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<?xml version="1.0" encoding="UTF-8"?>
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<title>Qt 4.8: Style Plugin Example</title>
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<a href="index.html" class="qtref"><span>Qt Reference Documentation</span></a>
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<li class="first"><a href="index.html">Home</a></li>
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<li><a href="all-examples.html">Examples</a></li>
<li>Style Plugin Example</li>
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<div class="toc">
<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#simplestyleplugin-class-definition">SimpleStylePlugin Class Definition</a></li>
<li class="level1"><a href="#simplestyleplugin-class-implementation">SimpleStylePlugin Class Implementation</a></li>
<li class="level1"><a href="#the-function">The <tt>main()</tt> function</a></li>
<li class="level1"><a href="#the-simple-style-plugin-profile">The Simple Style Plugin Profile</a></li>
<li class="level1"><a href="#related-articles-and-examples">Related articles and examples</a></li>
</ul>
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<h1 class="title">Style Plugin Example</h1>
<span class="subtitle"></span>
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<div class="descr"> <a name="details"></a>
<p>Files:</p>
<ul>
<li><a href="tools-styleplugin-plugin-simplestyle-cpp.html">tools/styleplugin/plugin/simplestyle.cpp</a></li>
<li><a href="tools-styleplugin-plugin-simplestyle-h.html">tools/styleplugin/plugin/simplestyle.h</a></li>
<li><a href="tools-styleplugin-plugin-simplestyleplugin-cpp.html">tools/styleplugin/plugin/simplestyleplugin.cpp</a></li>
<li><a href="tools-styleplugin-plugin-simplestyleplugin-h.html">tools/styleplugin/plugin/simplestyleplugin.h</a></li>
<li><a href="tools-styleplugin-stylewindow-stylewindow-cpp.html">tools/styleplugin/stylewindow/stylewindow.cpp</a></li>
<li><a href="tools-styleplugin-stylewindow-stylewindow-h.html">tools/styleplugin/stylewindow/stylewindow.h</a></li>
<li><a href="tools-styleplugin-stylewindow-main-cpp.html">tools/styleplugin/stylewindow/main.cpp</a></li>
<li><a href="tools-styleplugin-styleplugin-pro.html">tools/styleplugin/styleplugin.pro</a></li>
<li><a href="tools-styleplugin-plugin-plugin-pro.html">tools/styleplugin/plugin/plugin.pro</a></li>
<li><a href="tools-styleplugin-stylewindow-stylewindow-pro.html">tools/styleplugin/stylewindow/stylewindow.pro</a></li>
</ul>
<p>The Style Plugin example shows how to create a plugin that extends Qt with a new GUI look and feel.<p class="centerAlign"><img src="images/stylepluginexample.png" alt="" /></p><p>On some platforms, the native style will prevent the button from having a red background. In this case, try to run the example in another style (e.g., plastique).</p>
<p>A plugin in Qt is a class stored in a shared library that can be loaded by a <a href="qpluginloader.html">QPluginLoader</a> at run-time. When you create plugins in Qt, they either extend a Qt application or Qt itself. Writing a plugin that extends Qt itself is achieved by inheriting one of the plugin <a href="plugins.html">base classes</a>, reimplementing functions from that class, and adding a macro. In this example we extend Qt by adding a new GUI look and feel (i.e., making a new <a href="qstyle.html">QStyle</a> available). A high-level introduction to plugins is given in the plugin <a href="plugins-howto.html">overview document</a>.</p>
<p>Plugins that provide new styles inherit the <a href="qstyleplugin.html">QStylePlugin</a> base class. Style plugins are loaded by Qt and made available through <a href="qstylefactory.html">QStyleFactory</a>; we will look at this later. We have implemented <tt>SimpleStylePlugin</tt>, which provides <tt>SimpleStyle</tt>. The new style inherits <a href="qwindowsstyle.html">QWindowsStyle</a> and contributes to widget styling by drawing button backgrounds in red - not a major contribution, but it still makes a new style. We test the plugin with <tt>StyleWindow</tt>, in which we display a <a href="qpushbutton.html">QPushButton</a>.</p>
<p>The <tt>SimpleStyle</tt> and <tt>StyleWindow</tt> classes do not contain any plugin specific functionality and their implementations are trivial; we will therefore leap past them and head on to the <tt>SimpleStylePlugin</tt> and the <tt>main()</tt> function. After we have looked at that, we examine the plugin's profile.</p>
<a name="simplestyleplugin-class-definition"></a>
<h2>SimpleStylePlugin Class Definition</h2>
<p><tt>SimpleStylePlugin</tt> inherits <a href="qstyleplugin.html">QStylePlugin</a> and is the plugin class.</p>
<pre class="cpp"> <span class="keyword">class</span> SimpleStylePlugin : <span class="keyword">public</span> <span class="type"><a href="qstyleplugin.html">QStylePlugin</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
