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<title>Qt 4.8: Chapter 4: Using Custom Property Types</title>
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<h1 class="title">Chapter 4: Using Custom Property Types</h1>
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<!-- $$$declarative/tutorials/extending/chapter4-customPropertyTypes-description -->
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<p>Files:</p>
<ul>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-app-qml.html">declarative/tutorials/extending/chapter4-customPropertyTypes/app.qml</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-piechart-cpp.html">declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-piechart-h.html">declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.h</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-pieslice-cpp.html">declarative/tutorials/extending/chapter4-customPropertyTypes/pieslice.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-pieslice-h.html">declarative/tutorials/extending/chapter4-customPropertyTypes/pieslice.h</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-main-cpp.html">declarative/tutorials/extending/chapter4-customPropertyTypes/main.cpp</a></li>
<li><a href="declarative-tutorials-extending-chapter4-custompropertytypes-chapter4-custompropertytypes-pro.html">declarative/tutorials/extending/chapter4-customPropertyTypes/chapter4-customPropertyTypes.pro</a></li>
</ul>
<p>The <tt>PieChart</tt> type currently has a string-type property and a color-type property. It could have many other types of properties. For example, it could have an int-type property to store an identifier for each chart:</p>
<pre class="cpp"> <span class="comment">// C++</span>
<span class="keyword">class</span> PieChart : <span class="keyword">public</span> <span class="type"><a href="qdeclarativeitem.html">QDeclarativeItem</a></span>
{
Q_PROPERTY(<span class="type">int</span> chartId READ chartId WRITE setChartId NOTIFY chartIdChanged)
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="keyword">public</span>:
<span class="type">void</span> setChartId(<span class="type">int</span> chartId);
<span class="type">int</span> chartId() <span class="keyword">const</span>;
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="keyword">signals</span>:
<span class="type">void</span> chartIdChanged();
};
<span class="comment">// QML</span>
PieChart {
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
chartId: <span class="number">100</span>
}</pre>
<p>We can also use various other property types. QML has built-in support for the types listed in the <a href="qdeclarativebasictypes.html">QML Basic Types</a> documentation, which includes the following:</p>
<ul>
<li>bool, unsigned int, int, float, double, qreal</li>
<li><a href="qstring.html">QString</a>, <a href="qurl.html">QUrl</a>, <a href="qcolor.html">QColor</a></li>
<li><a href="qdate.html">QDate</a>, <a href="qtime.html">QTime</a>, <a href="qdatetime.html">QDateTime</a></li>
<li><a href="qpoint.html">QPoint</a>, <a href="qpointf.html">QPointF</a>, <a href="qsize.html">QSize</a>, <a href="qsizef.html">QSizeF</a>, <a href="qrect.html">QRect</a>, <a href="qrectf.html">QRectF</a></li>
<li><a href="qvariant.html">QVariant</a></li>
</ul>
<p>If we want to create a property whose type is not supported by QML by default, we need to register the type with QML.</p>
<p>For example, let's replace the use of the <tt>property</tt> with a type called "PieSlice" that has a <tt>color</tt> property. Instead of assigning a color, we assign an <tt>PieSlice</tt> value which itself contains a <tt>color</tt>:</p>
<pre class="qml"> import Charts 1.0
import QtQuick 1.0
<span class="type"><a href="qml-item.html">Item</a></span> {
<span class="name">width</span>: <span class="number">300</span>; <span class="name">height</span>: <span class="number">200</span>
<span class="type">PieChart</span> {
<span class="name">id</span>: <span class="name">chart</span>
<span class="name">anchors</span>.centerIn: <span class="name">parent</span>
<span class="name">width</span>: <span class="number">100</span>; <span class="name">height</span>: <span class="number">100</span>
<span class="name">pieSlice</span>: <span class="name">PieSlice</span> {
