAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_IBAZ, MamanIbazClass))
723 #define MAMAN_IS_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
724 #define MAMAN_IS_IBAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_TYPE ((vtable), MAMAN_TYPE_IBAZ))
725 #define MAMAN_IBAZ_GET_CLASS(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazClass))
728 typedef struct _MamanIbaz MamanIbaz; /* dummy object */
729 typedef struct _MamanIbazClass MamanIbazClass;
731 struct _MamanIbazClass {
732 GTypeInterface parent;
734 void (*do_action) (MamanIbaz *self);
737 GType maman_ibaz_get_type (void);
739 void maman_ibaz_do_action (MamanIbaz *self);
741 #endif /*MAMAN_IBAZ_H*/
743 This code is almost exactly similar to the code for a normal <type>GType</type>
744 which derives from a <type>GObject</type> except for a few details:
747 The <function>_GET_CLASS</function> macro is not implemented with
748 <function>G_TYPE_INSTANCE_GET_CLASS</function> but with <function>G_TYPE_INSTANCE_GET_INTERFACE</function>.
751 The instance type, <type>MamanIbaz</type> is not fully defined: it is used merely as an abstract
752 type which represents an instance of whatever object which implements the interface.
758 The implementation of the <type>MamanIbaz</type> type itself is trivial:
760 <listitem><para><function>maman_ibaz_get_type</function> registers the
761 type in the type system.
763 <listitem><para><function>maman_ibaz_base_init</function> is expected
764 to register the interface's signals if there are any (we will see a bit
765 (later how to use them). Make sure to use a static local boolean variable
766 to make sure not to run the initialization code twice (as described in
767 <xref linkend="gtype-non-instantiable-classed-init"/>,
768 <function>base_init</function> is run once for each interface implementation
769 instanciation)</para></listitem>
770 <listitem><para><function>maman_ibaz_do_action</function> de-references the class
771 structure to access its associated class function and calls it.
776 maman_ibaz_base_init (gpointer g_class)
778 static gboolean initialized = FALSE;
781 /* create interface signals here. */
787 maman_ibaz_get_type (void)
789 static GType type = 0;
791 static const GTypeInfo info = {
792 sizeof (MamanIbazClass),
793 maman_ibaz_base_init, /* base_init */
794 NULL, /* base_finalize */
795 NULL, /* class_init */
796 NULL, /* class_finalize */
797 NULL, /* class_data */
800 NULL /* instance_init */
802 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz", &info, 0);
807 void maman_ibaz_do_action (MamanIbaz *self)
809 MAMAN_IBAZ_GET_CLASS (self)->do_action (self);
815 <sect2 id="howto-interface-implement">
816 <title>How To define and implement an implementation of an Interface ?</title>
819 Once the interface is defined, implementing it is rather trivial. Source code showing how to do this
820 for the <type>IBaz</type> interface defined in the previous section is located in
821 <filename>sample/interface/maman-baz.{h|c}</filename>.
825 The first step is to define a normal GType. Here, we have decided to use a GType which derives from
826 GObject. Its name is <type>MamanBaz</type>:
831 #include <glib-object.h>
833 #define MAMAN_TYPE_BAZ (maman_baz_get_type ())
834 #define MAMAN_BAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
835 #define MAMAN_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_CAST ((vtable), MAMAN_TYPE_BAZ, MamanbazClass))
836 #define MAMAN_IS_BAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
837 #define MAMAN_IS_BAZ_CLASS(vtable) (G_TYPE_CHECK_CLASS_TYPE ((vtable), MAMAN_TYPE_BAZ))
838 #define MAMAN_BAZ_GET_CLASS(inst) (G_TYPE_INSTANCE_GET_CLASS ((inst), MAMAN_TYPE_BAZ, MamanbazClass))
841 typedef struct _MamanBaz MamanBaz;
842 typedef struct _MamanBazClass MamanBazClass;
849 struct _MamanBazClass {
853 GType maman_baz_get_type (void);
858 There is clearly nothing specifically weird or scary about this header: it does not define any weird API
859 or derives from a weird type.
