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 the section called “Object properties”, except that g_object_interface_install_property is used to declare the properties instead of g_object_class_install_property.

To include a property named 'name' of type string in the maman_ibaz interface example code above, we only need to add one [13] line in the maman_ibaz_base_init [14] as shown below:

static void
maman_ibaz_base_init (gpointer g_iface)
{
  static gboolean initialized = FALSE;

  if (!initialized) {
    /* create interface signals here. */

    g_object_interface_install_property (g_iface,
    			g_param_spec_string ("name",
			    	"maman_ibaz_name",
			    	"Name of the MamanIbaz",
			    	"maman",
			    	G_PARAM_READWRITE));
    initialized = TRUE;
  }
}

One point worth noting is that the declared property wasn't assigned an integer ID. The reason being that integer IDs of properities are utilized 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.

The story for the implementers of the interface is also quite trivial. An implementer shall declare and define it's properties in the usual way as explained in the section called “Object properties”, except for one small change: it shall declare the properties of the interface it implements using g_object_class_override_property instead of g_object_class_install_property. The following code snipet shows the modifications needed in the MamanBaz declaration and implementation above:


struct _MamanBaz {
  GObject parent;
  gint instance_member;
  gchar *name;		/* placeholder for property */
};

enum
{
  ARG_0,
  ARG_NAME
};

GType 
maman_baz_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBazClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      baz_class_init, /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBaz),
      0,      /* n_preallocs */
      baz_instance_init    /* instance_init */
    };
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
      NULL,               /* interface_finalize */
      NULL                /* interface_data */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBazType",
                                   &info, 0);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAZ,
                                 &ibaz_info);
  }
  return type;
}

static void
maman_baz_class_init (MamanBazClass * klass)
{
  GObjectClass *gobject_class;

  gobject_class = (GObjectClass *) klass;

  parent_class = g_type_class_ref (G_TYPE_OBJECT);

  gobject_class->set_property = maman_baz_set_property;
  gobject_class->get_property = maman_baz_get_property;

  g_object_class_override_property (gobject_class, ARG_NAME, "name");
}

static void
maman_baz_set_property (GObject * object, guint prop_id,
                        const GValue * value, GParamSpec * pspec)
{
  MamanBaz *baz;
  GObject *obj;

  /* it's not null if we got it, but it might not be ours */
  g_return_if_fail (G_IS_MAMAN_BAZ (object));

  baz = MAMAN_BAZ (object);

  switch (prop_id) {
    case ARG_NAME:
      baz->name = g_value_get_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;

  /* it's not null if we got it, but it might not be ours */
  g_return_if_fail (G_IS_TEXT_PLUGIN (object));

  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;
  }
}



[13] That really is one line extended to six for the sake of clarity

[14] The g_object_interface_install_property can also be called from class_init but it must not be called after that point.