1 <?xml version='1.0' encoding="ISO-8859-1"?>
2 <chapter id="chapter-signal">
3 <title>The GObject messaging system</title>
6 <title>Closures</title>
9 Closures are central to the concept of asynchronous signal delivery
10 which is widely used throughout GTK+ and Gnome applications. A Closure is an
11 abstraction, a generic representation of a callback. It is a small structure
12 which contains three objects:
14 <listitem><para>a function pointer (the callback itself) whose prototype looks like:
16 return_type function_callback (... , gpointer user_data);
20 the user_data pointer which is passed to the callback upon invocation of the closure
23 a function pointer which represents the destructor of the closure: whenever the
24 closure's refcount reaches zero, this function will be called before the closure
31 The <type><link linkend="GClosure">GClosure</link></type> structure represents the common functionality of all
32 closure implementations: there exist a different Closure implementation for
33 each separate runtime which wants to use the GObject type system.
35 In Practice, Closures sit at the boundary of language runtimes: if you are
36 writing python code and one of your Python callback receives a signal from
37 one of GTK+ widgets, the C code in GTK+ needs to execute your Python
38 code. The Closure invoked by the GTK+ object invokes the Python callback:
39 it behaves as a normal C object for GTK+ and as a normal Python object for
42 The GObject library provides a simple <type><link linkend="GCClosure">GCClosure</link></type> type which
43 is a specific implementation of closures to be used with C/C++ callbacks.
46 A <type><link linkend="GClosure">GClosure</link></type> provides simple services:
49 Invocation (<function><link linkend="g-closure-invoke">g_closure_invoke</link></function>): this is what closures
50 were created for: they hide the details of callback invocation from the
54 Notification: the closure notifies listeners of certain events such as
55 closure invocation, closure invalidation and closure finalization. Listeners
56 can be registered with <function><link linkend="g-closure-add-finalize-notifier">g_closure_add_finalize_notifier</link></function>
57 (finalization notification), <function><link linkend="g-closure-add-invalidate-notifier">g_closure_add_invalidate_notifier</link></function>
58 (invalidation notification) and
59 <function><link linkend="g-closure-add-marshal-guards">g_closure_add_marshal_guards</link></function> (invocation notification).
60 There exist symmetric de-registration functions for finalization and invalidation
61 events (<function><link linkend="g-closure-remove-finalize-notifier">g_closure_remove_finalize_notifier</link></function> and
62 <function><link linkend="g-closure-remove-invalidate-notifier">g_closure_remove_invalidate_notifier</link></function>) but not for the invocation
65 Closures are refcounted and notify listeners of their destruction in a two-stage
66 process: the invalidation notifiers are invoked before the finalization notifiers.
73 <title>C Closures</title>
76 If you are using C or C++
77 to connect a callback to a given event, you will either use the simple <type><link linkend="GCClosure">GCClosure</link></type>s
78 which have a pretty minimal API or the even simpler <function><link linkend="g-signal-connect">g_signal_connect</link></function>
79 functions (which will be presented a bit later :).
81 GClosure* g_cclosure_new (GCallback callback_func,
83 GClosureNotify destroy_data);
84 GClosure* g_cclosure_new_swap (GCallback callback_func,
86 GClosureNotify destroy_data);
87 GClosure* g_signal_type_cclosure_new (GType itype,
93 <function><link linkend="g-cclosure-new">g_cclosure_new</link></function> will create a new closure which can invoke the
94 user-provided callback_func with the user-provided user_data as last parameter. When the closure
95 is finalized (second stage of the destruction process), it will invoke the destroy_data function
96 if the user has supplied one.
100 <function><link linkend="g-cclosure-new-swap">g_cclosure_new_swap</link></function> will create a new closure which can invoke the
101 user-provided callback_func with the user-provided user_data as first parameter (instead of being the
102 last parameter as with <function><link linkend="g-cclosure-new">g_cclosure_new</link></function>). When the closure
103 is finalized (second stage of the destruction process), it will invoke the destroy_data
104 function if the user has supplied one.
109 <title>non-C closures (for the fearless).</title>
112 As was explained above, Closures hide the details of callback invocation. In C,
113 callback invocation is just like function invocation: it is a matter of creating
114 the correct stack frame for the called function and executing a <emphasis>call</emphasis>
115 assembly instruction.
