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<div class="sect1">
<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 'name' of type <span class="type">string</span> in the
<span class="type">MamanIbaz</span> interface example code above, we only need to
add one call in <code class="function">maman_ibaz_default_init</code> as shown
below:
</p>
<div class="informalexample">
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<td class="listing_code"><pre class="programlisting"><span class="gtkdoc kwb">static void</span>
<span class="function">maman_ibaz_default_init</span> <span class="gtkdoc opt">(</span>MamanIbazInterface <span class="gtkdoc opt">*</span>iface<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</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="gtkdoc opt">(</span>iface<span class="gtkdoc opt">,</span>
<span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-string">g_param_spec_string</a></span> <span class="gtkdoc opt">(</span><span class="string">"name"</span><span class="gtkdoc opt">,</span>
<span class="string">"Name"</span><span class="gtkdoc opt">,</span>
<span class="string">"Name of the MamanIbaz"</span><span class="gtkdoc opt">,</span>
<span class="string">"maman"</span><span class="gtkdoc opt">,</span>
G_PARAM_READWRITE<span class="gtkdoc opt">));</span>
<span class="gtkdoc opt">}</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">MamanBaz</span> declaration and implementation above:
</p>
<div class="informalexample">
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<td class="listing_code"><pre class="programlisting"><span class="gtkdoc kwb">struct</span> _MamanBaz
<span class="gtkdoc opt">{</span>
GObject parent_instance<span class="gtkdoc opt">;</span>
gint instance_member<span class="gtkdoc opt">;</span>
gchar <span class="gtkdoc opt">*</span>name<span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">};</span>
<span class="gtkdoc kwb">enum</span>
<span class="gtkdoc opt">{</span>
PROP_NAME <span class="gtkdoc opt">=</span> <span class="number">1</span><span class="gtkdoc opt">,</span>
N_PROPERTIES
<span class="gtkdoc opt">};</span>
<span class="gtkdoc kwb">static void</span>
<span class="function">maman_baz_set_property</span> <span class="gtkdoc opt">(</span>GObject <span class="gtkdoc opt">*</span>object<span class="gtkdoc opt">,</span>
guint prop_id<span class="gtkdoc opt">,</span>
<span class="gtkdoc kwb">const</span> GValue <span class="gtkdoc opt">*</span>value<span class="gtkdoc opt">,</span>
GParamSpec <span class="gtkdoc opt">*</span>pspec<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
MamanBaz <span class="gtkdoc opt">*</span>baz <span class="gtkdoc opt">=</span> <span class="function">MAMAN_BAZ</span> <span class="gtkdoc opt">(</span>object<span class="gtkdoc opt">);</span>
<span class="keyword">switch</span> <span class="gtkdoc opt">(</span>prop_id<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
<span class="keyword">case</span> PROP_NAME<span class="gtkdoc opt">:</span>
<span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-Memory-Allocation.html#g-free">g_free</a></span> <span class="gtkdoc opt">(</span>baz<span class="gtkdoc opt">-></span>name<span class="gtkdoc opt">);</span>
baz<span class="gtkdoc opt">-></span>name <span class="gtkdoc opt">=</span> <span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-value-dup-string">g_value_dup_string</a></span> <span class="gtkdoc opt">(</span>value<span class="gtkdoc opt">);</span>
<span class="keyword">break</span><span class="gtkdoc opt">;</span>
<span class="keyword">default</span><span class="gtkdoc opt">:</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="gtkdoc opt">(</span>object<span class="gtkdoc opt">,</span> prop_id<span class="gtkdoc opt">,</span> pspec<span class="gtkdoc opt">);</span>
<span class="keyword">break</span><span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span>
<span class="gtkdoc opt">}</span>
<span class="gtkdoc kwb">static void</span>
<span class="function">maman_baz_get_property</span> <span class="gtkdoc opt">(</span>GObject <span class="gtkdoc opt">*</span>object<span class="gtkdoc opt">,</span>
guint prop_id<span class="gtkdoc opt">,</span>
GValue <span class="gtkdoc opt">*</span>value<span class="gtkdoc opt">,</span>
GParamSpec <span class="gtkdoc opt">*</span>pspec<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
MamanBaz <span class="gtkdoc opt">*</span>baz <span class="gtkdoc opt">=</span> <span class="function">MAMAN_BAZ</span> <span class="gtkdoc opt">(</span>object<span class="gtkdoc opt">);</span>
<span class="keyword">switch</span> <span class="gtkdoc opt">(</span>prop_id<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
<span class="keyword">case</span> PROP_NAME<span class="gtkdoc opt">:</span>
<span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-value-set-string">g_value_set_string</a></span> <span class="gtkdoc opt">(</span>value<span class="gtkdoc opt">,</span> baz<span class="gtkdoc opt">-></span>name<span class="gtkdoc opt">);</span>
<span class="keyword">break</span><span class="gtkdoc opt">;</span>
<span class="keyword">default</span><span class="gtkdoc opt">:</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="gtkdoc opt">(</span>object<span class="gtkdoc opt">,</span> prop_id<span class="gtkdoc opt">,</span> pspec<span class="gtkdoc opt">);</span>
<span class="keyword">break</span><span class="gtkdoc opt">;</span>
<span class="gtkdoc opt">}</span>
<span class="gtkdoc opt">}</span>
<span class="gtkdoc kwb">static void</span>
<span class="function">maman_baz_class_init</span> <span class="gtkdoc opt">(</span>MamanBazClass <span class="gtkdoc opt">*</span>klass<span class="gtkdoc opt">)</span>
<span class="gtkdoc opt">{</span>
GObjectClass <span class="gtkdoc opt">*</span>object_class <span class="gtkdoc opt">=</span> <span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-CLASS:CAPS">G_OBJECT_CLASS</a></span> <span class="gtkdoc opt">(</span>klass<span class="gtkdoc opt">);</span>
object_class<span class="gtkdoc opt">-></span>set_property <span class="gtkdoc opt">=</span> maman_baz_set_property<span class="gtkdoc opt">;</span>
object_class<span class="gtkdoc opt">-></span>get_property <span class="gtkdoc opt">=</span> maman_baz_get_property<span class="gtkdoc opt">;</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="gtkdoc opt">(</span>object_class<span class="gtkdoc opt">,</span> PROP_NAME<span class="gtkdoc opt">,</span> <span class="string">"name"</span><span class="gtkdoc opt">);</span>
<span class="gtkdoc opt">}</span></pre></td>
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