## Interface Properties

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 [14] line in the `maman_ibaz_base_init` [15] 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 properties 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 snippet 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;
      }
    }

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

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