Embedding C++ Objects into QML with Context Properties

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.

The ability to inject C++ data into a QML object is made possible by the QQmlContext 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.

Setting a Simple Context Property

For example, here is a QML item that refers to a currentDateTime value that does not exist in the current scope:

// MyItem.qml
import QtQuick 2.0

Text { text: currentDateTime }

This currentDateTime value can be set directly by the C++ application that loads the QML component, using QQmlContext::setContextProperty():

QQuickView view;
view.rootContext()->setContextProperty("currentDateTime", QDateTime::currentDateTime());
view.setSource(QUrl::fromLocalFile("MyItem.qml"));
view.show();

Note: 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.

Setting an Object as a Context Property

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:

class ApplicationData : public QObject
{
    Q_OBJECT
public:
    Q_INVOKABLE QDateTime getCurrentDateTime() const {
        return QDateTime::currentDateTime();
    }
};

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

    QQuickView view;

    ApplicationData data;
    view.rootContext()->setContextProperty("applicationData", &data);

    view.setSource(QUrl::fromLocalFile("MyItem.qml"));
    view.show();

    return app.exec();
}

// MyItem.qml
import QtQuick 2.0

Text { text: applicationData.getCurrentDateTime() }

(Note that date/time values returned from C++ to QML can be formatted through Qt.formatDateTime() and associated functions.)

If the QML item needs to receive signals from the context property, it can connect to them using the Connections type. For example, if ApplicationData has a signal named dataChanged(), this signal can be connected to using an onDataChanged handler within a Connections object:

Text {
    text: applicationData.getCurrentDateTime()

    Connections {
        target: applicationData
        onDataChanged: console.log("The application data changed!")
    }
}

Context properties can be useful for using C++ based data models in a QML view. See the following examples:

• String ListModel
• Object ListModel
• AbstractItemModel

demonstrating the use of QStringList, QList<QObject*>-based models and QAbstractItemModel in QML views.

Also see the QQmlContext documentation for more information.