glib/docs/reference/gobject/tut_gsignal.xml

<?xml version='1.0' encoding="ISO-8859-1"?>
  <chapter id="chapter-signal">
    <title>The GObject messaging system</title>

    <sect1 id="closure">
      <title>Closures</title>

      <para>
        Closures are central to the concept of asynchronous signal delivery
        which is widely used throughout GTK+ and Gnome applications. A Closure is an 
        abstraction, a generic representation of a callback. It is a small structure
        which contains three objects:
        <itemizedlist>
          <listitem><para>a function pointer (the callback itself) whose prototype looks like:
<programlisting>
return_type function_callback (... , gpointer user_data);
</programlisting>
          </para></listitem>
          <listitem><para>
             the user_data pointer which is passed to the callback upon invocation of the closure
            </para></listitem>
          <listitem><para>
             a function pointer which represents the destructor of the closure: whenever the
             closure's refcount reaches zero, this function will be called before the closure
             structure is freed.
            </para></listitem>
        </itemizedlist>
      </para>

      <para>
	The <type><link linkend="GClosure">GClosure</link></type> structure represents the common functionality of all
	closure implementations: there exists a different Closure implementation for
	each separate runtime which wants to use the GObject type system.
	<footnote><para>
	    In Practice, Closures sit at the boundary of language runtimes: if you are
	    writing python code and one of your Python callback receives a signal from
	    one of GTK+ widgets, the C code in GTK+ needs to execute your Python
	    code. The Closure invoked by the GTK+ object invokes the Python callback:
	    it behaves as a normal C object for GTK+ and as a normal Python object for
	    python code.
	  </para></footnote>
	The GObject library provides a simple <type><link linkend="GCClosure">GCClosure</link></type> type which
	is a specific implementation of closures to be used with C/C++ callbacks.
      </para>
      <para>
	A <type><link linkend="GClosure">GClosure</link></type> provides simple services:
	<itemizedlist>
	  <listitem><para>
	      Invocation (<function><link linkend="g-closure-invoke">g_closure_invoke</link></function>): this is what closures 
	      were created for: they hide the details of callback invocation from the
	      callback invocator.
	    </para></listitem>
	  <listitem><para>
	      Notification: the closure notifies listeners of certain events such as
	      closure invocation, closure invalidation and closure finalization. Listeners
	      can be registered with <function><link linkend="g-closure-add-finalize-notifier">g_closure_add_finalize_notifier</link></function>
	      (finalization notification), <function><link linkend="g-closure-add-invalidate-notifier">g_closure_add_invalidate_notifier</link></function> 
	      (invalidation notification) and 
	      <function><link linkend="g-closure-add-marshal-guards">g_closure_add_marshal_guards</link></function> (invocation notification).
	      There exist symmetric de-registration functions for finalization and invalidation
	      events (<function><link linkend="g-closure-remove-finalize-notifier">g_closure_remove_finalize_notifier</link></function> and
	      <function><link linkend="g-closure-remove-invalidate-notifier">g_closure_remove_invalidate_notifier</link></function>) but not for the invocation 
	      process.
	      <footnote><para>
		  Closures are refcounted and notify listeners of their destruction in a two-stage
		  process: the invalidation notifiers are invoked before the finalization notifiers.
		</para></footnote>
	    </para></listitem>
	</itemizedlist>
      </para>

      <sect2>
	<title>C Closures</title>

	<para>
	  If you are using C or C++
	  to connect a callback to a given event, you will either use the simple <type><link linkend="GCClosure">GCClosure</link></type>s
	  which have a pretty minimal API or the even simpler <function><link linkend="g-signal-connect">g_signal_connect</link></function> 
	  functions (which will be presented a bit later :).
	  <programlisting>
GClosure* g_cclosure_new (GCallback        callback_func,
                          gpointer         user_data,
                          GClosureNotify   destroy_data);
GClosure* g_cclosure_new_swap (GCallback        callback_func,
                               gpointer         user_data,
                               GClosureNotify   destroy_data);
GClosure* g_signal_type_cclosure_new (GType  itype,
                                      guint  struct_offset);
	  </programlisting>
	</para>

