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<title>Test for modifier keys correctly: GTK+ 2 Reference Manual</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="checklist-modifiers"></a>Test for modifier keys correctly</h2></div></div></div>
<p><b>Why. </b>
With <a class="link" href="gtk2-Keyboard-Accelerators.html#gtk-accelerator-get-default-mod-mask" title="gtk_accelerator_get_default_mod_mask ()"><code class="function">gtk_accelerator_get_default_mod_mask()</code></a> you can test for
modifier keys reliably; this way your key event handlers will
work correctly even if <span class="keycap"><strong>NumLock</strong></span> or
<span class="keycap"><strong>CapsLock</strong></span> are activated.
</p>
<p>
In a <span class="structname">GdkEventKey</span>, the
<em class="structfield"><code>state</code></em> field is a bit mask which
indicates the modifier state at the time the key was pressed.
Modifiers are keys like <span class="keycap"><strong>Control</strong></span> and
<span class="keycap"><strong>NumLock</strong></span>. When implementing a
<a class="link" href="GtkWidget.html#GtkWidget-key-press-event" title="The “key-press-event” signal"><span class="type">“key-press-event”</span></a> handler, you should use
<a class="link" href="gtk2-Keyboard-Accelerators.html#gtk-accelerator-get-default-mod-mask" title="gtk_accelerator_get_default_mod_mask ()"><code class="function">gtk_accelerator_get_default_mod_mask()</code></a> to
test against modifier keys. This function returns a bit mask
which encompasses all the modifiers which the user may be
actively pressing, such as <span class="keycap"><strong>Control</strong></span>,
<span class="keycap"><strong>Shift</strong></span>, and <span class="keycap"><strong>Alt</strong></span>, but ignores
"innocuous" modifiers such as <span class="keycap"><strong>NumLock</strong></span> and
<span class="keycap"><strong>CapsLock</strong></span>.
</p>
<p>
Say you want to see if
<span class="keycap"><strong>Control</strong></span>+<span class="keycap"><strong>F10</strong></span>
was pressed. Doing a simple test like
<code class="literal">event->keysym == GDK_F10 &&
event->state == GDK_CONTROL_MASK</code> is not
enough. If <span class="keycap"><strong>CapsLock</strong></span> is pressed, then
<em class="structfield"><code>event->state</code></em> will be equal to
<code class="literal">GDK_CONTROL_MASK | GDK_LOCK_MASK</code>, and the
simple test will fail. By taking the logical-and of
<em class="structfield"><code>event->state</code></em> and
<a class="link" href="gtk2-Keyboard-Accelerators.html#gtk-accelerator-get-default-mod-mask" title="gtk_accelerator_get_default_mod_mask ()"><code class="function">gtk_accelerator_get_default_mod_mask()</code></a>, you
can ignore the modifiers which are not actively pressed by the
user at the same time as the base key.
</p>
<p>
The following example correctly tests for
<span class="keycap"><strong>Control</strong></span>+<span class="keycap"><strong>F10</strong></span>
being pressed.
</p>
<a name="default-mod-mask"></a><pre class="programlisting">
static gboolean
my_widget_key_press_event_handler (GtkWidget *widget, GdkEventKey *event)
{
guint modifiers;
modifiers = gtk_accelerator_get_default_mod_mask ();
if (event->keysym == GDK_F10
&& (event->state & modifiers) == GDK_CONTROL_MASK)
{
g_print ("Control-F10 was pressed\n");
return TRUE;
}
return FALSE;
}
</pre>
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