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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="howto-interface"></a>How to define and implement interfaces</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="sect1"><a href="howto-interface.html#howto-interface-define">Defining interfaces</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-implement.html">Implementing interfaces</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-prerequisite.html">Interface definition prerequisites</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-properties.html">Interface properties</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-override.html">Overriding interface methods</a></span></dt>
</dl></div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-define"></a>Defining interfaces</h2></div></div></div>
<p>
    The theory behind how GObject interfaces work is given in
    <a class="xref" href="gtype-non-instantiable-classed.html" title="Non-instantiable classed types: interfaces">the section called “Non-instantiable classed types: interfaces”</a>; this section covers how to
    define and implement an interface.
  </p>
<p>
    The first step is to get the header right. This interface
    defines two methods:
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
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        <td class="listing_code"><pre class="programlisting"><span class="comment">/*</span>
<span class="comment"> * Copyright/Licensing information.</span>
<span class="comment"> */</span>

<span class="preproc">#ifndef</span><span class="normal"> __VIEWER_EDITABLE_H__</span>
<span class="preproc">#define</span><span class="normal"> __VIEWER_EDITABLE_H__</span>

<span class="preproc">#include</span><span class="normal"> </span><span class="string">&lt;glib-object.h&gt;</span>

<span class="normal"><a href="../glib-Miscellaneous-Macros.html#G-BEGIN-DECLS:CAPS">G_BEGIN_DECLS</a></span>

<span class="preproc">#define</span><span class="normal"> </span><span class="usertype">VIEWER_TYPE_EDITABLE</span><span class="normal"> </span><span class="function">viewer_editable_get_type</span><span class="normal"> </span><span class="symbol">()</span>
<span class="function"><a href="gobject-Type-Information.html#G-DECLARE-INTERFACE:CAPS">G_DECLARE_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerEditable</span><span class="symbol">,</span><span class="normal"> viewer_editable</span><span class="symbol">,</span><span class="normal"> VIEWER</span><span class="symbol">,</span><span class="normal"> EDITABLE</span><span class="symbol">,</span><span class="normal"> <a href="gobject-The-Base-Object-Type.html#GObject-struct">GObject</a></span><span class="symbol">)</span>

<span class="keyword">struct</span><span class="normal"> </span><span class="classname">_ViewerEditableInterface</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">GTypeInterface</span><span class="normal"> parent_iface</span><span class="symbol">;</span>

<span class="normal">  </span><span class="type">void</span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">save</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                </span><span class="usertype">GError</span><span class="normal">         </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">);</span>
<span class="normal">  </span><span class="type">void</span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">undo</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                </span><span class="usertype">guint</span><span class="normal">            n_steps</span><span class="symbol">);</span>
<span class="normal">  </span><span class="type">void</span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">redo</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                </span><span class="usertype">guint</span><span class="normal">            n_steps</span><span class="symbol">);</span>
<span class="cbracket">}</span><span class="symbol">;</span>

<span class="type">void</span><span class="normal"> </span><span class="function">viewer_editable_save</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                           </span><span class="usertype">GError</span><span class="normal">         </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">);</span>
<span class="type">void</span><span class="normal"> </span><span class="function">viewer_editable_undo</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                           </span><span class="usertype">guint</span><span class="normal">            n_steps</span><span class="symbol">);</span>
<span class="type">void</span><span class="normal"> </span><span class="function">viewer_editable_redo</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                           </span><span class="usertype">guint</span><span class="normal">            n_steps</span><span class="symbol">);</span>

<span class="normal"><a href="../glib-Miscellaneous-Macros.html#G-END-DECLS:CAPS">G_END_DECLS</a></span>

<span class="preproc">#endif</span><span class="normal"> </span><span class="comment">/* __VIEWER_EDITABLE_H__ */</span></pre></td>
      </tr>
    </tbody>
  </table>
</div>