	<para>
	  <function><link linkend="g-cclosure-new">g_cclosure_new</link></function> will create a new closure which can invoke the
	  user-provided callback_func with the user-provided user_data as last parameter. When the closure
	  is finalized (second stage of the destruction process), it will invoke the destroy_data function 
	  if the user has supplied one.
	</para>

	<para>
	  <function><link linkend="g-cclosure-new-swap">g_cclosure_new_swap</link></function> will create a new closure which can invoke the
	  user-provided callback_func with the user-provided user_data as first parameter (instead of being the 
    last parameter as with <function><link linkend="g-cclosure-new">g_cclosure_new</link></function>). When the closure
	  is finalized (second stage of the destruction process), it will invoke the destroy_data 
	  function if the user has supplied one.
	</para>
      </sect2>

      <sect2>
	<title>non-C closures (for the fearless).</title>

	<para>
	  As was explained above, Closures hide the details of callback invocation. In C,
	  callback invocation is just like function invocation: it is a matter of creating
	  the correct stack frame for the called function and executing a <emphasis>call</emphasis>
	  assembly instruction.
	</para>

	<para>
	  C closure marshallers transform the array of GValues which represent 
	  the parameters to the target function into a C-style function parameter list, invoke
	  the user-supplied C function with this new parameter list, get the return value of the
	  function, transform it into a GValue and return this GValue to the marshaller caller.
	</para>

	<para>
	  The following code implements a simple marshaller in C for a C function which takes an
	  integer as first parameter and returns void.
	  <programlisting>
g_cclosure_marshal_VOID__INT (GClosure     *closure,
                              GValue       *return_value,
                              guint         n_param_values,
                              const GValue *param_values,
                              gpointer      invocation_hint,
                              gpointer      marshal_data)
{
  typedef void (*GMarshalFunc_VOID__INT) (gpointer     data1,
                                          gint         arg_1,
                                          gpointer     data2);
  register GMarshalFunc_VOID__INT callback;
  register GCClosure *cc = (GCClosure*) closure;
  register gpointer data1, data2;

  g_return_if_fail (n_param_values == 2);

  data1 = g_value_peek_pointer (param_values + 0);
  data2 = closure->data;

  callback = (GMarshalFunc_VOID__INT) (marshal_data ? marshal_data : cc->callback);

  callback (data1,
            g_marshal_value_peek_int (param_values + 1),
            data2);
}
	  </programlisting>
	</para>

	<para>
	  Of course, there exist other kinds of marshallers. For example, James Henstridge 
	  wrote a generic Python marshaller which is used by all python Closures (a python closure
	  is used to have python-based callback be invoked by the closure invocation process).
	  This python marshaller transforms the input GValue list representing the function 
	  parameters into a Python tupple which is the equivalent structure in python (you can
	  look in <function>pyg_closure_marshal</function> in <filename>pygtype.c</filename>
	  in the <emphasis>pygtk</emphasis> module in Gnome cvs server).
	</para>

      </sect2>
    </sect1>

    <sect1 id="signal">
      <title>Signals</title>

      <para>
	GObject's signals have nothing to do with standard UNIX signals: they connect 
	arbitrary application-specific events with any number of listeners.
	For example, in GTK+, every user event (keystroke or mouse move) is received
	from the X server and generates a GTK+ event under the form of a signal emission
	on a given object instance.
      </para>

      <para>
	Each signal is registered in the type system together with the type on which
	it can be emitted: users of the type are said to <emphasis>connect</emphasis>
	to the signal on a given type instance when they register a closure to be 
        invoked upon the signal emission. Users can also emit the signal by themselves 
        or stop the emission of the signal from within one of the closures connected 
        to the signal.
      </para>

      <para>
        When a signal is emitted on a given type instance, all the closures
        connected to this signal on this type instance will be invoked. All the closures
        connected to such a signal represent callbacks whose signature looks like:
<programlisting>
return_type function_callback (gpointer instance, ... , gpointer user_data);
</programlisting>
      </para>

