QQmlApplicationEngine Class

 #include <QGuiApplication>
 #include <QQmlApplicationEngine>

 int main(int argc, char *argv[])
 {
     QGuiApplication app(argc, argv);
     QQmlApplicationEngine engine("main.qml");
     return app.exec();
 }

Member Function Documentation

QQmlApplicationEngine::QQmlApplicationEngine(QObject *parent = nullptr)

Create a new QQmlApplicationEngine with the given parent. You will have to call load() later in order to load a QML file.

QQmlApplicationEngine::QQmlApplicationEngine(const QUrl &url, QObject *parent = nullptr)

Create a new QQmlApplicationEngine and loads the QML file at the given url. This is provided as a convenience, and is the same as using the empty constructor and calling load afterwards.

[explicit, since 6.5] QQmlApplicationEngine::QQmlApplicationEngine(QAnyStringView uri, QAnyStringView typeName, QObject *parent = nullptr)

Create a new QQmlApplicationEngine and loads the QML type specified by uri and typeName This is provided as a convenience, and is the same as using the empty constructor and calling loadFromModule afterwards.

This function was introduced in Qt 6.5.

QQmlApplicationEngine::QQmlApplicationEngine(const QString &filePath, QObject *parent = nullptr)

Create a new QQmlApplicationEngine and loads the QML file at the given filePath, which must be a local file path. If a relative path is given then it will be interpreted as relative to the working directory of the application.

This is provided as a convenience, and is the same as using the empty constructor and calling load afterwards.

[override virtual noexcept] QQmlApplicationEngine::~QQmlApplicationEngine()

Destroys the QQmlApplicationEngine and all QML objects it loaded.

[slot] void QQmlApplicationEngine::load(const QUrl &url)

Loads the root QML file located at url. The object tree defined by the file is created immediately for local file urls. Remote urls are loaded asynchronously, listen to the objectCreated signal to determine when the object tree is ready.

If an error occurs, the objectCreated signal is emitted with a null pointer as parameter and error messages are printed with qWarning.

[slot] void QQmlApplicationEngine::load(const QString &filePath)

Loads the root QML file located at filePath. filePath must be a path to a local file. If filePath is a relative path, it is taken as relative to the application's working directory. The object tree defined by the file is instantiated immediately.

If an error occurs, error messages are printed with qWarning.

[slot] void QQmlApplicationEngine::loadData(const QByteArray &data, const QUrl &url = QUrl())

Loads the QML given in data. The object tree defined by data is instantiated immediately.

If a url is specified it is used as the base url of the component. This affects relative paths within the data and error messages.

If an error occurs, error messages are printed with qWarning.

[slot, since 6.5] void QQmlApplicationEngine::loadFromModule(QAnyStringView uri, QAnyStringView typeName)

Loads the QML type typeName from the module specified by uri. If the type originates from a QML file located at a remote url, the type will be loaded asynchronously. Listen to the objectCreated signal to determine when the object tree is ready.

If an error occurs, the objectCreated signal is emitted with a null pointer as parameter and error messages are printed with qWarning.

 QQmlApplicationEngine engine;
 engine.loadFromModule("QtQuick", "Rectangle");

This function was introduced in Qt 6.5.

See also QQmlComponent::loadFromModule.

[signal] void QQmlApplicationEngine::objectCreated(QObject *object, const QUrl &url)

This signal is emitted when an object finishes loading. If loading was successful, object contains a pointer to the loaded object, otherwise the pointer is NULL.

The url to the component the object came from is also provided.

[signal, since 6.4] void QQmlApplicationEngine::objectCreationFailed(const QUrl &url)

This signal is emitted when loading finishes because an error occurred.

The url to the component that failed to load is provided as an argument.

 QGuiApplication app(argc, argv);
 QQmlApplicationEngine engine;

 // exit on error
 QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
     &app, []() { QCoreApplication::exit(-1); }, Qt::QueuedConnection);
 engine.load(QUrl());
 return app.exec();

See also QQmlApplicationEngine::objectCreated, which will be emitted in addition to this signal (even though creation failed).

This function was introduced in Qt 6.4.

QList<QObject *> QQmlApplicationEngine::rootObjects() const

Returns a list of all the root objects instantiated by the QQmlApplicationEngine. This will only contain objects loaded via load() or a convenience constructor.

[slot, since 6.0] void QQmlApplicationEngine::setExtraFileSelectors(const QStringList &extraFileSelectors)

Sets the extraFileSelectors to be passed to the internal QQmlFileSelector used for resolving URLs to local files. The extraFileSelectors are applied when the first QML file is loaded. Setting them afterwards has no effect.

This function was introduced in Qt 6.0.

See also QQmlFileSelector and QFileSelector::setExtraSelectors.

[slot] void QQmlApplicationEngine::setInitialProperties(const QVariantMap &initialProperties)

Sets the initialProperties with which the QML component gets initialized after it gets loaded.

 QQmlApplicationEngine engine;

 EventDatabase eventDatabase;
 EventMonitor eventMonitor;

 engine.setInitialProperties({
     { "eventDatabase", QVariant::fromValue(&eventDatabase) },
     { "eventMonitor", QVariant::fromValue(&eventMonitor) }
 });

See also QQmlComponent::setInitialProperties, QQmlApplicationEngine::load, and QQmlApplicationEngine::loadData.