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<h1 class="title">Integrating QML and C++</h1>
<span class="subtitle"></span>
<!-- $$$qtqml-cppintegration-topic.html-description -->
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<p>QML applications often need to handle more advanced and performance-intensive tasks in C++. The most common and quickest way to do this is to expose the C++ class to the QML runtime, provided the C++ implementation is derived from QObject. Assuming that you have Qt 5.7 or later installed, the following step-by-step instructions guide you through the process of using the C++ class, BackEnd, in a QML application:</p>
<ol class="1" type="1"><li>Create a new project using the "Qt Quick Application" template in Qt Creator<p><b>Note: </b>Uncheck the <b>With ui.qml file</b> option in the <b>Define Project Details</b> section of <b>New Project Wizard</b>.</p></li>
<li>Add a new C++ class called <code>BackEnd</code> to the project and replace its header file contents with:<pre class="cpp">
<span class="preprocessor">#ifndef BACKEND_H</span>
<span class="preprocessor">#define BACKEND_H</span>
<span class="preprocessor">#include <QObject></span>
<span class="preprocessor">#include <QString></span>
<span class="keyword">class</span> BackEnd : <span class="keyword">public</span> <span class="type">QObject</span>
{
Q_OBJECT
Q_PROPERTY(<span class="type">QString</span> userName READ userName WRITE setUserName NOTIFY userNameChanged)
<span class="keyword">public</span>:
<span class="keyword">explicit</span> BackEnd(<span class="type">QObject</span> <span class="operator">*</span>parent <span class="operator">=</span> nullptr);
<span class="type">QString</span> userName();
<span class="type">void</span> setUserName(<span class="keyword">const</span> <span class="type">QString</span> <span class="operator">&</span>userName);
<span class="keyword">signals</span>:
<span class="type">void</span> userNameChanged();
<span class="keyword">private</span>:
<span class="type">QString</span> m_userName;
};
<span class="preprocessor">#endif // BACKEND_H</span>
</pre>
<p>The <code>Q_PROPERTY</code> macro declares a property that could be accessed from QML.</p>
</li>
<li>Replace its C++ file contents with:<pre class="cpp">
<span class="preprocessor">#include "backend.h"</span>
BackEnd<span class="operator">::</span>BackEnd(<span class="type">QObject</span> <span class="operator">*</span>parent) :
<span class="type">QObject</span>(parent)
{
}
<span class="type">QString</span> BackEnd<span class="operator">::</span>userName()
{
<span class="keyword">return</span> m_userName;
}
<span class="type">void</span> BackEnd<span class="operator">::</span>setUserName(<span class="keyword">const</span> <span class="type">QString</span> <span class="operator">&</span>userName)
{
<span class="keyword">if</span> (userName <span class="operator">=</span><span class="operator">=</span> m_userName)
<span class="keyword">return</span>;
m_userName <span class="operator">=</span> userName;
<span class="keyword">emit</span> userNameChanged();
}
</pre>
<p>The <code>setUserName</code> function emits the <code>userNameChanged</code> signal every time <code>m_userName</code> value changes. The signal can be handled from QML using the <code>onUserNameChanged</code> handler.</p>
</li>
<li>Include <code>"backend.h"</code> in <code>main.cpp</code> and register the class as a QML type under a import URL as shown below:<pre class="cpp">
<span class="preprocessor">#include <QGuiApplication></span>
<span class="preprocessor">#include <QQmlApplicationEngine></span>
<span class="preprocessor">#include "backend.h"</span>
<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);
qmlRegisterType<span class="operator"><</span>BackEnd<span class="operator">></span>(<span class="string">"io.qt.examples.backend"</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">"BackEnd"</span>);
<span class="type"><a href="qqmlapplicationengine.html">QQmlApplicationEngine</a></span> engine;
engine<span class="operator">.</span>load(<span class="type">QUrl</span>(<span class="type">QStringLiteral</span>(<span class="string">"qrc:/main.qml"</span>)));
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}
</pre>
<p>The BackEnd class is registered as a type, which is accessible from QML by importing the URL, "<code>io.qt.examples.backend 1.0</code>".</p>
</li>
<li>Replace the contents of <code>main.qml</code> with the following code:<pre class="qml">
import QtQuick 2.6
import QtQuick.Controls 2.0
import io.qt.examples.backend 1.0
ApplicationWindow {
id: root
width: 300
height: 480
visible: true
BackEnd {
id: backend
}
TextField {
text: backend.userName
placeholderText: qsTr("User name")
anchors.centerIn: parent
onTextChanged: backend.userName = text
}
}
</pre>
<p>The <code>BackEnd</code> instance lets you access the <code>userName</code> property, which is updated when the TextField's <code>text</code> property changes.</p>
</li>
</ol>
<p>Now the application can be run.</p>
<div class="border"><p class="centerAlign"><img src="images/cppintegration-ex.png" alt="" /></p></div><p class="figCaption">Application running on Ubuntu</p>
<p>Qt offers several methods to integrate C++ with QML, and the method discussed in this tutorial is just one of them. For more details about these methods, refer to <a href="qtqml-cppintegration-overview.html">Overview - QML and C++ Integration</a>.</p>
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