How to define and implement interfaces: GObject Reference Manual

How to define and implement interfaces

Defining interfaces
Implementing interfaces
Interface definition prerequisites
Interface properties
Overriding interface methods

Defining interfaces

The theory behind how GObject interfaces work is given in the section called “Non-instantiable classed types: interfaces”; this section covers how to define and implement an interface.

The first step is to get the header right. This interface defines three methods:

/*
 * Copyright/Licensing information.
 */

#ifndef __VIEWER_EDITABLE_H__
#define __VIEWER_EDITABLE_H__

#include <glib-object.h>

G_BEGIN_DECLS

#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)

struct _ViewerEditableInterface
{
  GTypeInterface parent_iface;

  void (*save) (ViewerEditable  *self,
                GError         **error);
  void (*undo) (ViewerEditable  *self,
                guint            n_steps);
  void (*redo) (ViewerEditable  *self,
                guint            n_steps);
};

void viewer_editable_save (ViewerEditable  *self,
                           GError         **error);
void viewer_editable_undo (ViewerEditable  *self,
                           guint            n_steps);
void viewer_editable_redo (ViewerEditable  *self,
                           guint            n_steps);

G_END_DECLS

#endif /* __VIEWER_EDITABLE_H__ */

This code is the same as the code for a normal GType which derives from a GObject except for a few details:

The _GET_CLASS function is called _GET_IFACE (and is defined by G_DECLARE_INTERFACE).
The instance type, ViewerEditable, is not fully defined: it is used merely as an abstract type which represents an instance of whatever object which implements the interface.
The parent of the ViewerEditableInterface is GTypeInterface, not GObjectClass.

The implementation of the ViewerEditable type itself is trivial:

G_DEFINE_INTERFACE creates a viewer_editable_get_type function which registers the type in the type system. The third argument is used to define a prerequisite interface (which we'll talk about more later). Just pass 0 for this argument when an interface has no prerequisite.
viewer_editable_default_init is expected to register the interface's signals if there are any (we will see a bit later how to use them).
The interface methods viewer_editable_save, viewer_editable_undo and viewer_editable_redo dereference the interface structure to access its associated interface function and call it.

G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
    /* add properties and signals to the interface here */
}

void
viewer_editable_save (ViewerEditable  *self,
                      GError         **error)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->save != NULL);
  iface->save (self, error);
}

void
viewer_editable_undo (ViewerEditable *self,
                      guint           n_steps)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->undo != NULL);
  iface->undo (self, n_steps);
}

void
viewer_editable_redo (ViewerEditable *self,
                      guint           n_steps)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->redo != NULL);
  iface->redo (self, n_steps);
}