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| <div class="chapter" title="The GObject messaging system"> |
| <div class="titlepage"><div><div><h2 class="title"> |
| <a name="chapter-signal"></a>The GObject messaging system</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="sect1"><a href="chapter-signal.html#closure">Closures</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="chapter-signal.html#id563646">C Closures</a></span></dt> |
| <dt><span class="sect2"><a href="chapter-signal.html#id555455">Non-C closures (for the fearless)</a></span></dt> |
| </dl></dd> |
| <dt><span class="sect1"><a href="signal.html">Signals</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="signal.html#signal-registration">Signal registration</a></span></dt> |
| <dt><span class="sect2"><a href="signal.html#signal-connection">Signal connection</a></span></dt> |
| <dt><span class="sect2"><a href="signal.html#signal-emission">Signal emission</a></span></dt> |
| <dt><span class="sect2"><a href="signal.html#signal-detail">The <span class="emphasis"><em>detail</em></span> argument</a></span></dt> |
| </dl></dd> |
| </dl></div> |
| <div class="sect1" title="Closures"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="closure"></a>Closures</h2></div></div></div> |
| <p> |
| 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: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <p>a function pointer (the callback itself) whose prototype looks like: |
| </p> |
| <pre class="programlisting"> |
| return_type function_callback (... , gpointer user_data); |
| </pre> |
| <p> |
| </p> |
| </li> |
| <li class="listitem"><p> |
| the user_data pointer which is passed to the callback upon invocation of the closure |
| </p></li> |
| <li class="listitem"><p> |
| 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. |
| </p></li> |
| </ul></div> |
| <p> |
| </p> |
| <p> |
| The <span class="type"><a class="link" href="gobject-Closures.html#GClosure" title="GClosure">GClosure</a></span> 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. |
| <sup>[<a name="id573957" href="#ftn.id573957" class="footnote">6</a>]</sup> |
| The GObject library provides a simple <span class="type"><a class="link" href="gobject-Closures.html#GCClosure" title="GCClosure">GCClosure</a></span> type which |
| is a specific implementation of closures to be used with C/C++ callbacks. |
| </p> |
| <p> |
| A <span class="type"><a class="link" href="gobject-Closures.html#GClosure" title="GClosure">GClosure</a></span> provides simple services: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p> |
| Invocation (<code class="function"><a class="link" href="gobject-Closures.html#g-closure-invoke" title="g_closure_invoke ()">g_closure_invoke</a></code>): this is what closures |
| were created for: they hide the details of callback invocation from the |
| callback invoker.</p></li> |
| <li class="listitem"><p> |
| Notification: the closure notifies listeners of certain events such as |
| closure invocation, closure invalidation and closure finalization. Listeners |
| can be registered with <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-finalize-notifier" title="g_closure_add_finalize_notifier ()">g_closure_add_finalize_notifier</a></code> |
| (finalization notification), <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-invalidate-notifier" title="g_closure_add_invalidate_notifier ()">g_closure_add_invalidate_notifier</a></code> |
| (invalidation notification) and |
| <code class="function"><a class="link" href="gobject-Closures.html#g-closure-add-marshal-guards" title="g_closure_add_marshal_guards ()">g_closure_add_marshal_guards</a></code> (invocation notification). |
| There exist symmetric deregistration functions for finalization and invalidation |
| events (<code class="function"><a class="link" href="gobject-Closures.html#g-closure-remove-finalize-notifier" title="g_closure_remove_finalize_notifier ()">g_closure_remove_finalize_notifier</a></code> and |
| <code class="function"><a class="link" href="gobject-Closures.html#g-closure-remove-invalidate-notifier" title="g_closure_remove_invalidate_notifier ()">g_closure_remove_invalidate_notifier</a></code>) but not for the invocation |
| process. |
| <sup>[<a name="id563636" href="#ftn.id563636" class="footnote">7</a>]</sup></p></li> |
| </ul></div> |
| <p> |
| </p> |
| <div class="sect2" title="C Closures"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id563646"></a>C Closures</h3></div></div></div> |
| <p> |
| If you are using C or C++ |
| to connect a callback to a given event, you will either use simple <span class="type"><a class="link" href="gobject-Closures.html#GCClosure" title="GCClosure">GCClosure</a></span>s |
| which have a pretty minimal API or the even simpler <code class="function"><a class="link" href="gobject-Signals.html#g-signal-connect" title="g_signal_connect()">g_signal_connect</a></code> |
| functions (which will be presented a bit later :). |
| </p> |
| <pre class="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); |
| </pre> |
| <p> |
| </p> |
| <p> |
| <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new" title="g_cclosure_new ()">g_cclosure_new</a></code> 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. |
| </p> |
| <p> |
| <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new-swap" title="g_cclosure_new_swap ()">g_cclosure_new_swap</a></code> 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 <code class="function"><a class="link" href="gobject-Closures.html#g-cclosure-new" title="g_cclosure_new ()">g_cclosure_new</a></code>). When the closure |
| is finalized (second stage of the destruction process), it will invoke the destroy_data |
| function if the user has supplied one. |
| </p> |
| </div> |
| <div class="sect2" title="Non-C closures (for the fearless)"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id555455"></a>Non-C closures (for the fearless)</h3></div></div></div> |
| <p> |
| 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 <span class="emphasis"><em>call</em></span> |
| assembly instruction. |
| </p> |
| <p> |
| 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. |
| </p> |
| <p> |
| The following code implements a simple marshaller in C for a C function which takes an |
| integer as first parameter and returns void. |
| </p> |
| <pre class="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); |
| } |
| </pre> |
| <p> |
| </p> |
| <p> |
| 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 <code class="function">pyg_closure_marshal</code> in <code class="filename">pygtype.c</code> |
| in the <span class="emphasis"><em>pygobject</em></span> module in the GNOME source code repository). |
| </p> |
| </div> |
| </div> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a name="ftn.id573957" href="#id573957" class="para">6</a>] </sup> |
| 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. |
| </p></div> |
| <div class="footnote"><p><sup>[<a name="ftn.id563636" href="#id563636" class="para">7</a>] </sup> |
| Closures are reference counted and notify listeners of their destruction in a two-stage |
| process: the invalidation notifiers are invoked before the finalization notifiers. |
| </p></div> |
| </div> |
| </div> |
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