863 The second step is to implement <function>maman_baz_get_type</function>:
866 maman_baz_get_type (void)
868 static GType type = 0;
870 static const GTypeInfo info = {
871 sizeof (MamanBazClass),
872 NULL, /* base_init */
873 NULL, /* base_finalize */
874 NULL, /* class_init */
875 NULL, /* class_finalize */
876 NULL, /* class_data */
879 baz_instance_init /* instance_init */
881 static const GInterfaceInfo ibaz_info = {
882 (GInterfaceInitFunc) baz_interface_init, /* interface_init */
883 NULL, /* interface_finalize */
884 NULL /* interface_data */
886 type = g_type_register_static (G_TYPE_OBJECT,
889 g_type_add_interface_static (type,
896 This function is very much like all the similar functions we looked at previously. The only interface-specific
897 code present here is the call to <function>g_type_add_interface_static</function> which is used to inform
898 the type system that this just-registered <type>GType</type> also implements the interface
899 <function>MAMAN_TYPE_IBAZ</function>.
903 <function>baz_interface_init</function>, the interface initialization function, is also pretty simple:
905 static void baz_do_action (MamanBaz *self)
907 g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
910 baz_interface_init (gpointer g_iface,
913 MamanIbazClass *klass = (MamanIbazClass *)g_iface;
914 klass->do_action = (void (*) (MamanIbaz *self))baz_do_action;
917 baz_instance_init (GTypeInstance *instance,
920 MamanBaz *self = (MamanBaz *)instance;
921 self->instance_member = 0xdeadbeaf;
924 <function>baz_interface_init</function> merely initializes the interface methods to the implementations
925 defined by <type>MamanBaz</type>: <function>maman_baz_do_action</function> does nothing very useful
932 <title>Interface definition prerequisites</title>
936 <para>To specify that an interface requires the presence of other interfaces when implemented,
937 GObject introduces the concept of <emphasis>prerequisites</emphasis>: it is possible to associate
938 a list of prerequisite interfaces to an interface. For example, if object A wishes to implement interface
939 I1, and if interface I1 has a prerequisite on interface I2, A has to implement both I1 and I2.
942 <para>The mechanism described above is, in practice, very similar to Java's interface I1 extends
943 interface I2. The example below shows the GObject equivalent:
946 type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &info, 0);
947 /* Make the MamanIbar interface require MamanIbaz interface. */
948 g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
950 The code shown above adds the MamanIbaz interface to the list of prerequisites of MamanIbar while the
951 code below shows how an implementation can implement both interfaces and register their implementations:
953 static void ibar_do_another_action (MamanBar *self)
955 g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n", self->instance_member);
959 ibar_interface_init (gpointer g_iface,
962 MamanIbarClass *klass = (MamanIbarClass *)g_iface;
963 klass->do_another_action = (void (*) (MamanIbar *self))ibar_do_another_action;
967 static void ibaz_do_action (MamanBar *self)
969 g_print ("Bar implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
973 ibaz_interface_init (gpointer g_iface,
976 MamanIbazClass *klass = (MamanIbazClass *)g_iface;
977 klass->do_action = (void (*) (MamanIbaz *self))ibaz_do_action;
982 bar_instance_init (GTypeInstance *instance,
985 MamanBar *self = (MamanBar *)instance;
986 self->instance_member = 0x666;
991 maman_bar_get_type (void)
993 static GType type = 0;
995 static const GTypeInfo info = {
996 sizeof (MamanBarClass),
997 NULL, /* base_init */
998 NULL, /* base_finalize */
999 NULL, /* class_init */
1000 NULL, /* class_finalize */
1001 NULL, /* class_data */
1003 0, /* n_preallocs */
1004 bar_instance_init /* instance_init */
1006 static const GInterfaceInfo ibar_info = {
1007 (GInterfaceInitFunc) ibar_interface_init, /* interface_init */
1008 NULL, /* interface_finalize */
1009 NULL /* interface_data */
1011 static const GInterfaceInfo ibaz_info = {
1012 (GInterfaceInitFunc) ibaz_interface_init, /* interface_init */
1013 NULL, /* interface_finalize */
1014 NULL /* interface_data */
1016 type = g_type_register_static (G_TYPE_OBJECT,
1019 g_type_add_interface_static (type,
1022 g_type_add_interface_static (type,
1029 It is very important to notice that the order in which interface implementations are added to the main object
1030 is not random: <function>g_type_interface_static</function> must be invoked first on the interfaces which have
1031 no prerequisites and then on the others.
