Interacting with QML Objects from C++▲

All QML object types are QObject-derived types, whether they are internally implemented by the engine or defined by third-party sources. This means the QML engine can use the Qt Meta Object System to dynamically instantiate any QML object type and inspect the created objects.

This is useful for creating QML objects from C++ code, whether to display a QML object that can be visually rendered, or to integrate non-visual QML object data into a C++ application. Once a QML object is created, it can be inspected from C++ in order to read and write to properties, invoke methods and receive signal notifications.

For more information about C++ and the different QML integration methods, see the C++ and QML integration overview page.

Loading QML Objects from C++▲

A QML document can be loaded with QQmlComponent or QQuickView. QQmlComponent loads a QML document as a C++ object that can then be modified from C++ code. QQuickView also does this, but as QQuickView is a QWindow-derived class, the loaded object will also be rendered into a visual display; QQuickView is generally used to integrate a displayable QML object into an application's user interface.

For example, suppose there is a MyItem.qml file that looks like this:

 
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import QtQuick 2.0

Item {
    width: 100; height: 100
}

This QML document can be loaded with QQmlComponent or QQuickView with the following C++ code. Using a QQmlComponent requires calling QQmlComponent::create() to create a new instance of the component, while a QQuickView automatically creates an instance of the component, which is accessible via QQuickView::rootObject():

 
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// Using QQmlComponent
QQmlEngine engine;
QQmlComponent component(&engine,
        QUrl::fromLocalFile("MyItem.qml"));
QObject *object = component.create();
...
delete object;
 
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// Using QQuickView
QQuickView view;
view.setSource(QUrl::fromLocalFile("MyItem.qml"));
view.show();
QObject *object = view.rootObject();

This object is the instance of the MyItem.qml component that has been created. You can now modify the item's properties using QObject::setProperty() or QQmlProperty::write():

 
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object->setProperty("width", 500);
QQmlProperty(object, "width").write(500);

The difference between QObject::setProperty() and QQmlProperty::write() is that the latter will also remove the binding in addition to setting the property value. For example, suppose the width assignment above had been a binding to height:

 
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width: height

If the height of the Item changed after the object->setProperty("width", 500) call, the width would be updated again, as the binding remains active. However, if the height changes after the QQmlProperty(object, "width").write(500) call, the width will not be changed, as the binding does not exist anymore.

Alternatively, you can cast the object to its actual type and call methods with compile-time safety. In this case the base object of MyItem.qml is an Item, which is defined by the QQuic