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<title>How to define and implement interfaces: GObject Reference Manual</title>
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<link rel="up" href="pt02.html" title="Part IV. Tutorial">
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<link rel="prev" href="howto-gobject-chainup.html" title="Chaining up">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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How to define and implement interfaces
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Defining interfaces
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Implementing interfaces
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Interface definition prerequisites
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Interface properties
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Overriding interface methods
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Defining interfaces
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The theory behind how GObject interfaces work is given in
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the section called “Non-instantiable classed types: interfaces”; this section covers how to
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define and implement an interface.
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The first step is to get the header right. This interface
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defines three methods:
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#ifndef __VIEWER_EDITABLE_H__
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#define __VIEWER_EDITABLE_H__
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#include <glib-object.h>
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G_BEGIN_DECLS
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#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
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G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)
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struct _ViewerEditableInterface
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{
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GTypeInterface parent_iface;
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void (*save) (ViewerEditable *self,
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GError **error);
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void (*undo) (ViewerEditable *self,
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guint n_steps);
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void (*redo) (ViewerEditable *self,
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guint n_steps);
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};
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void viewer_editable_save (ViewerEditable *self,
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GError **error);
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void viewer_editable_undo (ViewerEditable *self,
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guint n_steps);
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void viewer_editable_redo (ViewerEditable *self,
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guint n_steps);
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G_END_DECLS
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#endif
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This code is the same as the code for a normal GType
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which derives from a GObject except for a few details:
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The _GET_CLASS function is called
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_GET_IFACE (and is defined by
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G_DECLARE_INTERFACE).
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The instance type, ViewerEditable, is not fully defined: it is
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used merely as an abstract type which represents an instance of
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whatever object which implements the interface.
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The parent of the ViewerEditableInterface is
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GTypeInterface, not GObjectClass.
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The implementation of the ViewerEditable type itself is trivial:
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G_DEFINE_INTERFACE
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creates a viewer_editable_get_type function which registers the
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type in the type system. The third argument is used to define a
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prerequisite interface
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(which we'll talk about more later). Just pass 0 for this
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argument when an interface has no prerequisite.
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viewer_editable_default_init is expected
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to register the interface's signals if there are any (we will see a bit
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later how to use them).
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The interface methods viewer_editable_save,
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viewer_editable_undo and viewer_editable_redo dereference the interface
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structure to access its associated interface function and call it.
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G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)
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static void
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viewer_editable_default_init (ViewerEditableInterface *iface)
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{
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}
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void
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viewer_editable_save (ViewerEditable *self,
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GError **error)
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{
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ViewerEditableInterface *iface;
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g_return_if_fail (VIEWER_IS_EDITABLE (self));
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g_return_if_fail (error == NULL || *error == NULL);
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iface = VIEWER_EDITABLE_GET_IFACE (self);
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g_return_if_fail (iface->save != NULL);
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iface->save (self, error);
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}
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void
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viewer_editable_undo (ViewerEditable *self,
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guint n_steps)
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{
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ViewerEditableInterface *iface;
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g_return_if_fail (VIEWER_IS_EDITABLE (self));
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iface = VIEWER_EDITABLE_GET_IFACE (self);
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g_return_if_fail (iface->undo != NULL);
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iface->undo (self, n_steps);
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}
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void
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viewer_editable_redo (ViewerEditable *self,
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guint n_steps)
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{
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ViewerEditableInterface *iface;
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g_return_if_fail (VIEWER_IS_EDITABLE (self));
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iface = VIEWER_EDITABLE_GET_IFACE (self);
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g_return_if_fail (iface->redo != NULL);
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iface->redo (self, n_steps);
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}
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