1035 Complete source code showing how to define the MamanIbar interface which requires MamanIbaz and how to
1036 implement the MamanIbar interface is located in <filename>sample/interface/maman-ibar.{h|c}</filename>
1037 and <filename>sample/interface/maman-bar.{h|c}</filename>.
1045 End Howto Interfaces
1054 <sect1 id="howto-signals">
1055 <title>Howto create and use signals</title>
1059 The signal system which was built in GType is pretty complex and flexible: it is possible for its users
1060 to connect at runtime any number of callbacks (implemented in any language for which a binding exists)
1062 <para>A python callback can be connected to any signal on any C-based GObject.
1066 to any signal and to stop the emission of any signal at any
1067 state of the signal emission process. This flexibility makes it possible to use GSignal for much more than
1068 just emit events which can be received by numerous clients.
1072 <title>Simple use of signals</title>
1074 <para>The most basic use of signals is to implement simple event notification: for example, if we have a
1075 MamanFile object, and if this object has a write method, we might wish to be notified whenever someone
1076 uses this method. The code below shows how the user can connect a callback to the write signal. Full code
1077 for this simple example is located in <filename>sample/signal/maman-file.{h|c}</filename> and
1078 in <filename>sample/signal/test.c</filename>
1080 file = g_object_new (MAMAN_FILE_TYPE, NULL);
1082 g_signal_connect (G_OBJECT (file), "write",
1083 (GCallback)write_event,
1086 maman_file_write (file, buffer, 50);
1091 The <type>MamanFile</type> signal is registered in the class_init function:
1093 klass->write_signal_id =
1094 g_signal_newv ("write",
1095 G_TYPE_FROM_CLASS (g_class),
1096 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1097 NULL /* class closure */,
1098 NULL /* accumulator */,
1099 NULL /* accu_data */,
1100 g_cclosure_marshal_VOID__VOID,
1101 G_TYPE_NONE /* return_type */,
1103 NULL /* param_types */);
1105 and the signal is emited in <function>maman_file_write</function>:
1107 void maman_file_write (MamanFile *self, guint8 *buffer, guint32 size)
1109 /* First write data. */
1110 /* Then, notify user of data written. */
1111 g_signal_emit (self, MAMAN_FILE_GET_CLASS (self)->write_signal_id,
1116 As shown above, you can safely set the details parameter to zero if you do not know what it can be used for.
1117 For a discussion of what you could used it for, see <xref linkend="signal-detail"/>
1127 <title>How to provide more flexibility to users ?</title>
1129 <para>The previous implementation does the job but the signal facility of GObject can be used to provide
1130 even more flexibility to this file change notification mechanism. One of the key ideas is to make the process
1131 of writing data to the file part of the signal emission process to allow users to be notified either
1132 before or after the data is written to the file.
1135 <para>To integrate the process of writing the data to the file into the signal emission mechanism, we can
1136 register a default class closure for this signal which will be invoked during the signal emission, just like
1137 any other user-connected signal handler.
1140 <para>The first step to implement this idea is to change the signature of the signal: we need to pass
1141 around the buffer to write and its size. To do this, we use our own marshaller which will be generated
1142 through glib's genmarshall tool. We thus create a file named <filename>marshall.list</filename> which contains
1143 the following single line:
1147 and use the Makefile provided in <filename>sample/signal/Makefile</filename> to generate the file named
1148 <filename>maman-file-complex-marshall.c</filename>. This C file is finally included in
1149 <filename>maman-file-complex.c</filename>.