119 C closure marshallers transform the array of GValues which represent
120 the parameters to the target function into a C-style function parameter list, invoke
121 the user-supplied C function with this new parameter list, get the return value of the
122 function, transform it into a GValue and return this GValue to the marshaller caller.
126 The following code implements a simple marshaller in C for a C function which takes an
127 integer as first parameter and returns void.
129 g_cclosure_marshal_VOID__INT (GClosure *closure,
130 GValue *return_value,
131 guint n_param_values,
132 const GValue *param_values,
133 gpointer invocation_hint,
134 gpointer marshal_data)
136 typedef void (*GMarshalFunc_VOID__INT) (gpointer data1,
139 register GMarshalFunc_VOID__INT callback;
140 register GCClosure *cc = (GCClosure*) closure;
141 register gpointer data1, data2;
143 g_return_if_fail (n_param_values == 2);
145 data1 = g_value_peek_pointer (param_values + 0);
146 data2 = closure->data;
148 callback = (GMarshalFunc_VOID__INT) (marshal_data ? marshal_data : cc->callback);
151 g_marshal_value_peek_int (param_values + 1),
158 Of course, there exist other kinds of marshallers. For example, James Henstridge
159 wrote a generic Python marshaller which is used by all python Closures (a python closure
160 is used to have python-based callback be invoked by the closure invocation process).
161 This python marshaller transforms the input GValue list representing the function
162 parameters into a Python tupple which is the equivalent structure in python (you can
163 look in <function>pyg_closure_marshal</function> in <filename>pygtype.c</filename>
164 in the <emphasis>pygtk</emphasis> module in Gnome cvs server).
171 <title>Signals</title>
174 GObject's signals have nothing to do with standard UNIX signals: they connect
175 arbitrary application-specific events with any number of listeners.
176 For example, in GTK+, every user event (keystroke or mouse move) is received
177 from the X server and generates a GTK+ event under the form of a signal emission
178 on a given object instance.
182 Each signal is registered in the type system together with the type on which
183 it can be emitted: users of the type are said to <emphasis>connect</emphasis>
184 to the signal on a given type instance when they register a closure to be
185 invoked upon the signal emission. Users can also emit the signal by themselves
186 or stop the emission of the signal from within one of the closures connected
191 When a signal is emitted on a given type instance, all the closures
192 connected to this signal on this type instance will be invoked. All the closures
193 connected to such a signal represent callbacks whose signature looks like:
195 return_type function_callback (gpointer instance, ... , gpointer user_data);
199 <sect2 id="signal-registration">
200 <title>Signal registration</title>
203 To register a new signal on an existing type, we can use any of <function><link linkend="g-signal-newv">g_signal_newv</link></function>,
204 <function><link linkend="g-signal-new-valist">g_signal_new_valist</link></function> or <function><link linkend="g-signal-new">g_signal_new</link></function> functions:
206 guint g_signal_newv (const gchar *signal_name,
208 GSignalFlags signal_flags,
209 GClosure *class_closure,
210 GSignalAccumulator accumulator,
212 GSignalCMarshaller c_marshaller,
217 The number of parameters to these functions is a bit intimidating but they are relatively
221 signal_name: is a string which can be used to uniquely identify a given signal.
224 itype: is the instance type on which this signal can be emitted.
227 signal_flags: partly defines the order in which closures which were connected to the
231 class_closure: this is the default closure for the signal: if it is not NULL upon
232 the signal emission, it will be invoked upon this emission of the signal. The
233 moment where this closure is invoked compared to other closures connected to that
234 signal depends partly on the signal_flags.
237 accumulator: this is a function pointer which is invoked after each closure
238 has been invoked. If it returns FALSE, signal emission is stopped. If it returns
239 TRUE, signal emission proceeds normally. It is also used to compute the return
240 value of the signal based on the return value of all the invoked closures.
243 accumulator_data: this pointer will be passed down to each invocation of the
244 accumulator during emission.
247 c_marshaller: this is the default C marshaller for any closure which is connected to
251 return_type: this is the type of the return value of the signal.
254 n_params: this is the number of parameters this signal takes.
257 param_types: this is an array of GTypes which indicate the type of each parameter
258 of the signal. The length of this array is indicated by n_params.
264 As you can see from the above definition, a signal is basically a description
265 of the closures which can be connected to this signal and a description of the
266 order in which the closures connected to this signal will be invoked.