<p>
    This code is the same as the code for a normal <a class="link" href="gobject-Type-Information.html#GType" title="GType"><span class="type">GType</span></a>
    which derives from a <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a> except for a few details:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
        The <code class="function">_GET_CLASS</code> function is called
        <code class="function">_GET_IFACE</code> (and is defined by
        <a class="link" href="gobject-Type-Information.html#G-DECLARE-INTERFACE:CAPS" title="G_DECLARE_INTERFACE()"><code class="function">G_DECLARE_INTERFACE</code></a>).
      </p></li>
<li class="listitem"><p>
        The instance type, <span class="type">ViewerEditable</span>, is not fully defined: it is
        used merely as an abstract type which represents an instance of
        whatever object which implements the interface.
      </p></li>
<li class="listitem"><p>
        The parent of the <span class="type">ViewerEditableInterface</span> is
        <span class="type">GTypeInterface</span>, not <span class="type">GObjectClass</span>.
      </p></li>
</ul></div>
<p>
  </p>
<p>
    The implementation of the <span class="type">ViewerEditable</span> type itself is trivial:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-INTERFACE:CAPS" title="G_DEFINE_INTERFACE()">G_DEFINE_INTERFACE</a></code>
       creates a <code class="function">viewer_editable_get_type</code> function which registers the
       type in the type system. The third argument is used to define a
       <a class="link" href="howto-interface-prerequisite.html" title="Interface definition prerequisites">prerequisite interface</a>
       (which we'll talk about more later). Just pass <code class="code">0</code> for this
       argument when an interface has no prerequisite.
       </p></li>
<li class="listitem"><p><code class="function">viewer_editable_default_init</code> is expected
      to register the interface's signals if there are any (we will see a bit
      later how to use them).</p></li>
<li class="listitem"><p>The interface methods <code class="function">viewer_editable_save</code>,
      <code class="function">viewer_editable_undo</code> and <code class="function">viewer_editable_redo</code> dereference the interface
      structure to access its associated interface function and call it.
      </p></li>
</ul></div>
<p>
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
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        <td class="listing_code"><pre class="programlisting"><span class="function"><a href="gobject-Type-Information.html#G-DEFINE-INTERFACE:CAPS">G_DEFINE_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerEditable</span><span class="symbol">,</span><span class="normal"> viewer_editable</span><span class="symbol">,</span><span class="normal"> <a href="gobject-Type-Information.html#G-TYPE-OBJECT:CAPS">G_TYPE_OBJECT</a></span><span class="symbol">);</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_editable_default_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">    </span><span class="comment">/* add properties and signals to the interface here */</span>
<span class="cbracket">}</span>

<span class="type">void</span>
<span class="function">viewer_editable_save</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                      </span><span class="usertype">GError</span><span class="normal">         </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">;</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_EDITABLE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">));</span>
<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </span><span class="symbol">==</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a> </span><span class="symbol">||</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">error </span><span class="symbol">==</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>

<span class="normal">  iface </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_EDITABLE_GET_IFACE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>
<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">iface</span><span class="symbol">-&gt;</span><span class="normal">save </span><span class="symbol">!=</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="function">save</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> error</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="type">void</span>
<span class="function">viewer_editable_undo</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                      </span><span class="usertype">guint</span><span class="normal">           n_steps</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">;</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_EDITABLE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">));</span>

<span class="normal">  iface </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_EDITABLE_GET_IFACE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>
<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">iface</span><span class="symbol">-&gt;</span><span class="normal">undo </span><span class="symbol">!=</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="function">undo</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> n_steps</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="type">void</span>
<span class="function">viewer_editable_redo</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">,</span>
<span class="normal">                      </span><span class="usertype">guint</span><span class="normal">           n_steps</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">;</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_EDITABLE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">));</span>

<span class="normal">  iface </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_EDITABLE_GET_IFACE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>
<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">iface</span><span class="symbol">-&gt;</span><span class="normal">redo </span><span class="symbol">!=</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="function">redo</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> n_steps</span><span class="symbol">);</span>
<span class="cbracket">}</span></pre></td>
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