1152 <para>Once the marshaller is present, we register the signal and its marshaller in the class_init function
1153 of the object <type>MamanFileComplex</type> (full source for this object is included in
1154 <filename>sample/signal/maman-file-complex.{h|c}</filename>):
1156 GClosure *default_closure;
1157 GType param_types[2];
1159 default_closure = g_cclosure_new (G_CALLBACK (default_write_signal_handler),
1160 (gpointer)0xdeadbeaf /* user_data */,
1161 NULL /* destroy_data */);
1163 param_types[0] = G_TYPE_POINTER;
1164 param_types[1] = G_TYPE_UINT;
1165 klass->write_signal_id =
1166 g_signal_newv ("write",
1167 G_TYPE_FROM_CLASS (g_class),
1168 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1169 default_closure /* class closure */,
1170 NULL /* accumulator */,
1171 NULL /* accu_data */,
1172 maman_file_complex_VOID__POINTER_UINT,
1173 G_TYPE_NONE /* return_type */,
1175 param_types /* param_types */);
1177 The code shown above first creates the closure which contains the code to complete the file write. This
1178 closure is registered as the default class_closure of the newly created signal.
1182 Of course, you need to implement completely the code for the default closure since I just provided
1186 default_write_signal_handler (GObject *obj, guint8 *buffer, guint size, gpointer user_data)
1188 g_assert (user_data == (gpointer)0xdeadbeaf);
1189 /* Here, we trigger the real file write. */
1190 g_print ("default signal handler: 0x%x %u\n", buffer, size);
1195 <para>Finally, the client code must invoke the <function>maman_file_complex_write</function> function which
1196 triggers the signal emission:
1198 void maman_file_complex_write (MamanFileComplex *self, guint8 *buffer, guint size)
1201 g_signal_emit (self,
1202 MAMAN_FILE_COMPLEX_GET_CLASS (self)->write_signal_id,
1209 <para>The client code (as shown in <filename>sample/signal/test.c</filename> and below) can now connect signal handlers before
1210 and after the file write is completed: since the default signal handler which does the write itself runs during the
1211 RUN_LAST phase of the signal emission, it will run after all handlers connected with <function>g_signal_connect</function>
1212 and before all handlers connected with <function>g_signal_connect_after</function>. If you intent to write a GObject
1213 which emits signals, I would thus urge you to create all your signals with the G_SIGNAL_RUN_LAST such that your users
1214 have a maximum of flexibility as to when to get the event. Here, we combined it with G_SIGNAL_NO_RECURSE and
1215 G_SIGNAL_NO_HOOKS to ensure our users will not try to do really weird things with our GObject. I strongly advise you
1216 to do the same unless you really know why (in which case you really know the inner workings of GSignal by heart and
1217 you are not reading this).
1222 static void complex_write_event_before (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1224 g_assert (user_data == NULL);
1225 g_print ("Complex Write event before: 0x%x, %u\n", buffer, size);
1228 static void complex_write_event_after (GObject *file, guint8 *buffer, guint size, gpointer user_data)
1230 g_assert (user_data == NULL);
1231 g_print ("Complex Write event after: 0x%x, %u\n", buffer, size);
1234 static void test_file_complex (void)
1239 file = g_object_new (MAMAN_FILE_COMPLEX_TYPE, NULL);
1241 g_signal_connect (G_OBJECT (file), "write",
1242 (GCallback)complex_write_event_before,
1245 g_signal_connect_after (G_OBJECT (file), "write",
1246 (GCallback)complex_write_event_after,
1249 maman_file_complex_write (MAMAN_FILE_COMPLEX (file), buffer, 50);
1251 g_object_unref (G_OBJECT (file));
1254 The code above generates the following output on my machine:
1256 Complex Write event before: 0xbfffe280, 50
1257 default signal handler: 0xbfffe280 50
1258 Complex Write event after: 0xbfffe280, 50
1264 <title>How most people do the same thing with less code</title>
1266 <para>For many historic reasons related to how the ancestor of GObject used to work in GTK+ 1.x versions,
1267 there is a much <emphasis>simpler</emphasis>
1269 <para>I personally think that this method is horribly mind-twisting: it adds a new indirection
1270 which unecessarily complicates the overall code path. However, because this method is widely used
1271 by all of GTK+ and GObject code, readers need to understand it. The reason why this is done that way
1272 in most of GTK+ is related to the fact that the ancestor of GObject did not provide any other way to
1273 create a signal with a default handler than this one. Some people have tried to justify that it is done
1274 that way because it is better, faster (I am extremly doubtfull about the faster bit. As a matter of fact,
1275 the better bit also mystifies me ;-). I have the feeling no one really knows and everyone does it
1276 because they copy/pasted code from code which did the same. It is probably better to leave this
1277 specific trivia to hacker legends domain...
