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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-properties"></a>Interface properties</h2></div></div></div>

 GObject interfaces can also have
 properties. Declaration of the interface properties is similar to
 declaring the properties of ordinary GObject types as explained in
 <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, except that
 <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-interface-install-property" title="g_object_interface_install_property ()">g_object_interface_install_property</a></code>
 is used to declare the properties instead of
 <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
 

 To include a property named 'autosave-frequency' of type gdouble in the 
 ViewerEditable interface example code above, we only need to
 add one call in <code class="function">viewer_editable_default_init</code> as shown
 below:

<div class="informalexample">
 <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
 <tbody>
 <tr>
 <td class="listing_lines" align="right"><pre>1
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 <td class="listing_code"><pre class="programlisting">static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
 g_object_interface_install_property (iface,
 g_param_spec_double ("autosave-frequency",
 "Autosave frequency",
 "Frequency (in per-seconds) to autosave backups of the editable content at. "
 "Or zero to disable autosaves.",
 0.0, /* minimum */
 G_MAXDOUBLE, /* maximum */
 0.0, /* default */
 G_PARAM_READWRITE));
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

One point worth noting is that the declared property wasn't assigned an 
 integer ID. The reason being that integer IDs of properties are used
 only inside the <code class="function">get_property</code> and
 <code class="function">set_property</code> virtual methods. Since interfaces
 declare but do not implement properties, there is no
 need to assign integer IDs to them.
 

 An implementation declares and defines its properties in the usual
 way as explained in <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, except for one
 small change: it can declare the properties of the interface it
 implements using <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-override-property" title="g_object_class_override_property ()">g_object_class_override_property</a></code>
 instead of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
 The following code snippet shows the modifications needed in the
 ViewerFile declaration and implementation above:

<div class="informalexample">
 <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
 <tbody>
 <tr>
 <td class="listing_lines" align="right"><pre>1
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 <td class="listing_code"><pre class="programlisting">struct _ViewerFile
{
 GObject parent_instance;

gdouble autosave_frequency;
};

enum
{
 PROP_AUTOSAVE_FREQUENCY = 1,
 N_PROPERTIES
};

static void
viewer_file_set_property (GObject *object,
 guint prop_id,
 const GValue *value,
 GParamSpec *pspec)
{
 ViewerFile *file = VIEWER_FILE (object);

switch (prop_id)
 {
 case PROP_AUTOSAVE_FREQUENCY:
 file->autosave_frequency = g_value_get_double (value);
 break;

default:
 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
 break;
 }
}

static void
viewer_file_get_property (GObject *object,
 guint prop_id,
 GValue *value,
 GParamSpec *pspec)
{
 ViewerFile *file = VIEWER_FILE (object);

switch (prop_id)
 {
 case PROP_AUTOSAVE_FREQUENCY:
 g_value_set_double (value, file->autosave_frequency);
 break;

default:
 G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
 break;
 }
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
 GObjectClass *object_class = G_OBJECT_CLASS (klass);

object_class->set_property = viewer_file_set_property;
 object_class->get_property = viewer_file_get_property;

g_object_class_override_property (object_class, PROP_AUTOSAVE_FREQUENCY, "autosave-frequency");
}</pre></td>
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 </tbody>
 </table>
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