Qt 5.13 | Qt QML | Interacting with QML Objects from C++

Qt 5.13.2 Reference Documentation

# 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.

## 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:

      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():

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

      // 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():

      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`:

      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 QQuickItem class:

      QQuickItem *item = qobject_cast<QQuickItem*>(object);
      item->setWidth(500);

You can also connect to any signals or call methods defined in the component using QMetaObject::invokeMethod() and QObject::connect(). See Invoking QML Methods and Connecting to QML Signals below for further details.

## Accessing Loaded QML Objects by Object Name

QML components are essentially object trees with children that have siblings and their own children. Child objects of QML components can be located using the QObject::objectName property with QObject::findChild(). For example, if the root item in `MyItem.qml` had a child Rectangle item:

      import QtQuick 2.0

      Item {
          width: 100; height: 100

          Rectangle {
              anchors.fill: parent
              objectName: "rect"
          }
      }

The child could be located like this:

      QObject *rect = object->findChild<QObject*>("rect");
      if (rect)
          rect->setProperty("color", "red");

Note that an object may have multiple children with the same `objectName`. For example, ListView creates multiple instances of its delegate, so if its delegate is declared with a particular objectName, the ListView will have multiple children with the same `objectName`. In this case, QObject::findChildren() can be used to find all children with a matching `objectName`.

Warning: Although it is possible to access QML objects from C++ and manipulate them, it is not the recommended approach, except for testing and prototyping purposes. One of the strengths of QML and C++ integration is the ability to implement UIs in QML separate from the C++ logic and dataset backend, and this fails if the C++ side starts manipulating QML directly. Such an approach also makes changing the QML UI difficult without affecting its C++ counterpart.

## Accessing Members of a QML Object Type from C++

### Properties

Any properties declared in a QML object are automatically accessible from C++. Given a QML item like this:

      // MyItem.qml
      import QtQuick 2.0

      Item {
          property int someNumber: 100
      }

The value of the `someNumber` property can be set and read using QQmlProperty, or QObject::setProperty() and QObject::property():

      QQmlEngine engine;
      QQmlComponent component(&engine, "MyItem.qml");
      QObject *object = component.create();

      qDebug() << "Property value:" << QQmlProperty::read(object, "someNumber").toInt();
      QQmlProperty::write(object, "someNumber", 5000);

      qDebug() << "Property value:" << object->property("someNumber").toInt();
      object->setProperty("someNumber", 100);

You should always use QObject::setProperty(), QQmlProperty or QMetaProperty::write() to change a QML property value, to ensure the QML engine is made aware of the property change. For example, say you have a custom type `PushButton` with a `buttonText` property that internally reflects the value of a `m_buttonText` member variable. Modifying the member variable directly like this is not a good idea:

      //bad code
      QQmlComponent component(engine, "MyButton.qml");
      PushButton *button = qobject_cast<PushButton*>(component.create());
      button->m_buttonText = "Click me";

Since the value is changed directly, this bypasses Qt's meta-object system and the QML engine is not made aware of the property change. This means property bindings to `buttonText` would not be updated, and any `onButtonTextChanged` handlers would not be called.

### Invoking QML Methods

All QML methods are exposed to the meta-object system and can be called from C++ using QMetaObject::invokeMethod(). Method parameters and return values passed from QML are always translated into QVariant values in C++.

Here is a C++ application that calls a QML method using QMetaObject::invokeMethod():

QML

      // MyItem.qml
      import QtQuick 2.0

      Item {
          function myQmlFunction(msg) {
              console.log("Got message:", msg)
              return "some return value"
          }
      }

C++

      // main.cpp
      QQmlEngine engine;
      QQmlComponent component(&engine, "MyItem.qml");
      QObject *object = component.create();

      QVariant returnedValue;
      QVariant msg = "Hello from C++";
      QMetaObject::invokeMethod(object, "myQmlFunction",
              Q_RETURN_ARG(QVariant, returnedValue),
              Q_ARG(QVariant, msg));

      qDebug() << "QML function returned:" << returnedValue.toString();
      delete object;

Notice the Q_RETURN_ARG() and Q_ARG() arguments for QMetaObject::invokeMethod() must be specified as QVariant types, as this is the generic data type used for QML method parameters and return values.

### Connecting to QML Signals

All QML signals are automatically available to C++, and can be connected to using QObject::connect() like any ordinary Qt C++ signal. In return, any C++ signal can be received by a QML object using signal handlers.

Here is a QML component with a signal named `qmlSignal` that is emitted with a string-type parameter. This signal is connected to a C++ object's slot using QObject::connect(), so that the `cppSlot()` method is called whenever the `qmlSignal` is emitted:

      // MyItem.qml
      import QtQuick 2.0

      Item {
          id: item
          width: 100; height: 100

          signal qmlSignal(string msg)

          MouseArea {
              anchors.fill: parent
              onClicked: item.qmlSignal("Hello from QML")
          }
      }

      class MyClass : public QObject
      {
          Q_OBJECT
      public slots:
          void cppSlot(const QString &msg) {
              qDebug() << "Called the C++ slot with message:" << msg;
          }
      };

      int main(int argc, char *argv[]) {
          QGuiApplication app(argc, argv);

          QQuickView view(QUrl::fromLocalFile("MyItem.qml"));
          QObject *item = view.rootObject();

          MyClass myClass;
          QObject::connect(item, SIGNAL(qmlSignal(QString)),
                           &myClass, SLOT(cppSlot(QString)));

          view.show();
          return app.exec();
      }

When a QML object type is used as a signal parameter, the parameter should use var as the type, and the value should be received in C++ using the QVariant type:

      // MyItem.qml
      import QtQuick 2.0

      Item {
          id: item
          width: 100; height: 100

          signal qmlSignal(var anObject)

          MouseArea {
              anchors.fill: parent
              onClicked: item.qmlSignal(item)
          }
      }

      class MyClass : public QObject
      {
          Q_OBJECT
      public slots:
          void cppSlot(const QVariant &v) {
             qDebug() << "Called the C++ slot with value:" << v;

             QQuickItem *item =
                 qobject_cast<QQuickItem*>(v.value<QObject*>());
             qDebug() << "Item dimensions:" << item->width()
                      << item->height();
          }
      };

      int main(int argc, char *argv[]) {
          QApplication app(argc, argv);

          QQuickView view(QUrl::fromLocalFile("MyItem.qml"));
          QObject *item = view.rootObject();

          MyClass myClass;
          QObject::connect(item, SIGNAL(qmlSignal(QVariant)),
                           &myClass, SLOT(cppSlot(QVariant)));

          view.show();
          return app.exec();
      }