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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>
<p>
Starting from version 2.4 of GLib, 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>.
</p>
<p>
To include a property named 'name' of type <span class="type">string</span> in the
<span class="type">maman_ibaz</span> interface example code above, we only need to
add one
<sup>[<a name="idp9381632" href="#ftn.idp9381632" class="footnote">12</a>]</sup>
line in the <code class="function">maman_ibaz_base_init</code>
<sup>[<a name="idp9382480" href="#ftn.idp9382480" class="footnote">13</a>]</sup>
as shown below:
</p>
<pre class="programlisting">
static void
maman_ibaz_base_init (gpointer g_iface)
{
static gboolean is_initialized = FALSE;
if (!is_initialized)
{
g_object_interface_install_property (g_iface,
g_param_spec_string ("name",
"Name",
"Name of the MamanIbaz",
"maman",
G_PARAM_READWRITE));
is_initialized = TRUE;
}
}
</pre>
<p>
</p>
<p>
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 get and set methods and since interfaces do not
implement properties, there is no need to assign integer IDs to
interface properties.
</p>
<p>
An implementation shall declare and define it's 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 must 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
<span class="type">MamanBaz</span> declaration and implementation above:
</p>
<pre class="programlisting">
struct _MamanBaz
{
GObject parent_instance;
gint instance_member;
gchar *name;
};
enum
{
PROP_0,
PROP_NAME
};
static void
maman_baz_set_property (GObject *object,
guint property_id,
const GValue *value,
GParamSpec *pspec)
{
MamanBaz *baz = MAMAN_BAZ (object);
GObject *obj;
switch (prop_id)
{
case ARG_NAME:
g_free (baz->name);
baz->name = g_value_dup_string (value);
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
break;
}
}
static void
maman_baz_get_property (GObject *object,
guint prop_id,
GValue *value,
GParamSpec *pspec)
{
MamanBaz *baz = MAMAN_BAZ (object);
switch (prop_id)
{
case ARG_NAME:
g_value_set_string (value, baz->name);
break;
default:
G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
break;
}
}
static void
maman_baz_class_init (MamanBazClass *klass)
{
GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
gobject_class->set_property = maman_baz_set_property;
gobject_class->get_property = maman_baz_get_property;
g_object_class_override_property (gobject_class, PROP_NAME, "name");
}
</pre>
<p>
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.idp9381632" href="#idp9381632" class="para">12</a>] </sup>
That really is one line extended to six for the sake of clarity
</p></div>
<div class="footnote"><p><sup>[<a id="ftn.idp9382480" href="#idp9382480" class="para">13</a>] </sup>
The <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>
can also be called from <code class="function">class_init</code> but it must
not be called after that point.
</p></div>
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