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<h1 class="title">Chapter 6: Writing an Extension Plugin</h1>
<span class="subtitle"></span>
<!-- $$$declarative/tutorials/extending/chapter6-plugins-description -->
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<p>Files:</p>
<ul>
<li><a href="declarative-tutorials-extending-chapter6-plugins-app-qml.html">declarative/tutorials/extending/chapter6-plugins/app.qml</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-chartsplugin-cpp.html">declarative/tutorials/extending/chapter6-plugins/chartsplugin.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-chartsplugin-h.html">declarative/tutorials/extending/chapter6-plugins/chartsplugin.h</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-piechart-cpp.html">declarative/tutorials/extending/chapter6-plugins/piechart.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-piechart-h.html">declarative/tutorials/extending/chapter6-plugins/piechart.h</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-pieslice-cpp.html">declarative/tutorials/extending/chapter6-plugins/pieslice.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-pieslice-h.html">declarative/tutorials/extending/chapter6-plugins/pieslice.h</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-chapter6-plugins-pro.html">declarative/tutorials/extending/chapter6-plugins/chapter6-plugins.pro</a></li>
<li><a href="declarative-tutorials-extending-chapter6-plugins-qmldir.html">declarative/tutorials/extending/chapter6-plugins/qmldir</a></li>
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<p>Currently the <tt>PieChart</tt> and <tt>PieSlice</tt> types are used by <tt>app.qml</tt>, which is displayed using a <a href="qdeclarativeview.html">QDeclarativeView</a> in a C++ application. An alternative way to use our QML extension is to create a plugin library to make it available to the QML engine. This allows <tt>app.qml</tt> to be loaded with the <a href="qmlviewer.html">QML Viewer</a> (or some other QML <a href="qmlruntime.html">runtime</a> application) instead of writing a <tt>main.cpp</tt> file and loading our own C++ application.</p>
<p>To create a plugin library, we need:</p>
<ul>
<li>A plugin class that registers our QML types</li>
<li>A project file that describes the plugin</li>
<li>A <a href="qdeclarativemodules.html#writing-a-qmldir-file">qmldir</a> file that tells the QML engine to load the plugin</li>
</ul>
<p>First, we create a plugin class named <tt>ChartsPlugin</tt>. It subclasses <a href="qdeclarativeextensionplugin.html">QDeclarativeExtensionPlugin</a> and registers our QML types in the inherited <a href="qdeclarativeextensionplugin.html#registerTypes">registerTypes()</a> method. It also calls <a href="qtplugin.html#Q_EXPORT_PLUGIN2#q-export-plugin2">Q_EXPORT_PLUGIN2</a> for Qt's <a href="plugins-howto.html">plugin system</a>.</p>
<p>Here is the <tt>ChartsPlugin</tt> definition in <tt>chartsplugin.h</tt>:</p>
<pre class="cpp"> <span class="preprocessor">#include <QDeclarativeExtensionPlugin></span>
<span class="keyword">class</span> ChartsPlugin : <span class="keyword">public</span> <span class="type"><a href="qdeclarativeextensionplugin.html">QDeclarativeExtensionPlugin</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
<span class="type">void</span> registerTypes(<span class="keyword">const</span> <span class="type">char</span> <span class="operator">*</span>uri);
};</pre>
<p>And its implementation in <tt>chartsplugin.cpp</tt>:</p>
<pre class="cpp"> <span class="preprocessor">#include "piechart.h"</span>
<span class="preprocessor">#include "pieslice.h"</span>
<span class="preprocessor">#include <qdeclarative.h></span>
<span class="type">void</span> ChartsPlugin<span class="operator">::</span>registerTypes(<span class="keyword">const</span> <span class="type">char</span> <span class="operator">*</span>uri)
{
qmlRegisterType<span class="operator"><</span>PieChart<span class="operator">></span>(uri<span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> <span class="string">"PieChart"</span>);
qmlRegisterType<span class="operator"><</span>PieSlice<span class="operator">></span>(uri<span class="operator">,</span> <span class="number">1</span><span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> <span class="string">"PieSlice"</span>);
}
<a href="qtplugin.html#Q_EXPORT_PLUGIN2">Q_EXPORT_PLUGIN2</a>(chartsplugin<span class="operator">,</span> ChartsPlugin);</pre>
<p>Then, we write a <tt>.pro</tt> project file that defines the project as a plugin library and specifies with DESTDIR that library files should be built into a "lib" subdirectory:</p>
<pre class="cpp"> TEMPLATE = lib
CONFIG += qt plugin
QT += declarative
DESTDIR = lib
OBJECTS_DIR = tmp
MOC_DIR = tmp
HEADERS += piechart.h \
pieslice.h \
chartsplugin.h
SOURCES += piechart.cpp \
pieslice.cpp \
chartsplugin.cpp
symbian {
include($$QT_SOURCE_TREE/examples/symbianpkgrules.pri)
TARGET.EPOCALLOWDLLDATA = 1
}
maemo5: include($$QT_SOURCE_TREE/examples/maemo5pkgrules.pri)</pre>
<p>Finally, we add a <a href="qdeclarativemodules.html#writing-a-qmldir-file">qmldir</a> file that is automatically parsed by the QML engine. In this file, we specify that a plugin named "chapter6-plugin" (the name of the example project) can be found in the "lib" subdirectory:</p>
<pre class="cpp"> plugin chapter6<span class="operator">-</span>plugins lib</pre>
<p>Now we have a plugin, and instead of having a main.cpp and an executable, we can build the project and then load the QML file in the <a href="qmlviewer.html">QML Viewer</a>:</p>
<pre class="cpp"> qmlviewer app<span class="operator">.</span>qml</pre>
<p>(On Mac OS X, you can launch the "QMLViewer" application instead.)</p>
<p>Notice the "import Charts 1.0" statement has disappeared from <tt>app.qml</tt>. This is because the <tt>qmldir</tt> file is in the same directory as <tt>app.qml</tt>: this is equivalent to having PieChart.qml and PieSlice.qml files inside the project directory, which could both be used by <tt>app.qml</tt> without import statements.</p>
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