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<?xml version="1.0" encoding="UTF-8"?>
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<title>Qt 4.8: Integrating QML Code with Existing Qt UI Code</title>
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<li>Integrating QML Code with Existing Qt UI Code</li>
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<li class="level1"><a href="#integrating-with-a-qwidget-based-ui">Integrating with a <a href="qwidget.html">QWidget</a>-based UI</a></li>
<li class="level1"><a href="#integrating-with-a-qgraphicsview-based-ui">Integrating with a QGraphicsView-based UI</a></li>
<li class="level2"><a href="#adding-qml-widgets-to-a-qgraphicsscene">Adding QML widgets to a QGraphicsScene</a></li>
<li class="level2"><a href="#loading-qgraphicswidget-objects-in-qml">Loading QGraphicsWidget objects in QML</a></li>
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<h1 class="title">Integrating QML Code with Existing Qt UI Code</h1>
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<p>There are a number of ways to integrate QML into <a href="qwidget.html">QWidget</a>-based UI applications, depending on the characteristics of your existing UI code.</p>
<a name="integrating-with-a-qwidget-based-ui"></a>
<h2>Integrating with a <a href="qwidget.html">QWidget</a>-based UI</h2>
<p>If you have an existing <a href="qwidget.html">QWidget</a>-based UI, QML widgets can be integrated into it using <a href="qdeclarativeview.html">QDeclarativeView</a>. <a href="qdeclarativeview.html">QDeclarativeView</a> is a subclass of <a href="qwidget.html">QWidget</a> so you can add it to your user interface like any other <a href="qwidget.html">QWidget</a>. Use <a href="qdeclarativeview.html#source-prop">QDeclarativeView::setSource</a>() to load a QML file into the view, then add the view to your UI:</p>
<pre class="cpp"> <span class="type"><a href="qdeclarativeview.html">QDeclarativeView</a></span> <span class="operator">*</span>qmlView <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qdeclarativeview.html">QDeclarativeView</a></span>;
qmlView<span class="operator">-</span><span class="operator">></span>setSource(<span class="type"><a href="qurl.html">QUrl</a></span><span class="operator">::</span>fromLocalFile(<span class="string">"myqml.qml"</span>));
<span class="type"><a href="qwidget.html">QWidget</a></span> <span class="operator">*</span>widget <span class="operator">=</span> myExistingWidget();
<span class="type"><a href="qvboxlayout.html">QVBoxLayout</a></span> <span class="operator">*</span>layout <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qvboxlayout.html">QVBoxLayout</a></span>(widget);
layout<span class="operator">-</span><span class="operator">></span>addWidget(qmlView);</pre>
<p>The one drawback to this approach is that <a href="qdeclarativeview.html">QDeclarativeView</a> is slower to initialize and uses more memory than a <a href="qwidget.html">QWidget</a>, and creating large numbers of <a href="qdeclarativeview.html">QDeclarativeView</a> objects may lead to performance degradation. If this is the case, it may be better to rewrite your widgets in QML, and load the widgets from a main QML widget instead of using <a href="qdeclarativeview.html">QDeclarativeView</a>.</p>
<p>Keep in mind that QWidgets were designed for a different type of user interface than QML, so it is not always a good idea to port a <a href="qwidget.html">QWidget</a>-based application to QML. QWidgets are a better choice if your UI is comprised of a small number of complex and static elements, and QML is a better choice if your UI is comprised of a large number of simple and dynamic elements.</p>
<a name="integrating-with-a-qgraphicsview-based-ui"></a>
<h2>Integrating with a QGraphicsView-based UI</h2>
<a name="adding-qml-widgets-to-a-qgraphicsscene"></a>
<h3>Adding QML widgets to a QGraphicsScene</h3>
<p>If you have an existing UI based on the <a href="graphicsview.html">Graphics View Framework</a>, you can integrate QML widgets directly into your <a href="qgraphicsscene.html">QGraphicsScene</a>. Use <a href="qdeclarativecomponent.html">QDeclarativeComponent</a> to create a <a href="qgraphicsobject.html">QGraphicsObject</a> from a QML file, and place the graphics object into your scene using <a href="qgraphicsscene.html#addItem">QGraphicsScene::addItem</a>(), or reparent it to an item already in the <a href="qgraphicsscene.html">QGraphicsScene</a>.</p>
<p>For example:</p>
<pre class="cpp"> <span class="type"><a href="qgraphicsscene.html">QGraphicsScene</a></span><span class="operator">*</span> scene <span class="operator">=</span> myExistingGraphicsScene();
<span class="type"><a href="qdeclarativeengine.html">QDeclarativeEngine</a></span> <span class="operator">*</span>engine <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qdeclarativeengine.html">QDeclarativeEngine</a></span>;
<span class="type"><a href="qdeclarativecomponent.html">QDeclarativeComponent</a></span> component(engine<span class="operator">,</span> <span class="type"><a href="qurl.html">QUrl</a></span><span class="operator">::</span>fromLocalFile(<span class="string">"myqml.qml"</span>));
<span class="type"><a href="qgraphicsobject.html">QGraphicsObject</a></span> <span class="operator">*</span>object <span class="operator">=</span>
qobject_cast<span class="operator"><</span><span class="type"><a href="qgraphicsobject.html">QGraphicsObject</a></span> <span class="operator">*</span><span class="operator">></span>(component<span class="operator">.</span>create());
scene<span class="operator">-</span><span class="operator">></span>addItem(object);</pre>
<p>The following <a href="qgraphicsview.html">QGraphicsView</a> options are recommended for optimal performance of QML UIs:</p>
<ul>
<li>QGraphicsView::setOptimizationFlags(<a href="qgraphicsview.html#OptimizationFlag-enum">QGraphicsView::DontSavePainterState</a>)</li>
<li>QGraphicsView::setViewportUpdateMode(<a href="qgraphicsview.html#ViewportUpdateMode-enum">QGraphicsView::BoundingRectViewportUpdate</a>)</li>
<li>QGraphicsScene::setItemIndexMethod(<a href="qgraphicsscene.html#ItemIndexMethod-enum">QGraphicsScene::NoIndex</a>)</li>
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<h3>Loading QGraphicsWidget objects in QML</h3>
<p>An alternative approach is to expose your existing <a href="qgraphicswidget.html">QGraphicsWidget</a> objects to QML and construct your scene in QML instead. See the <a href="declarative-cppextensions-qgraphicslayouts.html">graphics layouts example</a> which shows how to expose Qt's graphics layout classes to QML in order to use <a href="qgraphicswidget.html">QGraphicsWidget</a> with classes like <a href="qgraphicslinearlayout.html">QGraphicsLinearLayout</a> and <a href="qgraphicsgridlayout.html">QGraphicsGridLayout</a>.</p>
<p>To expose your existing <a href="qgraphicswidget.html">QGraphicsWidget</a> classes to QML, use <a href="qdeclarativeengine.html#qmlRegisterType">qmlRegisterType</a>(). See <a href="qml-extending.html">Extending QML Functionalities using C++</a> for further information on how to use C++ types in QML.</p>
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