<?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 callbacks receives a signal from
a GTK+ widget, 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 invoker.</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 deregistration 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 reference counted 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 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 tuple which is the equivalent structure in Python (you can
look in <function>pyg_closure_marshal</function> in <filename>pygtype.c</filename>
in the <emphasis>pygobject</emphasis> module in the GNOME Subversion 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>
</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.
If the closures' detail does not match the detail provided by the user, they will not be invoked
(even though they are connected to a signal which is being emitted).
</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 in 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>