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<title>Overriding interface methods: GObject Reference Manual</title>
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Overriding interface methods
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      If a base class already implements an interface and a derived
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      class needs to implement the same interface but needs to override certain
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      methods, you must reimplement the interface and set only the interface
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      methods which need overriding.
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      In this example, ViewerAudioFile is derived from
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      ViewerFile. Both implement the ViewerEditable
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      interface. ViewerAudioFile only implements one method of the
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      ViewerEditable interface and uses the base class implementation of
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      the other.
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static void
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viewer_audio_file_editable_save (ViewerEditable  *editable,
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                                 GError         **error)
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{
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  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);
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  g_print ("Audio file implementation of editable interface save method.\n");
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}
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static void
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viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
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{
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  /* Override the implementation of save(). */
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  iface->save = viewer_audio_file_editable_save;
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  /*
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   * Leave iface->undo and ->redo alone, they are already set to the
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   * base class implementation.
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   */
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}
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G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
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                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
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                                                viewer_audio_file_editable_interface_init))
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static void
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viewer_audio_file_class_init (ViewerAudioFileClass *klass)
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{
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  /* Nothing here. */
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}
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static void
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viewer_audio_file_init (ViewerAudioFile *self)
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{
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  /* Nothing here. */
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}
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      To access the base class interface implementation use
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      g_type_interface_peek_parent
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      from within an interface's default_init function.
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      To call the base class implementation of an interface
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      method from an derived class where than interface method has been
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      overridden, stash away the pointer returned from
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      g_type_interface_peek_parent
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      in a global variable.
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      In this example ViewerAudioFile overrides the
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      save interface method. In its overridden method
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      it calls the base class implementation of the same interface method.
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static ViewerEditableInterface *viewer_editable_parent_interface = NULL;
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static void
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viewer_audio_file_editable_save (ViewerEditable  *editable,
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                                 GError         **error)
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{
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  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);
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  g_print ("Audio file implementation of editable interface save method.\n");
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  /* Now call the base implementation */
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  viewer_editable_parent_interface->save (editable, error);
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}
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static void
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viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
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{
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  viewer_editable_parent_interface = g_type_interface_peek_parent (iface);
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  iface->save = viewer_audio_file_editable_save;
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}
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G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
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                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
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                                                viewer_audio_file_editable_interface_init))
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static void
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viewer_audio_file_class_init (ViewerAudioFileClass *klass)
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{
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  /* Nothing here. */
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}
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static void
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viewer_audio_file_init (ViewerAudioFile *self)
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{
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  /* Nothing here. */
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}
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