1280 way to create a signal with a default handler than to create
1281 a closure by hand and to use the <function>g_signal_newv</function>.
1284 <para>For example, <function>g_signal_new</function> can be used to create a signal which uses a default
1285 handler which is stored in the class structure of the object. More specifically, the class structure
1286 contains a function pointer which is accessed during signal emission to invoke the default handler and
1287 the user is expected to provide to <function>g_signal_new</function> the offset from the start of the
1288 class structure to the function pointer.
1290 <para>I would like to point out here that the reason why the default handler of a signal is named everywhere
1291 a class_closure is probably related to the fact that it used to be really a function pointer stored in
1292 the class structure.
1297 <para>The following code shows the declaration of the <type>MamanFileSimple</type> class structure which contains
1298 the <function>write</function> function pointer.
1300 struct _MamanFileSimpleClass {
1301 GObjectClass parent;
1303 guint write_signal_id;
1305 /* signal default handlers */
1306 void (*write) (MamanFileSimple *self, guint8 *buffer, guint size);
1309 The <function>write</function> function pointer is initialied in the class_init function of the object
1310 to <function>default_write_signal_handler</function>:
1313 maman_file_simple_class_init (gpointer g_class,
1314 gpointer g_class_data)
1316 GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
1317 MamanFileSimpleClass *klass = MAMAN_FILE_SIMPLE_CLASS (g_class);
1319 klass->write = default_write_signal_handler;
1321 Finally, the signal is created with <function>g_signal_new</function> in the same class_init function:
1323 klass->write_signal_id =
1324 g_signal_new ("write",
1325 G_TYPE_FROM_CLASS (g_class),
1326 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
1327 G_STRUCT_OFFSET (MamanFileSimpleClass, write),
1328 NULL /* accumulator */,
1329 NULL /* accu_data */,
1330 maman_file_complex_VOID__POINTER_UINT,
1331 G_TYPE_NONE /* return_type */,
1336 Of note, here, is the 4th argument to the function: it is an integer calculated by the <function>G_STRUCT_OFFSET</function>
1337 macro which indicates the offset of the member <emphasis>write</emphasis> from the start of the
1338 <type>MamanFileSimpleClass</type> class structure.
1340 <para>GSignal uses this offset to create a special wrapper closure
1341 which first retrieves the target function pointer before calling it.
1347 While the complete code for this type of default handler looks less clutered as shown in
1348 <filename>sample/signal/maman-file-simple.{h|c}</filename>, it contains numerous subtleties.
1349 The main subtle point which everyone must be aware of is that the signature of the default
1350 handler created that way does not have a user_data argument:
1351 <function>default_write_signal_handler</function> is different in
1352 <filename>sample/signal/maman-file-complex.c</filename> and in
1353 <filename>sample/signal/maman-file-simple.c</filename>.
1356 <para>If you have doubts about which method to use, I would advise you to use the second one which
1357 involves <function>g_signal_new</function> rather than <function>g_signal_newv</function>:
1358 it is better to write code which looks like the vast majority of other GTK+/Gobject code than to
1359 do it your own way. However, now, you know why.
1370 <title>How users can abuse signals (and why some think it is good)</title>
1372 <para>Now that you know how to create signals to which the users can connect easily and at any point in
1373 the signal emission process thanks to <function>g_signal_connect</function>,
1374 <function>g_signal_connect_after</function> and G_SIGNAL_RUN_LAST, it is time to look into how your
1375 users can and will screw you. This is also interesting to know how you too, can screw other people.