      <sect2 id="signal-registration">
	<title>Signal registration</title>

	<para>
	  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>,
	  <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:
<programlisting>
guint                 g_signal_newv         (const gchar        *signal_name,
                                             GType               itype,
                                             GSignalFlags        signal_flags,
                                             GClosure           *class_closure,
                                             GSignalAccumulator  accumulator,
                                             gpointer            accu_data,
                                             GSignalCMarshaller  c_marshaller,
                                             GType               return_type,
                                             guint               n_params,
                                             GType              *param_types);
</programlisting>
	The number of parameters to these functions is a bit intimidating but they are relatively
	simple:
	<itemizedlist>
	  <listitem><para>
	      signal_name: is a string which can be used to uniquely identify a given signal.
	    </para></listitem>
	  <listitem><para>
	      itype: is the instance type on which this signal can be emitted.
	    </para></listitem>
	  <listitem><para>
	      signal_flags: partly defines the order in which closures which were connected to the
	      signal are invoked.
	    </para></listitem>
	  <listitem><para>
	      class_closure: this is the default closure for the signal: if it is not NULL upon
	      the signal emission, it will be invoked upon this emission of the signal. The 
	      moment where this closure is invoked compared to other closures connected to that 
	      signal depends partly on the signal_flags.
	    </para></listitem>
	    <listitem><para>
	      accumulator: this is a function pointer which is invoked after each closure
	      has been invoked. If it returns FALSE, signal emission is stopped. If it returns
	      TRUE, signal emission proceeds normally. It is also used to compute the return
	      value of the signal based on the return value of all the invoked closures.
	    </para></listitem>
	  <listitem><para>
	      accumulator_data: this pointer will be passed down to each invocation of the
	      accumulator during emission.
	    </para></listitem>
	  <listitem><para>
	      c_marshaller: this is the default C marshaller for any closure which is connected to
		this signal.
	    </para></listitem>
	  <listitem><para>
	      return_type: this is the type of the return value of the signal.
	    </para></listitem>
	  <listitem><para>
	      n_params: this is the number of parameters this signal takes.
	    </para></listitem>
	  <listitem><para>
	      param_types: this is an array of GTypes which indicate the type of each parameter
	      of the signal. The length of this array is indicated by n_params.
	    </para></listitem>
	</itemizedlist>
      </para>

	<para>
	  As you can see from the above definition, a signal is basically a description
	  of the closures which can be connected to this signal and a description of the
	  order in which the closures connected to this signal will be invoked.
	</para>

      </sect2>

      <sect2 id="signal-connection">
	<title>Signal connection</title>

	<para>
	  If you want to connect to a signal with a closure, you have three possibilities:
	  <itemizedlist>
	    <listitem><para>
		You can register a class closure at signal registration: this is a
		system-wide operation. i.e.: the class_closure will be invoked during each emission
		of a given signal on all the instances of the type which supports that signal.
	      </para></listitem>
	    <listitem><para>
		You can use <function><link linkend="g-signal-override-class-closure">g_signal_override_class_closure</link></function> which
		overrides the class_closure of a given type. It is possible to call this function
		only on a derived type of the type on which the signal was registered.
		This function is of use only to language bindings.
	      </para></listitem>
	    <listitem><para>
		You can register a closure with the <function><link linkend="g-signal-connect">g_signal_connect</link></function>
		family of functions. This is an instance-specific operation: the closure
		will be invoked only during emission of a given signal on a given instance.
	      </para></listitem>
	  </itemizedlist>
	  It is also possible to connect a different kind of callback on a given signal: 
	  emission hooks are invoked whenever a given signal is emitted whatever the instance on 
	  which it is emitted. Emission hooks are used for example to get all mouse_clicked
	  emissions in an application to be able to emit the small mouse click sound.
	  Emission hooks are connected with <function><link linkend="g-signal-add-emission-hook">g_signal_add_emission_hook</link></function>
	  and removed with <function><link linkend="g-signal-remove-emission-hook">g_signal_remove_emission_hook</link></function>.
	</para>