271 <sect2 id="signal-connection">
272 <title>Signal connection</title>
275 If you want to connect to a signal with a closure, you have three possibilities:
278 You can register a class closure at signal registration: this is a
279 system-wide operation. i.e.: the class_closure will be invoked during each emission
280 of a given signal on all the instances of the type which supports that signal.
283 You can use <function><link linkend="g-signal-override-class-closure">g_signal_override_class_closure</link></function> which
284 overrides the class_closure of a given type. It is possible to call this function
285 only on a derived type of the type on which the signal was registered.
286 This function is of use only to language bindings.
289 You can register a closure with the <function><link linkend="g-signal-connect">g_signal_connect</link></function>
290 family of functions. This is an instance-specific operation: the closure
291 will be invoked only during emission of a given signal on a given instance.
294 It is also possible to connect a different kind of callback on a given signal:
295 emission hooks are invoked whenever a given signal is emitted whatever the instance on
296 which it is emitted. Emission hooks are used for example to get all mouse_clicked
297 emissions in an application to be able to emit the small mouse click sound.
298 Emission hooks are connected with <function><link linkend="g-signal-add-emission-hook">g_signal_add_emission_hook</link></function>
299 and removed with <function><link linkend="g-signal-remove-emission-hook">g_signal_remove_emission_hook</link></function>.
307 <sect2 id="signal-emission">
308 <title>Signal emission</title>
311 Signal emission is done through the use of the <function><link linkend="g-signal-emit">g_signal_emit</link></function> family
314 void g_signal_emitv (const GValue *instance_and_params,
317 GValue *return_value);
321 The instance_and_params array of GValues contains the list of input
322 parameters to the signal. The first element of the array is the
323 instance pointer on which to invoke the signal. The following elements of
324 the array contain the list of parameters to the signal.
327 signal_id identifies the signal to invoke.
330 detail identifies the specific detail of the signal to invoke. A detail is a kind of
331 magic token/argument which is passed around during signal emission and which is used
332 by closures connected to the signal to filter out unwanted signal emissions. In most
333 cases, you can safely set this value to zero. See <xref linkend="signal-detail"/> for
334 more details about this parameter.
337 return_value holds the return value of the last closure invoked during emission if
338 no accumulator was specified. If an accumulator was specified during signal creation,
339 this accumulator is used to calculate the return_value as a function of the return
340 values of all the closures invoked during emission.
342 James (again!!) gives a few non-trivial examples of accumulators:
344 For instance, you may have an accumulator that ignores NULL returns from
345 closures, and only accumulates the non-NULL ones. Another accumulator may try
346 to return the list of values returned by the closures.
349 If no closure is invoked during
350 emission, the return_value is nonetheless initialized to zero/null.
356 Internally, the GValue array is passed to the emission function proper,
357 <function>signal_emit_unlocked_R</function> (implemented in <filename>gsignal.c</filename>).
358 Signal emission can be decomposed in 5 steps:
361 <emphasis>RUN_FIRST</emphasis>: if the G_SIGNAL_RUN_FIRST flag was used
362 during signal registration and if there exist a class_closure for this signal,
363 the class_closure is invoked. Jump to <emphasis>EMISSION_HOOK</emphasis> state.
366 <emphasis>EMISSION_HOOK</emphasis>: if any emission hook was added to
367 the signal, they are invoked from first to last added. Accumulate return values
368 and jump to <emphasis>HANDLER_RUN_FIRST</emphasis> state.
371 <emphasis>HANDLER_RUN_FIRST</emphasis>: if any closure were connected
372 with the <function><link linkend="g-signal-connect">g_signal_connect</link></function> family of
373 functions, and if they are not blocked (with the <function><link linkend="g-signal-handler-block">g_signal_handler_block</link></function>
374 family of functions) they are run here, from first to last connected.
375 Jump to <emphasis>RUN_LAST</emphasis> state.
378 <emphasis>RUN_LAST</emphasis>: if the G_SIGNAL_RUN_LAST
379 flag was set during registration and if a class_closure
380 was set, it is invoked here. Jump to
381 <emphasis>HANDLER_RUN_LAST</emphasis> state.
384 <emphasis>HANDLER_RUN_LAST</emphasis>: if any closure were connected
385 with the <function>g_signal_connect_after</function> family of
386 functions, if they were not invoked during HANDLER_RUN_FIRST and if they
387 are not blocked, they are run here, from first to last connected.
