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<!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 definition prerequisites: 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-implement.html" title="Implementing interfaces"> <link rel="next" href="howto-interface-properties.html" title="Interface properties"> <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-implement.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="howto-interface-properties.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-prerequisite"></a>Interface definition prerequisites</h2></div></div></div> <p> To specify that an interface requires the presence of other interfaces when implemented, GObject introduces the concept of <span class="emphasis"><em>prerequisites</em></span>: 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. </p> <p> The mechanism described above is, in practice, very similar to Java's interface I1 extends interface I2. The example below shows the GObject equivalent: </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</pre></td> <td class="listing_code"><pre class="programlisting"><span class="cm">/* Make the ViewerEditableLossy interface require ViewerEditable interface. */</span> <span class="n">G_DEFINE_INTERFACE</span> <span class="p">(</span><span class="n">ViewerEditableLossy</span><span class="p">,</span> <span class="n">viewer_editable_lossy</span><span class="p">,</span> <span class="n">VIEWER_TYPE_EDITABLE</span><span class="p">)</span></pre></td> </tr> </tbody> </table> </div> <p> 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 <span class="type">ViewerEditable</span> interface is a prerequisite of <span class="type">ViewerEditableLossy</span>. The code below shows how an implementation can implement both interfaces and register their implementations: </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 64 65 66 67 68 69 70</pre></td> <td class="listing_code"><pre class="programlisting"><span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_lossy_compress</span> <span class="p">(</span><span class="n">ViewerEditableLossy</span> <span class="o">*</span><span class="n">editable</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"File implementation of lossy editable interface compress method: %s.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">self</span><span class="o">-></span><span class="n">filename</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_lossy_interface_init</span> <span class="p">(</span><span class="n">ViewerEditableLossyInterface</span> <span class="o">*</span><span class="n">iface</span><span class="p">)</span> <span class="p">{</span> <span class="n">iface</span><span class="o">-></span><span class="n">compress</span> <span class="o">=</span> <span class="n">viewer_file_editable_lossy_compress</span><span class="p">;</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_save</span> <span class="p">(</span><span class="n">ViewerEditable</span> <span class="o">*</span><span class="n">editable</span><span class="p">,</span> <span class="n">GError</span> <span class="o">**</span><span class="n">error</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"File implementation of editable interface save method: %s.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">self</span><span class="o">-></span><span class="n">filename</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_undo</span> <span class="p">(</span><span class="n">ViewerEditable</span> <span class="o">*</span><span class="n">editable</span><span class="p">,</span> <span class="n">guint</span> <span class="n">n_steps</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"File implementation of editable interface undo method: %s.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">self</span><span class="o">-></span><span class="n">filename</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_redo</span> <span class="p">(</span><span class="n">ViewerEditable</span> <span class="o">*</span><span class="n">editable</span><span class="p">,</span> <span class="n">guint</span> <span class="n">n_steps</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"File implementation of editable interface redo method: %s.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">self</span><span class="o">-></span><span class="n">filename</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_editable_interface_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">iface</span><span class="o">-></span><span class="n">save</span> <span class="o">=</span> <span class="n">viewer_file_editable_save</span><span class="p">;</span> <span class="n">iface</span><span class="o">-></span><span class="n">undo</span> <span class="o">=</span> <span class="n">viewer_file_editable_undo</span><span class="p">;</span> <span class="n">iface</span><span class="o">-></span><span class="n">redo</span> <span class="o">=</span> <span class="n">viewer_file_editable_redo</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="cm">/* Nothing here. */</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_file_init</span> <span class="p">(</span><span class="n">ViewerFile</span> <span class="o">*</span><span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Instance variable initialisation code. */</span> <span class="p">}</span> <span class="n">G_DEFINE_TYPE_WITH_CODE</span> <span class="p">(</span><span class="n">ViewerFile</span><span class="p">,</span> <span class="n">viewer_file</span><span class="p">,</span> <span class="n">G_TYPE_OBJECT</span><span class="p">,</span> <span class="n">G_IMPLEMENT_INTERFACE</span> <span class="p">(</span><span class="n">VIEWER_TYPE_EDITABLE</span><span class="p">,</span> <span class="n">viewer_file_editable_interface_init</span><span class="p">)</span> <span class="n">G_IMPLEMENT_INTERFACE</span> <span class="p">(</span><span class="n">VIEWER_TYPE_EDITABLE_LOSSY</span><span class="p">,</span> <span class="n">viewer_file_editable_lossy_interface_init</span><span class="p">))</span></pre></td> </tr> </tbody> </table> </div> <p> 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. </p> </div> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>