        <para>
        </para>

      </sect2>

      <sect2 id="signal-emission">
	<title>Signal emission</title>

	<para>
	  Signal emission is done through the use of the <function><link linkend="g-signal-emit">g_signal_emit</link></function> family 
	  of functions.
<programlisting>
void                  g_signal_emitv        (const GValue       *instance_and_params,
					     guint               signal_id,
					     GQuark              detail,
					     GValue             *return_value);
</programlisting>
	  <itemizedlist>
	    <listitem><para>
		The instance_and_params array of GValues contains the list of input
		parameters to the signal. The first element of the array is the 
		instance pointer on which to invoke the signal. The following elements of
		the array contain the list of parameters to the signal.
	      </para></listitem>
	    <listitem><para>
		signal_id identifies the signal to invoke.
	      </para></listitem>
	    <listitem><para>
		detail identifies the specific detail of the signal to invoke. A detail is a kind of 
                magic token/argument which is passed around during signal emission and which is used
                by closures connected to the signal to filter out unwanted signal emissions. In most 
                cases, you can safely set this value to zero. See <xref linkend="signal-detail"/> for
                more details about this parameter.
	      </para></listitem>
	    <listitem><para>
		return_value holds the return value of the last closure invoked during emission if
		no accumulator was specified. If an accumulator was specified during signal creation,
		this accumulator is used to calculate the return_value as a function of the return
		values of all the closures invoked during emission. 
		<footnote><para>
		    James (again!!) gives a few non-trivial examples of accumulators:
		    <quote>
		      For instance, you may have an accumulator that ignores NULL returns from 
		      closures, and only accumulates the non-NULL ones. Another accumulator may try
		      to return the list of values returned by the closures.
		    </quote>
		  </para></footnote>
		If no closure is invoked during
		emission, the return_value is nonetheless initialized to zero/null.
	      </para></listitem>
	    </itemizedlist>
	</para>

	<para>
	  Internally, the GValue array is passed to the emission function proper, 
	  <function>signal_emit_unlocked_R</function> (implemented in <filename>gsignal.c</filename>).
	  Signal emission can be decomposed in 5 steps:
	  <itemizedlist>
	    <listitem><para>
		<emphasis>RUN_FIRST</emphasis>: if the G_SIGNAL_RUN_FIRST flag was used
		during signal registration and if there exist a class_closure for this signal,
		the class_closure is invoked. Jump to <emphasis>EMISSION_HOOK</emphasis> state.
	      </para></listitem>
	    <listitem><para>
		<emphasis>EMISSION_HOOK</emphasis>: if any emission hook was added to
		the signal, they are invoked from first to last added. Accumulate return values
                and jump to <emphasis>HANDLER_RUN_FIRST</emphasis> state. 
	      </para></listitem>
	    <listitem><para>
		<emphasis>HANDLER_RUN_FIRST</emphasis>: if any closure were connected
		with the <function><link linkend="g-signal-connect">g_signal_connect</link></function> family of 
		functions, and if they are not blocked (with the <function><link linkend="g-signal-handler-block">g_signal_handler_block</link></function>
		family of functions) they are run here, from first to last connected.
                Jump to <emphasis>RUN_LAST</emphasis> state.
	      </para></listitem>
	    <listitem><para>
		<emphasis>RUN_LAST</emphasis>: if the G_SIGNAL_RUN_LAST
		flag was set during registration and if a class_closure
		was set, it is invoked here. Jump to 
                <emphasis>HANDLER_RUN_LAST</emphasis> state.
	      </para></listitem>
	    <listitem><para>
		<emphasis>HANDLER_RUN_LAST</emphasis>: if any closure were connected
		with the <function>g_signal_connect_after</function> family of 
		functions, if they were not invoked during HANDLER_RUN_FIRST and if they 
		are not blocked, they are run here, from first to last connected.
                Jump to  <emphasis>RUN_CLEANUP</emphasis> state.
	      </para></listitem>
	    <listitem><para>
		<emphasis>RUN_CLEANUP</emphasis>: if the G_SIGNAL_RUN_CLEANUP flag
		was set during registration and if a class_closure was set,
		it is invoked here. Signal emission is completed here.
	      </para></listitem>
	  </itemizedlist>	  
	</para>

	<para>
	  If, at any point during emission (except in RUN_CLEANUP state), one of the 
	  closures or emission hook stops the signal emission with 
	  <function><link linkend="g-signal-stop">g_signal_stop</link></function>, emission jumps to CLEANUP state.
	</para>

	<para>
	  If, at any point during emission, one of the closures or emission hook 
	  emits the same signal on the same instance, emission is restarted from
	  the RUN_FIRST state.
	</para>