388 Jump to <emphasis>RUN_CLEANUP</emphasis> state.
391 <emphasis>RUN_CLEANUP</emphasis>: if the G_SIGNAL_RUN_CLEANUP flag
392 was set during registration and if a class_closure was set,
393 it is invoked here. Signal emission is completed here.
399 If, at any point during emission (except in RUN_CLEANUP state), one of the
400 closures or emission hook stops the signal emission with
401 <function><link linkend="g-signal-stop">g_signal_stop</link></function>, emission jumps to CLEANUP state.
405 If, at any point during emission, one of the closures or emission hook
406 emits the same signal on the same instance, emission is restarted from
411 The accumulator function is invoked in all states, after invocation
412 of each closure (except in EMISSION_HOOK and CLEANUP). It accumulates
413 the closure return value into the signal return value and returns TRUE or
414 FALSE. If, at any point, it does not return TRUE, emission jumps to CLEANUP state.
418 If no accumulator function was provided, the value returned by the last handler
419 run will be returned by <function><link linkend="g-signal-emit">g_signal_emit</link></function>.
425 <sect2 id="signal-detail">
426 <title>The <emphasis>detail</emphasis> argument</title>
428 <para>All the functions related to signal emission or signal connection have a parameter
429 named the <emphasis>detail</emphasis>. Sometimes, this parameter is hidden by the API
430 but it is always there, under one form or another.
434 Of the three main connection functions,
435 only one has an explicit detail parameter as a <type><link linkend="GQuark">GQuark</link></type>
437 <para>A GQuark is an integer which uniquely represents a string. It is possible to transform
438 back and forth between the integer and string representations with the functions
439 <function><link linkend="g-quark-from-string">g_quark_from_string</link></function> and <function><link linkend="g-quark-to-string">g_quark_to_string</link></function>.
443 gulong g_signal_connect_closure_by_id (gpointer instance,
449 The two other functions hide the detail parameter in the signal name identification:
451 gulong g_signal_connect_closure (gpointer instance,
452 const gchar *detailed_signal,
455 gulong g_signal_connect_data (gpointer instance,
456 const gchar *detailed_signal,
459 GClosureNotify destroy_data,
460 GConnectFlags connect_flags);
462 Their detailed_signal parameter is a string which identifies the name of the signal
463 to connect to. However, the format of this string is structured to look like
464 <emphasis>signal_name::detail_name</emphasis>. Connecting to the signal
465 named <emphasis>notify::cursor_position</emphasis> will actually connect to the signal
466 named <emphasis>notify</emphasis> with the <emphasis>cursor_position</emphasis> name.
467 Internally, the detail string is transformed to a GQuark if it is present.
471 Of the four main signal emission functions, three have an explicit detail parameter as a
472 <type><link linkend="GQuark">GQuark</link></type> again:
474 void g_signal_emitv (const GValue *instance_and_params,
477 GValue *return_value);
478 void g_signal_emit_valist (gpointer instance,
482 void g_signal_emit (gpointer instance,
487 The fourth function hides it in its signal name parameter:
489 void g_signal_emit_by_name (gpointer instance,
490 const gchar *detailed_signal,
493 The format of the detailed_signal parameter is exactly the same as the format used by
494 the <function><link linkend="g-signal-connect">g_signal_connect</link></function> functions: <emphasis>signal_name::detail_name</emphasis>.
498 If a detail is provided by the user to the emission function, it is used during emission to match
499 against the closures which also provide a detail. The closures which provided a detail will not
500 be invoked (even though they are connected to a signal which is being emitted) if their detail
501 does not match the detail provided by the user.
504 <para>This completely optional filtering mechanism is mainly used as an optimization for signals
505 which are often emitted for many different reasons: the clients can filter out which events they are
506 interested into before the closure's marshalling code runs. For example, this is used extensively
507 by the <emphasis>notify</emphasis> signal of GObject: whenever a property is modified on a GObject,
508 instead of just emitting the <emphasis>notify</emphasis> signal, GObject associates as a detail to this
509 signal emission the name of the property modified. This allows clients who wish to be notified of changes
510 to only one property to filter most events before receiving them.
513 <para>As a simple rule, users can and should set the detail parameter to zero: this will disable completely
514 this optional filtering.