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<title>Interface definition prerequisites: GObject Reference Manual</title>
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Interface definition prerequisites
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      To specify that an interface requires the presence of other interfaces
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      when implemented, GObject introduces the concept of
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      prerequisites: it is possible to associate
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      a list of prerequisite types to an interface. For example, if
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      object A wishes to implement interface I1, and if interface I1 has a
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      prerequisite on interface I2, A has to implement both I1 and I2.
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      The mechanism described above is, in practice, very similar to
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      Java's interface I1 extends interface I2. The example below shows
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      the GObject equivalent:
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/* Make the ViewerEditableLossy interface require ViewerEditable interface. */
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G_DEFINE_INTERFACE (ViewerEditableLossy, viewer_editable_lossy, VIEWER_TYPE_EDITABLE)
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      In the G_DEFINE_INTERFACE
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      call above, the third parameter defines the prerequisite type. This
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      is the GType of either an interface or a class. In this case
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      the ViewerEditable interface is a prerequisite of
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      ViewerEditableLossy. The code
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      below shows how an implementation can implement both interfaces and
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      register their implementations:
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static void
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viewer_file_editable_lossy_compress (ViewerEditableLossy *editable)
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{
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  ViewerFile *self = VIEWER_FILE (editable);
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  g_print ("File implementation of lossy editable interface compress method: %s.\n",
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           self->filename);
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}
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static void
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viewer_file_editable_lossy_interface_init (ViewerEditableLossyInterface *iface)
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{
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  iface->compress = viewer_file_editable_lossy_compress;
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}
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static void
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viewer_file_editable_save (ViewerEditable  *editable,
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                           GError         **error)
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{
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  ViewerFile *self = VIEWER_FILE (editable);
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  g_print ("File implementation of editable interface save method: %s.\n",
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           self->filename);
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}
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static void
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viewer_file_editable_undo (ViewerEditable *editable,
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                           guint           n_steps)
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{
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  ViewerFile *self = VIEWER_FILE (editable);
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  g_print ("File implementation of editable interface undo method: %s.\n",
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           self->filename);
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}
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static void
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viewer_file_editable_redo (ViewerEditable *editable,
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                           guint           n_steps)
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{
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  ViewerFile *self = VIEWER_FILE (editable);
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  g_print ("File implementation of editable interface redo method: %s.\n",
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           self->filename);
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}
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static void
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viewer_file_editable_interface_init (ViewerEditableInterface *iface)
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{
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  iface->save = viewer_file_editable_save;
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  iface->undo = viewer_file_editable_undo;
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  iface->redo = viewer_file_editable_redo;
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}
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static void
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viewer_file_class_init (ViewerFileClass *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_file_init (ViewerFile *self)
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{
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  /* Instance variable initialisation code. */
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}
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G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
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                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
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                                                viewer_file_editable_interface_init)
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                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE_LOSSY,
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                                                viewer_file_editable_lossy_interface_init))
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      It is very important to notice that the order in which interface
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      implementations are added to the main object is not random:
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      g_type_add_interface_static,
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      which is called by
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      G_IMPLEMENT_INTERFACE,
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      must be invoked first on the interfaces which have no prerequisites and then on
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      the others.
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