QML documents are loaded and executed by the QML runtime. This includes the Declarative UI engine along with the built-in QML elements and plugin modules, and it also provides access to third-party QML elements and modules.

Applications that use QML need to invoke the QML runtime in order to execute QML documents. This can be done by creating a QDeclarativeView or a QDeclarativeEngine, as described below. In addition, the Declarative UI package includes the Qt QML Viewer tool, which loads .qml files. This tool is useful for developing and testing QML code without the need to write a C++ application to load the QML runtime.

Deploying QML-based applications

To deploy an application that uses QML, the QML runtime must be invoked by the application. This is done by writing a Qt C++ application that loads the QDeclarativeEngine by either:

• Loading the QML file through a QDeclarativeView instance, or
• Creating a QDeclarativeEngine instance and loading QML files with QDeclarativeComponent

Deploying with QDeclarativeView

QDeclarativeView is a QWidget-based class that is able to load QML files. For example, if there is a QML file, application.qml, like this:

import QtQuick 1.0

Rectangle { width: 100; height: 100; color: "red" }

It can be loaded in a Qt application's main.cpp file like this:

#include <QApplication>
#include <QDeclarativeView>

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

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

    return app.exec();
}

This creates a QWidget-based view that displays the contents of application.qml.

The application's .pro project file must specify the declarative module for the QT variable. For example:

TEMPLATE += app
QT += gui declarative
SOURCES += main.cpp

Creating a QDeclarativeEngine directly

If application.qml does not have any graphical components, or if it is preferred to avoid QDeclarativeView for other reasons, the QDeclarativeEngine can be constructed directly instead. In this case, application.qml is loaded as a QDeclarativeComponent instance rather than placed into a view:

#include <QApplication>
#include <QDeclarativeEngine>
#include <QDeclarativeContext>
#include <QDeclarativeComponent>

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

    QDeclarativeEngine engine;
    QDeclarativeContext *objectContext = new QDeclarativeContext(engine.rootContext());

    QDeclarativeComponent component(&engine, "application.qml");
    QObject *object = component.create(objectContext);

    // ... delete object and objectContext when necessary

    return app.exec();
}

See Using QML Bindings in C++ Applications for more information about using QDeclarativeEngine, QDeclarativeContext and QDeclarativeComponent, as well as details on including QML files through Qt's Resource system.

Developing and prototyping with QML Viewer

The Declarative UI package includes a QML runtime tool, the Qt QML Viewer, which loads and displays QML documents. This is useful during the application development phase for prototyping QML-based applications without writing your own C++ applications to invoke the QML runtime.

See the QML Viewer documentation for more details.