glib/docs/reference/gobject/html/howto-gobject-construction.html - nest-cam/4320010/glib - Git at Google

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 <div class="sect1">
 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 <a name="howto-gobject-construction"></a>Object construction</h2></div></div></div>
 <p>
       People often get confused when trying to construct their GObjects because of the
       sheer number of different ways to hook into the objects's construction process: it is
       difficult to figure which is the <span class="emphasis"><em>correct</em></span>, recommended way.
     </p>
 <p>
       <a class="xref" href="chapter-gobject.html#gobject-construction-table" title="Table 4. g_object_new">Table 4, “g_object_new”</a> shows what user-provided functions
       are invoked during object instantiation and in which order they are invoked.
       A user looking for the equivalent of the simple C++ constructor function should use
       the <code class="function">instance_init</code> method. It will be invoked after
       all the parents’ <code class="function">instance_init</code>
       functions have been invoked. It cannot take arbitrary construction parameters
       (as in C++) but if your object needs arbitrary parameters to complete initialization,
       you can use construction properties.
     </p>
 <p>
       Construction properties will be set only after all
       <code class="function">instance_init</code> functions have run.
       No object reference will be returned to the client of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>
       until all the construction properties have been set.
     </p>
 <p>
       It is important to note that object construction cannot <span class="emphasis"><em>ever</em></span>
       fail. If you require a fallible GObject construction, you can use the
       <a href="https://developer.gnome.org/gio/unstable/GInitable.html#GInitable-struct"><span class="type">GInitable</span></a> and
       <a href="https://developer.gnome.org/gio/unstable/GAsyncInitable.html#GAsyncInitable-struct"><span class="type">GAsyncInitable</span></a>
       interfaces provided by the GIO library.
     </p>
 <p>
       You should write the following code first:
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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         <td class="listing_code"><pre class="programlisting"><span class="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-PRIVATE:CAPS">G_DEFINE_TYPE_WITH_PRIVATE</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="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="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="usertype">ViewerFilePrivate</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">priv </span><span class="symbol">=</span><span class="normal"> </span><span class="function">viewer_file_get_instance_private</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>

 <span class="normal">  </span><span class="comment">/* initialize all public and private members to reasonable default values.</span>
 <span class="comment">   * They are all automatically initialized to 0 to begin with. */</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
     </p>
 <p>
       If you need special construction properties (with
       <a class="link" href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS"><code class="function">G_PARAM_CONSTRUCT_ONLY</code></a>
       set), install the properties in
       the <code class="function">class_init()</code> function, override the <code class="function">set_property()</code>
       and <code class="function">get_property()</code> methods of the GObject class,
       and implement them as described by <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>.
     </p>
 <p>
       Property IDs must start from 1, as 0 is reserved for internal use by
       GObject.
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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 37</pre></td>
         <td class="listing_code"><pre class="programlisting"><span class="keyword">enum</span>
 <span class="cbracket">{</span>
 <span class="normal">  PROP_FILENAME </span><span class="symbol">=</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
 <span class="normal">  PROP_ZOOM_LEVEL</span><span class="symbol">,</span>
 <span class="normal">  N_PROPERTIES</span>
 <span class="cbracket">}</span><span class="symbol">;</span>

 <span class="keyword">static</span><span class="normal"> </span><span class="usertype">GParamSpec</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">obj_properties</span><span class="symbol">[</span><span class="normal">N_PROPERTIES</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</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="usertype">GObjectClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">object_class </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-CLASS:CAPS">G_OBJECT_CLASS</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">);</span>

 <span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">set_property </span><span class="symbol">=</span><span class="normal"> viewer_file_set_property</span><span class="symbol">;</span>
 <span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">get_property </span><span class="symbol">=</span><span class="normal"> viewer_file_get_property</span><span class="symbol">;</span>

 <span class="normal">  obj_properties</span><span class="symbol">[</span><span class="normal">PROP_FILENAME</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
 <span class="normal">    </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="normal"> </span><span class="symbol">(</span><span class="string">"filename"</span><span class="symbol">,</span>
 <span class="normal">                         </span><span class="string">"Filename"</span><span class="symbol">,</span>
 <span class="normal">                         </span><span class="string">"Name of the file to load and display from."</span><span class="symbol">,</span>
 <span class="normal">                         <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a>  </span><span class="comment">/* default value */</span><span class="symbol">,</span>
 <span class="normal">                         <a href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS">G_PARAM_CONSTRUCT_ONLY</a> </span><span class="symbol">|</span><span class="normal"> <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

