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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-prerequisite"></a>Interface definition prerequisites</h2></div></div></div>

 To specify that an interface requires the presence of other interfaces
 when implemented, GObject introduces the concept of
 prerequisites: it is possible to associate
 a list of prerequisite types to an interface. For example, if
 object A wishes to implement interface I1, and if interface I1 has a
 prerequisite on interface I2, A has to implement both I1 and I2.
 

 The mechanism described above is, in practice, very similar to
 Java's interface I1 extends interface I2. The example below shows
 the GObject equivalent:

<div class="informalexample">
 <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
 <tbody>
 <tr>
 <td class="listing_lines" align="right"><pre>1
2</pre></td>
 <td class="listing_code"><pre class="programlisting">/* Make the ViewerEditableLossy interface require ViewerEditable interface. */
G_DEFINE_INTERFACE (ViewerEditableLossy, viewer_editable_lossy, VIEWER_TYPE_EDITABLE)</pre></td>
 </tr>
 </tbody>
 </table>
</div>

In the <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-INTERFACE:CAPS" title="G_DEFINE_INTERFACE()">G_DEFINE_INTERFACE</a></code>
 call above, the third parameter defines the prerequisite type. This
 is the GType of either an interface or a class. In this case
 the ViewerEditable interface is a prerequisite of
 ViewerEditableLossy. The code
 below shows how an implementation can implement both interfaces and
 register their implementations:

<div class="informalexample">
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 <td class="listing_code"><pre class="programlisting">static void
viewer_file_editable_lossy_compress (ViewerEditableLossy *editable)
{
 ViewerFile *self = VIEWER_FILE (editable);

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 = viewer_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_editable_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 (ViewerFile *self)
{
 /* Instance variable initialisation code. */
}

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_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))</pre></td>
 </tr>
 </tbody>
 </table>
</div>

It is very important to notice that the order in which interface
 implementations are added to the main object is not random:
 <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-add-interface-static" title="g_type_add_interface_static ()">g_type_add_interface_static</a></code>,
 which is called by
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-INTERFACE:CAPS" title="G_DEFINE_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>,
 must be invoked first on the interfaces which have no prerequisites and then on
 the others.
 
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