<span class="name">anchors</span>.fill: <span class="name">parent</span>
<span class="name">color</span>: <span class="string">"red"</span>
}
}
<span class="name">Component</span>.onCompleted: <span class="name">console</span>.<span class="name">log</span>(<span class="string">"The pie is colored "</span> <span class="operator">+</span> <span class="name">chart</span>.<span class="name">pieSlice</span>.<span class="name">color</span>)
}</pre>
<p>Like <tt>PieChart</tt>, this new <tt>PieSlice</tt> type inherits from <a href="qdeclarativeitem.html">QDeclarativeItem</a> and declares its properties with <a href="qobject.html#Q_PROPERTY">Q_PROPERTY</a>():</p>
<pre class="cpp"> <span class="keyword">class</span> PieSlice : <span class="keyword">public</span> <span class="type"><a href="qdeclarativeitem.html">QDeclarativeItem</a></span>
{
Q_OBJECT
Q_PROPERTY(<span class="type"><a href="qcolor.html">QColor</a></span> color READ color WRITE setColor)
<span class="keyword">public</span>:
PieSlice(<span class="type"><a href="qdeclarativeitem.html">QDeclarativeItem</a></span> <span class="operator">*</span>parent <span class="operator">=</span> <span class="number">0</span>);
<span class="type"><a href="qcolor.html">QColor</a></span> color() <span class="keyword">const</span>;
<span class="type">void</span> setColor(<span class="keyword">const</span> <span class="type"><a href="qcolor.html">QColor</a></span> <span class="operator">&</span>color);
<span class="type">void</span> paint(<span class="type"><a href="qpainter.html">QPainter</a></span> <span class="operator">*</span>painter<span class="operator">,</span> <span class="keyword">const</span> <span class="type"><a href="qstyleoptiongraphicsitem.html">QStyleOptionGraphicsItem</a></span> <span class="operator">*</span>option<span class="operator">,</span> <span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>widget <span class="operator">=</span> <span class="number">0</span>);
<span class="keyword">private</span>:
<span class="type"><a href="qcolor.html">QColor</a></span> m_color;
};</pre>
<p>To use it in <tt>PieChart</tt>, we modify the <tt>color</tt> property declaration and associated method signatures:</p>
<pre class="cpp"> <span class="keyword">class</span> PieChart : <span class="keyword">public</span> <span class="type"><a href="qdeclarativeitem.html">QDeclarativeItem</a></span>
{
Q_OBJECT
Q_PROPERTY(PieSlice<span class="operator">*</span> pieSlice READ pieSlice WRITE setPieSlice)
...
<span class="keyword">public</span>:
...
PieSlice <span class="operator">*</span>pieSlice() <span class="keyword">const</span>;
<span class="type">void</span> setPieSlice(PieSlice <span class="operator">*</span>pieSlice);
...
};</pre>
<p>There is one thing to be aware of when implementing <tt>setPieSlice()</tt>. The <tt>PieSlice</tt> is a visual item, so it must be set as a child of the <tt>PieChart</tt> using <a href="qdeclarativeitem.html#parent-prop">QDeclarativeItem::setParentItem</a>() so that the <tt>PieChart</tt> knows to paint this child item when its contents are drawn:</p>
<pre class="cpp"> <span class="type">void</span> PieChart<span class="operator">::</span>setPieSlice(PieSlice <span class="operator">*</span>pieSlice)
{
m_pieSlice <span class="operator">=</span> pieSlice;
pieSlice<span class="operator">-</span><span class="operator">></span>setParentItem(<span class="keyword">this</span>);
}</pre>
<p>Like the <tt>PieChart</tt> type, the <tt>PieSlice</tt> type has to be registered using <a href="qdeclarativeengine.html#qmlRegisterType">qmlRegisterType</a>() to be used from QML. As with <tt>PieChart</tt>, we'll add the type to the "Charts" module, version 1.0:</p>
<pre class="cpp"> <span class="type">int</span> main(<span class="type">int</span> argc<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>argv<span class="operator">[</span><span class="operator">]</span>)
{
...
qmlRegisterType<span class="operator"><</span>PieSlice<span class="operator">></span>(<span class="string">"Charts"</span><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>);
...
}</pre>
<p>Try it out with the code in Qt's <tt>examples/tutorials/extending/chapter4-customPropertyTypes</tt> directory.</p>
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