	<para>
	  The accumulator function is invoked in all states, after invocation
	  of each closure (except in EMISSION_HOOK and CLEANUP). It accumulates
	  the closure return value into the signal return value and returns TRUE or
	  FALSE. If, at any point, it does not return TRUE, emission jumps to CLEANUP state.
	</para>

        <para>
          If no accumulator function was provided, the value returned by the last handler
          run will be returned by <function><link linkend="g-signal-emit">g_signal_emit</link></function>.
        </para>

      </sect2>


      <sect2 id="signal-detail">
        <title>The <emphasis>detail</emphasis> argument</title>

        <para>All the functions related to signal emission or signal connection have a parameter
          named the <emphasis>detail</emphasis>. Sometimes, this parameter is hidden by the API
          but it is always there, under one form or another. 
        </para>

        <para>
         Of the three main connection functions,
          only one has an explicit detail parameter as a <type><link linkend="GQuark">GQuark</link></type>
          <footnote>
            <para>A GQuark is an integer which uniquely represents a string. It is possible to transform
             back and forth between the integer and string representations with the functions 
             <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>.
            </para>
          </footnote>:
<programlisting>
gulong	 g_signal_connect_closure_by_id	      (gpointer		  instance,
					       guint		  signal_id,
					       GQuark		  detail,
					       GClosure		 *closure,
					       gboolean		  after);
</programlisting>
         The two other functions hide the detail parameter in the signal name identification:
<programlisting>
gulong	 g_signal_connect_closure	      (gpointer		  instance,
					       const gchar       *detailed_signal,
					       GClosure		 *closure,
					       gboolean		  after);
gulong	 g_signal_connect_data		      (gpointer		  instance,
					       const gchar	 *detailed_signal,
					       GCallback	  c_handler,
					       gpointer		  data,
					       GClosureNotify	  destroy_data,
					       GConnectFlags	  connect_flags);
</programlisting>
          Their detailed_signal parameter is a string which identifies the name of the signal
          to connect to. However, the format of this string is structured to look like 
          <emphasis>signal_name::detail_name</emphasis>. Connecting to the signal
          named <emphasis>notify::cursor_position</emphasis> will actually connect to the signal
          named <emphasis>notify</emphasis> with the <emphasis>cursor_position</emphasis> name.
          Internally, the detail string is transformed to a GQuark if it is present.
        </para>

        <para>
          Of the four main signal emission functions, three have an explicit detail parameter as a 
          <type><link linkend="GQuark">GQuark</link></type> again:
<programlisting>
void                  g_signal_emitv        (const GValue       *instance_and_params,
					     guint               signal_id,
					     GQuark              detail,
					     GValue             *return_value);
void                  g_signal_emit_valist  (gpointer            instance,
					     guint               signal_id,
					     GQuark              detail,
					     va_list             var_args);
void                  g_signal_emit         (gpointer            instance,
					     guint               signal_id,
					     GQuark              detail,
					     ...);
</programlisting>
         The fourth function hides it in its signal name parameter:
<programlisting>
void                  g_signal_emit_by_name (gpointer            instance,
					     const gchar        *detailed_signal,
					     ...);
</programlisting>
         The format of the detailed_signal parameter is exactly the same as the format used by
         the <function><link linkend="g-signal-connect">g_signal_connect</link></function> functions: <emphasis>signal_name::detail_name</emphasis>.
        </para>

        <para>
         If a detail is provided by the user to the emission function, it is used during emission to match
         against the closures which also provide a detail. The closures which provided a detail will not
         be invoked (even though they are connected to a signal which is being emitted) if their detail
         does not match the detail provided by the user.
        </para>

        <para>This completely optional filtering mechanism is mainly used as an optimization for signals
         which are often emitted for many different reasons: the clients can filter out which events they are
         interested into before the closure's marshalling code runs. For example, this is used extensively
         by the <emphasis>notify</emphasis> signal of GObject: whenever a property is modified on a GObject,
         instead of just emitting the <emphasis>notify</emphasis> signal, GObject associates as a detail to this
         signal emission the name of the property modified. This allows clients who wish to be notified of changes
         to only one property to filter most events before receiving them.
        </para>

        <para>As a simple rule, users can and should set the detail parameter to zero: this will disable completely
         this optional filtering.
        </para>

      </sect2>

    </sect1>
  </chapter>