SimpleStylePlugin() {};
<span class="type"><a href="qstringlist.html">QStringList</a></span> keys() <span class="keyword">const</span>;
<span class="type"><a href="qstyle.html">QStyle</a></span> <span class="operator">*</span>create(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>key);
};</pre>
<p><tt>keys()</tt> returns a list of style names that this plugin can create, while <tt>create()</tt> takes such a string and returns the <a href="qstyle.html">QStyle</a> corresponding to the key. Both functions are pure virtual functions reimplemented from <a href="qstyleplugin.html">QStylePlugin</a>. When an application requests an instance of the <tt>SimpleStyle</tt> style, which this plugin creates, Qt will create it with this plugin.</p>
<a name="simplestyleplugin-class-implementation"></a>
<h2>SimpleStylePlugin Class Implementation</h2>
<p>Here is the implementation of <tt>keys()</tt>:</p>
<pre class="cpp"> <span class="type"><a href="qstringlist.html">QStringList</a></span> SimpleStylePlugin<span class="operator">::</span>keys() <span class="keyword">const</span>
{
<span class="keyword">return</span> <span class="type"><a href="qstringlist.html">QStringList</a></span>() <span class="operator"><</span><span class="operator"><</span> <span class="string">"SimpleStyle"</span>;
}</pre>
<p>Since this plugin only supports one style, we return a <a href="qstringlist.html">QStringList</a> with the class name of that style.</p>
<p>Here is the <tt>create()</tt> function:</p>
<pre class="cpp"> <span class="type"><a href="qstyle.html">QStyle</a></span> <span class="operator">*</span>SimpleStylePlugin<span class="operator">::</span>create(<span class="keyword">const</span> <span class="type"><a href="qstring.html">QString</a></span> <span class="operator">&</span>key)
{
<span class="keyword">if</span> (key<span class="operator">.</span>toLower() <span class="operator">=</span><span class="operator">=</span> <span class="string">"simplestyle"</span>)
<span class="keyword">return</span> <span class="keyword">new</span> SimpleStyle;
<span class="keyword">return</span> <span class="number">0</span>;
}</pre>
<p>Note that the key for style plugins are case insensitive. The case sensitivity varies from plugin to plugin, so you need to check this when implementing new plugins.</p>
<a name="the-function"></a>
<h2>The <tt>main()</tt> function</h2>
<pre class="cpp"> <span class="type">int</span> main(<span class="type">int</span> argv<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>args<span class="operator">[</span><span class="operator">]</span>)
{
<span class="type"><a href="qapplication.html">QApplication</a></span> app(argv<span class="operator">,</span> args);
<span class="type"><a href="qapplication.html">QApplication</a></span><span class="operator">::</span>setStyle(<span class="type"><a href="qstylefactory.html">QStyleFactory</a></span><span class="operator">::</span>create(<span class="string">"simplestyle"</span>));
StyleWindow window;
window<span class="operator">.</span>resize(<span class="number">200</span><span class="operator">,</span> <span class="number">50</span>);
window<span class="operator">.</span>show();
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}</pre>
<p>Qt loads the available style plugins when the <a href="qapplication.html">QApplication</a> object is initialized. The <a href="qstylefactory.html">QStyleFactory</a> class knows about all styles and produces them with <a href="qstylefactory.html#create">create()</a> (it is a wrapper around all the style plugins).</p>
<a name="the-simple-style-plugin-profile"></a>
<h2>The Simple Style Plugin Profile</h2>
<p>The <tt>SimpleStylePlugin</tt> lives in its own directory and have its own profile:</p>
<pre class="cpp"> TEMPLATE = lib
CONFIG += plugin
HEADERS = simplestyle.h \
simplestyleplugin.h
SOURCES = simplestyle.cpp \
simplestyleplugin.cpp
TARGET = simplestyleplugin</pre>
<p>In the plugin profile we need to set the lib template as we are building a shared library instead of an executable. We must also set the config to plugin. We set the library to be stored in the styles folder under stylewindow because this is a path in which Qt will search for style plugins.</p>
<a name="related-articles-and-examples"></a>
<h2>Related articles and examples</h2>
<p>In addition to the plugin <a href="plugins-howto.html">overview document</a>, we have other examples and articles that concern plugins.</p>
<p>In the <a href="tools-echoplugin.html">echo plugin example</a> we show how to implement plugins that extends Qt applications rather than Qt itself, which is the case with the style plugin of this example. The <a href="tools-plugandpaint.html">plug & paint</a> example shows how to implement a static plugin as well as being a more involved example on plugins that extend applications.</p>
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