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  22 <div class="chapter" title="How to create and use signals">
  23 <div class="titlepage"><div><div><h2 class="title">
  24 <a name="howto-signals"></a>How to create and use signals</h2></div></div></div>
  25 <div class="toc"><dl><dt><span class="sect1"><a href="howto-signals.html#howto-simple-signals">Simple use of signals</a></span></dt></dl></div>
  26 <p>
  27     The signal system which was built in GType is pretty complex and
  28     flexible: it is possible for its users to connect at runtime any
  29     number of callbacks (implemented in any language for which a binding
  30     exists)
  31     <sup>[<a name="id740741" href="#ftn.id740741" class="footnote">14</a>]</sup>
  32     to any signal and to stop the emission of any signal at any
  33     state of the signal emission process. This flexibility makes it
  34     possible to use GSignal for much more than just emit signals which
  35     can be received by numerous clients.
  36   </p>
  37 <div class="sect1" title="Simple use of signals">
  38 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  39 <a name="howto-simple-signals"></a>Simple use of signals</h2></div></div></div>
  40 <p>
  41       The most basic use of signals is to implement simple event
  42       notification: for example, if we have a MamanFile object, and
  43       if this object has a write method, we might wish to be notified
  44       whenever someone has changed something via our MamanFile instance.
  45       The code below shows how the user can connect a callback to the
  46       "changed" signal.
  47 </p>
  48 <pre class="programlisting">
  49 file = g_object_new (MAMAN_FILE_TYPE, NULL);
  50
  51 g_signal_connect (file, "changed", G_CALLBACK (changed_event), NULL);
  52
  53 maman_file_write (file, buffer, strlen (buffer));
  54 </pre>
  55 <p>
  56     </p>
  57 <p>
  58       The <span class="type">MamanFile</span> signal is registered in the class_init
  59       function:
  60 </p>
  61 <pre class="programlisting">
  62 file_signals[CHANGED] =
  63   g_signal_newv ("changed",
  64                  G_TYPE_FROM_CLASS (gobject_class),
  65                  G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
  66                  NULL /* closure */,
  67                  NULL /* accumulator */,
  68                  NULL /* accumulator data */,
  69                  g_cclosure_marshal_VOID__VOID,
  70                  G_TYPE_NONE /* return_type */,
  71                  0     /* n_params */,
  72                  NULL  /* param_types */);
  73 </pre>
  74 <p>
  75       and the signal is emitted in <code class="function">maman_file_write</code>:
  76 </p>
  77 <pre class="programlisting">
  78 void
  79 maman_file_write (MamanFile    *self,
  80                   const guchar *buffer,
  81                   gssize        size)
  82 {
  83   /* First write data. */
  84
  85   /* Then, notify user of data written. */
  86   g_signal_emit (self, file_signals[CHANGED], 0 /* details */);
  87 }
  88 </pre>
  89 <p>
  90       As shown above, you can safely set the details parameter to zero if
  91       you do not know what it can be used for. For a discussion of what you
  92       could used it for, see <a class="xref" href="signal.html#signal-detail" title="The detail argument">the section called “The <span class="emphasis"><em>detail</em></span> argument”</a>
  93     </p>
  94 <p>
  95       The signature of the signal handler in the above example is defined as
  96       <code class="function">g_cclosure_marshal_VOID__VOID</code>. Its name follows
  97       a simple convention which encodes the function parameter and return value
  98       types in the function name. Specifically, the value in front of the
  99       double underscore is the type of the return value, while the value(s)
 100       after the double underscore denote the parameter types.
 101     </p>
 102 <p>
 103       The header <code class="filename">gobject/gmarshal.h</code> defines a set of
 104       commonly needed closures that one can use. If you want to have complex
 105       marshallers for your signals you should probably use glib-genmarshal
 106       to autogenerate them from a file containing their return and
 107       parameter types.
 108     </p>
 109 </div>
 110 <div class="footnotes">
 111 <br><hr width="100" align="left">
 112 <div class="footnote"><p><sup>[<a name="ftn.id740741" href="#id740741" class="para">14</a>] </sup>A Python callback can be connected to any signal on any
 113       C-based GObject.
 114       </p></div>
 115 </div>
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