Prerequisites and properties for the GObject interface definition

1. Prerequisites for GObject Interface definition

If you want to specify that an interface requires the presence of other interfaces when implemented, GObject introduces the concept of prerequisites: You can associate a series of prerequisite types with an interface. For example, if object A wants to implement an interface I1, and if an interface I1 has a prerequisite that an interface I2 is required, a must implement I1 and I2.

The above mechanism is very similar in practice to the Java interface I1 extension interface I2:

/* Make the Viewereditablelossy interface require viewereditable interface. *
/G_define_interface (Viewereditablelossy, Viewer_editable_lossy, viewer_type_editable);

In the above G_define_interface call, the third parameter defines the precondition type. This is the gtype of the interface or class. In this case, the Viewereditable interface is a precondition for viewereditablelossy. The following code shows how the implementation implements the two interfaces and registers their implementations:

static void Viewer_file_editable_lossy_compress (Viewereditablelossy *editable) {viewerfile *self = Viewer_file (editab

  Le);
G_print ("File implementation of lossy editable interface compress method:%s.\n", self->filename); } static void Viewer_file_editable_lossy_interface_init (Viewereditablelossyinterface *iface) {iface->compress = VI
ewer_file_editable_lossy_compress;
  } static void Viewer_file_editable_save (Viewereditable *editable, Gerror **error) {

  Viewerfile *self = viewer_file (editable);
G_print ("File implementation of Editable interface Save method:%s.\n", self->filename);
  } static void Viewer_file_editable_undo (Viewereditable *editable, Guint n_steps) {

  Viewerfile *self = viewer_file (editable);
G_print ("File implementation of Editable interface Undo method:%s.\n", self->filename); } static void Viewer_file_editable_redo (ViewerediTable *editable, Guint n_steps) {viewerfile *self = viewer_file (editable);
G_print ("File implementation of Editable interface Redo method:%s.\n", self->filename); } static void Viewer_file_editable_interface_init (Viewereditableinterface *iface) {iface->save = Viewer_file_edita
  Ble_save;
  Iface->undo = Viewer_file_editable_undo;
Iface->redo = Viewer_file_editable_redo; } static void Viewer_file_class_init (Viewerfileclass *klass) {/* nothing here. */} static void Viewer_file_init (Vi Ewerfile *self) {/* Instance variable initialisation code. */} G_define_type_with_code (Viewerfile, Viewer_file, g_ty
                                                Pe_object, G_implement_interface (viewer_type_editable,
                                                Viewer_file_editable_interface_init) G_implement_interface (Viewer_type_editable_lossy, Viewer_file_editablE_lossy_interface_init)) 

It is important to note that the order in which the interface implementations are added to the main object is not random: the g_type_add_interface_static that calls G_implement_interface must first call an interface that has no other interface as a prerequisite.