 <span class="normal">  obj_properties</span><span class="symbol">[</span><span class="normal">PROP_ZOOM_LEVEL</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
 <span class="normal">    </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-uint">g_param_spec_uint</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"zoom-level"</span><span class="symbol">,</span>
 <span class="normal">                       </span><span class="string">"Zoom level"</span><span class="symbol">,</span>
 <span class="normal">                       </span><span class="string">"Zoom level to view the file at."</span><span class="symbol">,</span>
 <span class="normal">                       </span><span class="number">0</span><span class="normal">  </span><span class="comment">/* minimum value */</span><span class="symbol">,</span>
 <span class="normal">                       </span><span class="number">10</span><span class="normal"> </span><span class="comment">/* maximum value */</span><span class="symbol">,</span>
 <span class="normal">                       </span><span class="number">2</span><span class="normal">  </span><span class="comment">/* default value */</span><span class="symbol">,</span>
 <span class="normal">                       <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

 <span class="normal">  </span><span class="function"><a href="gobject-The-Base-Object-Type.html#g-object-class-install-properties">g_object_class_install_properties</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object_class</span><span class="symbol">,</span>
 <span class="normal">                                     N_PROPERTIES</span><span class="symbol">,</span>
 <span class="normal">                                     obj_properties</span><span class="symbol">);</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
       If you need this, make sure you can build and run code similar to the
       code shown above. Also, make sure your construct properties can be set
       without side effects during construction.
     </p>
 <p>
       Some people sometimes need to complete the initialization of a instance
       of a type only after the properties passed to the constructors have been
       set. This is possible through the use of the <code class="function">constructor()</code>
       class method as described in <a class="xref" href="chapter-gobject.html#gobject-instantiation" title="Object instantiation">the section called “Object instantiation”</a> or,
       more simply, using the <code class="function">constructed()</code> class method.
       Note that the <code class="function">constructed()</code>
       virtual function will only be invoked after the properties marked as
       <code class="function">G_PARAM_CONSTRUCT_ONLY</code> or
       <code class="function">G_PARAM_CONSTRUCT</code> have been consumed, but
       before the regular properties passed to <code class="function">g_object_new()</code>
       have been set.
     </p>
 </div>
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	<div class="sect1">
	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
	<a name="howto-gobject-construction"></a>Object construction</h2></div></div></div>
	<p>
	People often get confused when trying to construct their GObjects because of the
	sheer number of different ways to hook into the objects's construction process: it is
	difficult to figure which is the <span class="emphasis"><em>correct</em></span>, recommended way.
	</p>
	<p>
	<a class="xref" href="chapter-gobject.html#gobject-construction-table" title="Table 4. g_object_new">Table 4, “g_object_new”</a> shows what user-provided functions
	are invoked during object instantiation and in which order they are invoked.
	A user looking for the equivalent of the simple C++ constructor function should use
	the <code class="function">instance_init</code> method. It will be invoked after
	all the parents’ <code class="function">instance_init</code>
	functions have been invoked. It cannot take arbitrary construction parameters
	(as in C++) but if your object needs arbitrary parameters to complete initialization,
	you can use construction properties.
	</p>
	<p>
	Construction properties will be set only after all
	<code class="function">instance_init</code> functions have run.
	No object reference will be returned to the client of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>
	until all the construction properties have been set.
	</p>
	<p>
	It is important to note that object construction cannot <span class="emphasis"><em>ever</em></span>
	fail. If you require a fallible GObject construction, you can use the
	<a href="https://developer.gnome.org/gio/unstable/GInitable.html#GInitable-struct"><span class="type">GInitable</span></a> and
	<a href="https://developer.gnome.org/gio/unstable/GAsyncInitable.html#GAsyncInitable-struct"><span class="type">GAsyncInitable</span></a>
	interfaces provided by the GIO library.
	</p>
	<p>
	You should write the following code first:
	</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
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	15</pre></td>
	<td class="listing_code"><pre class="programlisting"><span class="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-PRIVATE:CAPS">G_DEFINE_TYPE_WITH_PRIVATE</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="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="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="usertype">ViewerFilePrivate</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">priv </span><span class="symbol">=</span><span class="normal"> </span><span class="function">viewer_file_get_instance_private</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>

	<span class="normal"> </span><span class="comment">/* initialize all public and private members to reasonable default values.</span>
	<span class="comment"> * They are all automatically initialized to 0 to begin with. */</span>
	<span class="cbracket">}</span></pre></td>
	</tr>
	</tbody>
	</table>
	</div>

