docs/reference/gobject/html/howto-interface-prerequisite.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 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 V1.25.1 (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="comment">/* Make the ViewerEditableLossy interface require ViewerEditable interface. */</span>
<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">ViewerEditableLossy</span><span class="symbol">,</span><span class="normal"> viewer_editable_lossy</span><span class="symbol">,</span><span class="normal"> VIEWER_TYPE_EDITABLE</span><span class="symbol">);</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="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_editable_lossy_compress</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditableLossy</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">editable</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">self </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">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"File implementation of lossy editable interface compress method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_editable_lossy_interface_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditableLossyInterface</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">  iface</span><span class="symbol">-&gt;</span><span class="normal">compress </span><span class="symbol">=</span><span class="normal"> viewer_file_editable_lossy_compress</span><span class="symbol">;</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_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">editable</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">ViewerFile</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">VIEWER_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"File implementation of editable interface save method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_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">editable</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">ViewerFile</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">VIEWER_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"File implementation of editable interface undo method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_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">editable</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">ViewerFile</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">VIEWER_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"File implementation of editable interface redo method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_editable_interface_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">  iface</span><span class="symbol">-&gt;</span><span class="normal">save </span><span class="symbol">=</span><span class="normal"> viewer_file_editable_save</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"> viewer_file_editable_undo</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"> viewer_file_editable_redo</span><span class="symbol">;</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="comment">/* Nothing here. */</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_file_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Instance variable initialisation code. */</span>
<span class="cbracket">}</span>

<span class="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS">G_DEFINE_TYPE_WITH_CODE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerFile</span><span class="symbol">,</span><span class="normal"> viewer_file</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="normal">                         </span><span class="function"><a href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS">G_IMPLEMENT_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">VIEWER_TYPE_EDITABLE</span><span class="symbol">,</span>
<span class="normal">                                                viewer_file_editable_interface_init</span><span class="symbol">)</span>
<span class="normal">                         </span><span class="function"><a href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS">G_IMPLEMENT_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">VIEWER_TYPE_EDITABLE_LOSSY</span><span class="symbol">,</span>
<span class="normal">                                                viewer_file_editable_lossy_interface_init</span><span class="symbol">))</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 V1.25.1</div>
</body>
</html>