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<td >Qt 5.12</td><td ><a href="qtqml-index.html">Qt QML</a></td><td >Embedding C++ Objects into QML with Context Properties</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right"><a href="qtqml-index.html">Qt 5.12.6 Reference Documentation</a></td>
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<h3><a name="toc">Contents</a></h3>
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<li class="level1"><a href="#setting-a-simple-context-property">Setting a Simple Context Property</a></li>
<li class="level1"><a href="#setting-an-object-as-a-context-property">Setting an Object as a Context Property</a></li>
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<h1 class="title">Embedding C++ Objects into QML with Context Properties</h1>
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<!-- $$$qtqml-cppintegration-contextproperties.html-description -->
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<p>When loading a QML object into a C++ application, it can be useful to directly embed some C++ data that can be used from within the QML code. This makes it possible, for example, to invoke a C++ method on the embedded object, or use a C++ object instance as a data model for a QML view.</p>
<p>The ability to inject C++ data into a QML object is made possible by the <a href="qqmlcontext.html">QQmlContext</a> class. This class exposes data to the context of a QML object so that the data can be referred to directly from within the scope of the QML code.</p>
<a name="setting-a-simple-context-property"></a>
<h2 id="setting-a-simple-context-property">Setting a Simple Context Property</h2>
<p>For example, here is a QML item that refers to a <code>currentDateTime</code> value that does not exist in the current scope:</p>
<pre class="qml">
// MyItem.qml
import QtQuick 2.0
Text { text: currentDateTime }
</pre>
<p>This <code>currentDateTime</code> value can be set directly by the C++ application that loads the QML component, using <a href="qqmlcontext.html#setContextProperty">QQmlContext::setContextProperty</a>():</p>
<pre class="cpp">
<span class="type">QQuickView</span> view;
view<span class="operator">.</span>rootContext()<span class="operator">-</span><span class="operator">></span>setContextProperty(<span class="string">"currentDateTime"</span><span class="operator">,</span> <span class="type">QDateTime</span><span class="operator">::</span>currentDateTime());
view<span class="operator">.</span>setSource(<span class="type">QUrl</span><span class="operator">::</span>fromLocalFile(<span class="string">"MyItem.qml"</span>));
view<span class="operator">.</span>show();
</pre>
<p><b>Note: </b>Since all expressions evaluated in QML are evaluated in a particular context, if the context is modified, all bindings in that context will be re-evaluated. Thus, context properties should be used with care outside of application initialization, as this may lead to decreased application performance.</p><a name="setting-an-object-as-a-context-property"></a>
<h2 id="setting-an-object-as-a-context-property">Setting an Object as a Context Property</h2>
<p>Context properties can hold either QVariant or QObject* values. This means custom C++ objects can also be injected using this approach, and these objects can be modified and read directly in QML. Here, we modify the above example to embed a QObject instance instead of a QDateTime value, and the QML code invokes a method on the object instance:</p>
<div class="table"><table class="generic">
<tr valign="top" class="odd"><td >C++</td><td ><pre class="cpp">
<span class="keyword">class</span> ApplicationData : <span class="keyword">public</span> <span class="type">QObject</span>
{
Q_OBJECT
<span class="keyword">public</span>:
Q_INVOKABLE <span class="type">QDateTime</span> getCurrentDateTime() <span class="keyword">const</span> {
<span class="keyword">return</span> <span class="type">QDateTime</span><span class="operator">::</span>currentDateTime();
}
};
<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>) {
<span class="type">QGuiApplication</span> app(argc<span class="operator">,</span> argv);
<span class="type">QQuickView</span> view;
ApplicationData data;
view<span class="operator">.</span>rootContext()<span class="operator">-</span><span class="operator">></span>setContextProperty(<span class="string">"applicationData"</span><span class="operator">,</span> <span class="operator">&</span>data);
view<span class="operator">.</span>setSource(<span class="type">QUrl</span><span class="operator">::</span>fromLocalFile(<span class="string">"MyItem.qml"</span>));
view<span class="operator">.</span>show();
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}
</pre>
</td></tr>
<tr valign="top" class="even"><td >QML</td><td ><pre class="qml">
// MyItem.qml
import QtQuick 2.0
Text { text: applicationData.getCurrentDateTime() }
</pre>
</td></tr>
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<p>(Note that date/time values returned from C++ to QML can be formatted through <a href="qml-qtqml-qt.html#formatDateTime-method">Qt.formatDateTime()</a> and associated functions.)</p>
<p>If the QML item needs to receive signals from the context property, it can connect to them using the <a href="qml-qtqml-connections.html">Connections</a> type. For example, if <code>ApplicationData</code> has a signal named <code>dataChanged()</code>, this signal can be connected to using an <code>onDataChanged</code> handler within a <a href="qml-qtqml-connections.html">Connections</a> object:</p>
<pre class="qml">
Text {
text: applicationData.getCurrentDateTime()
Connections {
target: applicationData
onDataChanged: console.log("The application data changed!")
}
}
</pre>
<p>Context properties can be useful for using C++ based data models in a QML view. See the following examples:</p>
<ul>
<li>String ListModel</li>
<li>Object ListModel</li>
<li>AbstractItemModel</li>
</ul>
<p>demonstrating the use of QStringList, QList<QObject*>-based models and QAbstractItemModel in QML views.</p>
<p>Also see the <a href="qqmlcontext.html">QQmlContext</a> documentation for more information.</p>
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