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<div class="sect1" title="X Event signals">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-xeventsignals"></a>X Event signals</h2></div></div></div>
<p>
The <code class="classname">Widget</code> class has some special signals which
correspond to the underlying X-Windows events. These are suffixed by
<code class="literal">_event</code>; for instance,
<code class="methodname">Widget::signal_button_pressed_event()</code>.
</p>
<p>
You might occasionally find it useful to handle X events when there's something
you can't accomplish with normal signals. <code class="classname">Gtk::Button</code>,
for example, does not send mouse-pointer coordinates with its
<code class="literal">clicked</code> signal, but you could handle
<code class="literal">button_pressed_event</code> if you needed this
information. X events are also often used to handle key-presses.
</p>
<p>
These signals behave slightly differently. The value returned from the signal handler indicates whether it has fully "handled"
the event. If the value is <code class="literal">false</code> then <span class="application">gtkmm</span> will pass the event on to the next signal handler. If the value is <code class="literal">true</code> then no other signal handlers will need to be called.
</p>
<p>
Handling an X event doesn't affect the Widget's other signals. If you handle
<code class="literal">button_pressed_event</code> for
<code class="classname">Gtk::Button</code>, you'll still be able to get the
<code class="literal">clicked</code> signal. They are emitted at (nearly) the same time.
</p>
<p>Note also that not all widgets recieve all X events by default. To receive additional
X events, you can use <code class="methodname">Gtk::Widget::set_events()</code> before showing the
widget, or <code class="methodname">Gtk::Widget::add_events()</code> after showing the widget. However,
some widgets must first be placed inside an <code class="classname">EventBox</code> widget. See
the <a class="link" href="chapter-widgets-without-xwindows.html" title="Chapter 13. Widgets Without X-Windows">Widgets Without X-Windows</a> chapter.
</p>
<p>
Here's a simple example:
</p>
<pre class="programlisting">
bool on_button_press(GdkEventButton* event);
Gtk::Button button("label");
button.signal_button_press_event().connect( sigc::ptr_fun(&on_button_press) );
</pre>
<p>
</p>
<p>
When the mouse is over the button and a mouse button is pressed,
<code class="methodname">on_button_press()</code> will be called.
</p>
<p>
<span class="type">GdkEventButton</span> is a structure containing the event's parameters,
such as the coordinates of the mouse pointer at the time the button was
pressed. There are several different types of <span class="type">GdkEvent</span> structures
for the various events.
</p>
<div class="sect2" title="Signal Handler sequence">
<div class="titlepage"><div><div><h3 class="title">
<a name="signal-handler-sequence"></a>Signal Handler sequence</h3></div></div></div>
<p>By default, your signal handlers are called after any previously-connected signal handlers. However, this can be a problem with the X Event signals. For instance, the existing signal handlers, or the default signal handler, might return true to stop other signal handlers from being called. To specify that your signal handler should be called before the other signal handlers, so that will always be called, you can specify <code class="literal">false</code> for the optional <code class="literal">after</code> parameter. For instance,
</p>
<pre class="programlisting">
button.signal_button_press_event().connect( sigc::ptr_fun(&on_mywindow_button_press), false );
</pre>
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