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howto-interface-properties.html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Interface properties: GObject Reference Manual</title> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="index.html" title="GObject Reference Manual"> <link rel="up" href="howto-interface.html" title="How to define and implement interfaces"> <link rel="prev" href="howto-interface-prerequisite.html" title="Interface definition prerequisites"> <link rel="next" href="howto-interface-override.html" title="Overriding interface methods"> <meta name="generator" content="GTK-Doc V1.32 (XML mode)"> <link rel="stylesheet" href="style.css" type="text/css"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle"> <td width="100%" align="left" class="shortcuts"></td> <td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td> <td><a accesskey="u" href="howto-interface.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td> <td><a accesskey="p" href="howto-interface-prerequisite.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="howto-interface-override.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td> </tr></table> <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 '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"> <table class="listing_frame" border="0" cellpadding="0" cellspacing="0"> <tbody> <tr> <td class="listing_lines" align="right"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13</pre></td> <td class="listing_code"><pre class="programlisting"><span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_editable_default_init</span> <span class="p">(</span><span class="n">ViewerEditableInterface</span> <span class="o">*</span><span class="n">iface</span><span class="p">)</span> <span class="p">{</span> <span class="n">g_object_interface_install_property</span> <span class="p">(</span><span class="n">iface</span><span class="p">,</span> <span class="n">g_param_spec_double</span> <span class="p">(</span><span class="s">"autosave-frequency"</span><span class="p">,</span> <span class="s">"Autosave frequency"</span><span class="p">,</span> <span class="s">"Frequency (in per-seconds) to autosave backups of the editable content at. "</span> <span class="s">"Or zero to disable autosaves."</span><span class="p">,</span> <span class="mf">0.0</span><span class="p">,</span> <span class="cm">/* minimum */</span> <span class="n">G_MAXDOUBLE</span><span class="p">,</span> <span class="cm">/* maximum */</span> <span class="mf">0.0</span><span class="p">,</span> <span class="cm">/* default */</span> <span class="n">G_PARAM_READWRITE</span><span class="p">));</span> <span class="p">}</span></pre></td> </tr> </tbody> </table> </div> <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"> <table class="listing_frame" border="0" cellpadding="0" cellspacing="0"> <tbody> <tr> <td class="listing_lines" align="right"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63</pre></td> <td class="listing_code"><pre class="programlisting"><span class="k">struct</span> <span class="n">_ViewerFile</span> <span class="p">{</span> <span class="n">GObject</span> <span class="n">parent_instance</span><span class="p">;</span> <span class="n">gdouble</span> <span class="n">autosave_frequency</span><span class="p">;</span> <span class="p">};</span> <span class="k">enum</span> <span class="p">{</span> <span class="n">PROP_AUTOSAVE_FREQUENCY</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">N_PROPERTIES</span> <span class="p">};</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_set_property</span> <span class="p">(</span><span class="n">GObject</span> <span class="o">*</span><span class="n">object</span><span class="p">,</span> <span class="n">guint</span> <span class="n">prop_id</span><span class="p">,</span> <span class="k">const</span> <span class="n">GValue</span> <span class="o">*</span><span class="n">value</span><span class="p">,</span> <span class="n">GParamSpec</span> <span class="o">*</span><span class="n">pspec</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">file</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">object</span><span class="p">);</span> <span class="k">switch</span> <span class="p">(</span><span class="n">prop_id</span><span class="p">)</span> <span class="p">{</span> <span class="k">case</span> <span class="nl">PROP_AUTOSAVE_FREQUENCY</span><span class="p">:</span> <span class="n">file</span><span class="o">-></span><span class="n">autosave_frequency</span> <span class="o">=</span> <span class="n">g_value_get_double</span> <span class="p">(</span><span class="n">value</span><span class="p">);</span> <span class="k">break</span><span class="p">;</span> <span class="k">default</span><span class="o">:</span> <span class="n">G_OBJECT_WARN_INVALID_PROPERTY_ID</span> <span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="n">prop_id</span><span class="p">,</span> <span class="n">pspec</span><span class="p">);</span> <span class="k">break</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_get_property</span> <span class="p">(</span><span class="n">GObject</span> <span class="o">*</span><span class="n">object</span><span class="p">,</span> <span class="n">guint</span> <span class="n">prop_id</span><span class="p">,</span> <span class="n">GValue</span> <span class="o">*</span><span class="n">value</span><span class="p">,</span> <span class="n">GParamSpec</span> <span class="o">*</span><span class="n">pspec</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">file</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">object</span><span class="p">);</span> <span class="k">switch</span> <span class="p">(</span><span class="n">prop_id</span><span class="p">)</span> <span class="p">{</span> <span class="k">case</span> <span class="nl">PROP_AUTOSAVE_FREQUENCY</span><span class="p">:</span> <span class="n">g_value_set_double</span> <span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="n">file</span><span class="o">-></span><span class="n">autosave_frequency</span><span class="p">);</span> <span class="k">break</span><span class="p">;</span> <span class="k">default</span><span class="o">:</span> <span class="n">G_OBJECT_WARN_INVALID_PROPERTY_ID</span> <span class="p">(</span><span class="n">object</span><span class="p">,</span> <span class="n">prop_id</span><span class="p">,</span> <span class="n">pspec</span><span class="p">);</span> <span class="k">break</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_class_init</span> <span class="p">(</span><span class="n">ViewerFileClass</span> <span class="o">*</span><span class="n">klass</span><span class="p">)</span> <span class="p">{</span> <span class="n">GObjectClass</span> <span class="o">*</span><span class="n">object_class</span> <span class="o">=</span> <span class="n">G_OBJECT_CLASS</span> <span class="p">(</span><span class="n">klass</span><span class="p">);</span> <span class="n">object_class</span><span class="o">-></span><span class="n">set_property</span> <span class="o">=</span> <span class="n">viewer_file_set_property</span><span class="p">;</span> <span class="n">object_class</span><span class="o">-></span><span class="n">get_property</span> <span class="o">=</span> <span class="n">viewer_file_get_property</span><span class="p">;</span> <span class="n">g_object_class_override_property</span> <span class="p">(</span><span class="n">object_class</span><span class="p">,</span> <span class="n">PROP_AUTOSAVE_FREQUENCY</span><span class="p">,</span> <span class="s">"autosave-frequency"</span><span class="p">);</span> <span class="p">}</span></pre></td> </tr> </tbody> </table> </div> <p> </p> </div> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>