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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-properties"></a>Interface properties</h2></div></div></div>
<p>
      GObject interfaces can also have
      properties. Declaration of the interface properties is similar to
      declaring the properties of ordinary GObject types as explained in
      <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, except that
      <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-interface-install-property" title="g_object_interface_install_property ()">g_object_interface_install_property</a></code>
      is used to declare the properties instead of
      <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
    </p>
<p>
      To include a property named 'autosave-frequency' of type <span class="type">gdouble</span> in the 
      <span class="type">ViewerEditable</span> interface example code above, we only need to
      add one call in <code class="function">viewer_editable_default_init</code> as shown
      below:
</p>
<div class="informalexample">
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        <td class="listing_code"><pre class="programlisting"><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="function"><a href="gobject-The-Base-Object-Type.html#g-object-interface-install-property">g_object_interface_install_property</a></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="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-double">g_param_spec_double</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"autosave-frequency"</span><span class="symbol">,</span>
<span class="normal">                                                            </span><span class="string">"Autosave frequency"</span><span class="symbol">,</span>
<span class="normal">                                                            </span><span class="string">"Frequency (in per-seconds) to autosave backups of the editable content at. "</span>
<span class="normal">                                                            </span><span class="string">"Or zero to disable autosaves."</span><span class="symbol">,</span>
<span class="normal">                                                            </span><span class="number">0.0</span><span class="symbol">,</span><span class="normal">  </span><span class="comment">/* minimum */</span>
<span class="normal">                                                            <a href="../glib-Basic-Types.html#G-MAXDOUBLE:CAPS">G_MAXDOUBLE</a></span><span class="symbol">,</span><span class="normal">  </span><span class="comment">/* maximum */</span>
<span class="normal">                                                            </span><span class="number">0.0</span><span class="symbol">,</span><span class="normal">  </span><span class="comment">/* default */</span>
<span class="normal">                                                            <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>
<span class="cbracket">}</span></pre></td>
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<p>
    </p>
<p>
      One point worth noting is that the declared property wasn't assigned an 
      integer ID. The reason being that integer IDs of properties are used
      only inside the <code class="function">get_property</code> and
      <code class="function">set_property</code> virtual methods. Since interfaces
      declare but do not <span class="emphasis"><em>implement</em></span> properties, there is no
      need to assign integer IDs to them.
    </p>
<p>
      An implementation declares and defines its properties in the usual
      way as explained in <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>, except for one
      small change: it can declare the properties of the interface it
      implements using <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-override-property" title="g_object_class_override_property ()">g_object_class_override_property</a></code>
      instead of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-class-install-property" title="g_object_class_install_property ()">g_object_class_install_property</a></code>.
      The following code snippet shows the modifications needed in the
      <span class="type">ViewerFile</span> declaration and implementation above:
</p>
<div class="informalexample">
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        <td class="listing_code"><pre class="programlisting"><span class="keyword">struct</span><span class="normal"> </span><span class="classname">_ViewerFile</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">GObject</span><span class="normal"> parent_instance</span><span class="symbol">;</span>

<span class="normal">  </span><span class="usertype">gdouble</span><span class="normal"> autosave_frequency</span><span class="symbol">;</span>
<span class="cbracket">}</span><span class="symbol">;</span>

<span class="keyword">enum</span>
<span class="cbracket">{</span>
<span class="normal">  PROP_AUTOSAVE_FREQUENCY </span><span class="symbol">=</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
<span class="normal">  N_PROPERTIES</span>
<span class="cbracket">}</span><span class="symbol">;</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_set_property</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">GObject</span><span class="normal">      </span><span class="symbol">*</span><span class="normal">object</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="usertype">guint</span><span class="normal">         prop_id</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GValue</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">value</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="usertype">GParamSpec</span><span class="normal">   </span><span class="symbol">*</span><span class="normal">pspec</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">file </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object</span><span class="symbol">);</span>

<span class="normal">  </span><span class="keyword">switch</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">prop_id</span><span class="symbol">)</span>
<span class="normal">    </span><span class="cbracket">{</span>
<span class="normal">    </span><span class="keyword">case</span><span class="normal"> PROP_AUTOSAVE_FREQUENCY</span><span class="symbol">:</span>
<span class="normal">      file</span><span class="symbol">-&gt;</span><span class="normal">autosave_frequency </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-value-get-double">g_value_get_double</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">value</span><span class="symbol">);</span>
<span class="normal">      </span><span class="keyword">break</span><span class="symbol">;</span>

<span class="label">    default:</span>
<span class="normal">      </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-WARN-INVALID-PROPERTY-ID:CAPS">G_OBJECT_WARN_INVALID_PROPERTY_ID</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object</span><span class="symbol">,</span><span class="normal"> prop_id</span><span class="symbol">,</span><span class="normal"> pspec</span><span class="symbol">);</span>
<span class="normal">      </span><span class="keyword">break</span><span class="symbol">;</span>
<span class="normal">    </span><span class="cbracket">}</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_get_property</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">GObject</span><span class="normal">    </span><span class="symbol">*</span><span class="normal">object</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="usertype">guint</span><span class="normal">       prop_id</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="usertype">GValue</span><span class="normal">     </span><span class="symbol">*</span><span class="normal">value</span><span class="symbol">,</span>
<span class="normal">                          </span><span class="usertype">GParamSpec</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">pspec</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">file </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object</span><span class="symbol">);</span>

<span class="normal">  </span><span class="keyword">switch</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">prop_id</span><span class="symbol">)</span>
<span class="normal">    </span><span class="cbracket">{</span>
<span class="normal">    </span><span class="keyword">case</span><span class="normal"> PROP_AUTOSAVE_FREQUENCY</span><span class="symbol">:</span>
<span class="normal">      </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-value-set-double">g_value_set_double</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">value</span><span class="symbol">,</span><span class="normal"> file</span><span class="symbol">-&gt;</span><span class="normal">autosave_frequency</span><span class="symbol">);</span>
<span class="normal">      </span><span class="keyword">break</span><span class="symbol">;</span>

<span class="label">    default:</span>
<span class="normal">      </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-WARN-INVALID-PROPERTY-ID:CAPS">G_OBJECT_WARN_INVALID_PROPERTY_ID</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object</span><span class="symbol">,</span><span class="normal"> prop_id</span><span class="symbol">,</span><span class="normal"> pspec</span><span class="symbol">);</span>
<span class="normal">      </span><span class="keyword">break</span><span class="symbol">;</span>
<span class="normal">    </span><span class="cbracket">}</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_class_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">GObjectClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">object_class </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-CLASS:CAPS">G_OBJECT_CLASS</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">);</span>

<span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">set_property </span><span class="symbol">=</span><span class="normal"> viewer_file_set_property</span><span class="symbol">;</span>
<span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">get_property </span><span class="symbol">=</span><span class="normal"> viewer_file_get_property</span><span class="symbol">;</span>

<span class="normal">  </span><span class="function"><a href="gobject-The-Base-Object-Type.html#g-object-class-override-property">g_object_class_override_property</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object_class</span><span class="symbol">,</span><span class="normal"> PROP_AUTOSAVE_FREQUENCY</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"autosave-frequency"</span><span class="symbol">);</span>
<span class="cbracket">}</span></pre></td>
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