1376 This will make you feel good and eleet.
1379 <para>The users can:
1381 <listitem><para>stop the emission of the signal at anytime</para></listitem>
1382 <listitem><para>override the default handler of the signal if it is stored as a function
1383 pointer in the class structure (which is the prefered way to create a default signal handler,
1384 as discussed in the previous section).</para></listitem>
1388 <para>In both cases, the original programmer should be as careful as possible to write code which is
1389 resistant to the fact that the default handler of the signal might not able to run. This is obviously
1390 not the case in the example used in the previous sections since the write to the file depends on whether
1391 or not the default handler runs (however, this might be your goal: to allow the user to prevent the file
1392 write if he wishes to).
1395 <para>If all you want to do is to stop the signal emission from one of the callbacks you connected yourself,
1396 you can call <function>g_signal_stop_by_name</function>. Its use is very simple which is why I won't detail
1400 <para>If the signal's default handler is just a class function pointer, it is also possible to override
1401 it yourself from the class_init function of a type which derives from the parent. That way, when the signal
1402 is emitted, the parent class will use the function provided by the child as a signal default handler.
1403 Of course, it is also possible (and recommended) to chain up from the child to the parent's default signal
1404 handler to ensure the integrity of the parent object.
1407 <para>Overriding a class method and chaining up was demonstrated in <xref linkend="howto-gobject-methods"/>
1408 which is why I won't bother to show exactly how to do it here again.</para>
1417 <title>Warning on signal creation and default closure</title>
1420 Most of the existing code I have seen up to now (in both GTK+, Gnome libraries and
1421 many GTK+ and Gnome applications) using signals uses a small
1422 variation of the default handler pattern I have shown in the previous section.
1426 Usually, the <function>g_signal_new</function> function is preferred over
1427 <function>g_signal_newv</function>. When <function>g_signal_new</function>
1428 is used, the default closure is exported as a class function. For example,
1429 <filename>gobject.h</filename> contains the declaration of <type>GObjectClass</type>
1430 whose notify class function is the default handler for the <emphasis>notify</emphasis>
1433 struct _GObjectClass
1435 GTypeClass g_type_class;
1437 /* class methods and other stuff. */
1440 void (*notify) (GObject *object,
1447 <filename>gobject.c</filename>'s <function>g_object_do_class_init</function> function
1448 registers the <emphasis>notify</emphasis> signal and initializes this class function
1452 g_object_do_class_init (GObjectClass *class)
1457 class->notify = NULL;
1459 gobject_signals[NOTIFY] =
1460 g_signal_new ("notify",
1461 G_TYPE_FROM_CLASS (class),
1462 G_SIGNAL_RUN_FIRST | G_SIGNAL_NO_RECURSE | G_SIGNAL_DETAILED | G_SIGNAL_NO_HOOKS,
1463 G_STRUCT_OFFSET (GObjectClass, notify),
1465 g_cclosure_marshal_VOID__PARAM,
1470 <function>g_signal_new</function> creates a <type>GClosure</type> which de-references the
1471 type's class structure to access the class function pointer and invoke it if it not NULL. The
1472 class function is ignored it is set to NULL.
1476 To understand the reason for such a complex scheme to access the signal's default handler,
1477 you must remember the whole reason for the use of these signals. The goal here is to delegate
1478 a part of the process to the user without requiring the user to subclass the object to override
1479 one of the class functions. The alternative to subclassing, that is, the use of signals
1480 to delegate processing to the user, is, however, a bit less optimal in terms of speed: rather
1481 than just de-referencing a function pointer in a class structure, you must start the whole
1482 process of signal emission which is a bit heavyweight.
1486 This is why some people decided to use class functions for some signal's default handlers:
1487 rather than having users connect a handler to the signal and stop the signal emission
1488 from within that handler, you just need to override the default class function which is
1489 supposedly more efficient.
1497 <sect1 id="howto-doc">
1498 <title>How to generate API documentation for your type ?</title>