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| <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> |
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| 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">-></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">-></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">-></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">-></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">-></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">-></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">-></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">-></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> |
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