	<p>
	</p>
	<p>
	If you need special construction properties (with
	<a class="link" href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS"><code class="function">G_PARAM_CONSTRUCT_ONLY</code></a>
	set), install the properties in
	the <code class="function">class_init()</code> function, override the <code class="function">set_property()</code>
	and <code class="function">get_property()</code> methods of the GObject class,
	and implement them as described by <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>.
	</p>
	<p>
	Property IDs must start from 1, as 0 is reserved for internal use by
	GObject.
	</p>
	<div class="informalexample">
	<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
	<tbody>
	<tr>
	<td class="listing_lines" align="right"><pre>1
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	16
	17
	18
	19
	20
	21
	22
	23
	24
	25
	26
	27
	28
	29
	30
	31
	32
	33
	34
	35
	36
	37</pre></td>
	<td class="listing_code"><pre class="programlisting"><span class="keyword">enum</span>
	<span class="cbracket">{</span>
	<span class="normal"> PROP_FILENAME </span><span class="symbol">=</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
	<span class="normal"> PROP_ZOOM_LEVEL</span><span class="symbol">,</span>
	<span class="normal"> N_PROPERTIES</span>
	<span class="cbracket">}</span><span class="symbol">;</span>

	<span class="keyword">static</span><span class="normal"> </span><span class="usertype">GParamSpec</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">obj_properties</span><span class="symbol">[</span><span class="normal">N_PROPERTIES</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</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="usertype">GObjectClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">object_class </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-CLASS:CAPS">G_OBJECT_CLASS</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">);</span>

	<span class="normal"> object_class</span><span class="symbol">-></span><span class="normal">set_property </span><span class="symbol">=</span><span class="normal"> viewer_file_set_property</span><span class="symbol">;</span>
	<span class="normal"> object_class</span><span class="symbol">-></span><span class="normal">get_property </span><span class="symbol">=</span><span class="normal"> viewer_file_get_property</span><span class="symbol">;</span>

	<span class="normal"> obj_properties</span><span class="symbol">[</span><span class="normal">PROP_FILENAME</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
	<span class="normal"> </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="normal"> </span><span class="symbol">(</span><span class="string">"filename"</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="string">"Filename"</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="string">"Name of the file to load and display from."</span><span class="symbol">,</span>
	<span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a> </span><span class="comment">/* default value */</span><span class="symbol">,</span>
	<span class="normal"> <a href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS">G_PARAM_CONSTRUCT_ONLY</a> </span><span class="symbol">\|</span><span class="normal"> <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

	<span class="normal"> obj_properties</span><span class="symbol">[</span><span class="normal">PROP_ZOOM_LEVEL</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
	<span class="normal"> </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-uint">g_param_spec_uint</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"zoom-level"</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="string">"Zoom level"</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="string">"Zoom level to view the file at."</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="number">0</span><span class="normal"> </span><span class="comment">/* minimum value */</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="number">10</span><span class="normal"> </span><span class="comment">/* maximum value */</span><span class="symbol">,</span>
	<span class="normal"> </span><span class="number">2</span><span class="normal"> </span><span class="comment">/* default value */</span><span class="symbol">,</span>
	<span class="normal"> <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

	<span class="normal"> </span><span class="function"><a href="gobject-The-Base-Object-Type.html#g-object-class-install-properties">g_object_class_install_properties</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object_class</span><span class="symbol">,</span>
	<span class="normal"> N_PROPERTIES</span><span class="symbol">,</span>
	<span class="normal"> obj_properties</span><span class="symbol">);</span>
	<span class="cbracket">}</span></pre></td>
	</tr>
	</tbody>
	</table>
	</div>

	<p>
	If you need this, make sure you can build and run code similar to the
	code shown above. Also, make sure your construct properties can be set
	without side effects during construction.
	</p>
	<p>
	Some people sometimes need to complete the initialization of a instance
	of a type only after the properties passed to the constructors have been
	set. This is possible through the use of the <code class="function">constructor()</code>
	class method as described in <a class="xref" href="chapter-gobject.html#gobject-instantiation" title="Object instantiation">the section called “Object instantiation”</a> or,
	more simply, using the <code class="function">constructed()</code> class method.
	Note that the <code class="function">constructed()</code>
	virtual function will only be invoked after the properties marked as
	<code class="function">G_PARAM_CONSTRUCT_ONLY</code> or
	<code class="function">G_PARAM_CONSTRUCT</code> have been consumed, but
	before the regular properties passed to <code class="function">g_object_new()</code>
	have been set.
	</p>
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