- Qt 6.10
- Qt Qml
- C++ Classes
- QQmlApplicationEngine
- Qt 6.10.2 Reference Documentation

# QQmlApplicationEngine Class

QQmlApplicationEngine provides a convenient way to load an application from a single QML file. More...

Header: | `#include <QQmlApplicationEngine>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Qml)` `target_link_libraries(mytarget PRIVATE Qt6::Qml)`
qmake: | `QT += qml`
Inherits: | QQmlEngine

- List of all members, including inherited members

## Public Functions

| QQmlApplicationEngine(QObject *parent = nullptr)
| QQmlApplicationEngine(const QString &filePath, QObject *parent = nullptr)
| QQmlApplicationEngine(const QUrl &url, QObject *parent = nullptr)
`(since 6.5)` | QQmlApplicationEngine(QAnyStringView uri, QAnyStringView typeName, QObject *parent = nullptr)
virtual | ~QQmlApplicationEngine() override
QList<QObject *> | rootObjects() const

## Public Slots

void | load(const QString &filePath)
void | load(const QUrl &url)
void | loadData(const QByteArray &data, const QUrl &url = QUrl())
`(since 6.5)` void | loadFromModule(QAnyStringView uri, QAnyStringView typeName)
`(since 6.0)` void | setExtraFileSelectors(const QStringList &extraFileSelectors)
void | setInitialProperties(const QVariantMap &initialProperties)

## Signals

void | objectCreated(QObject *object, const QUrl &url)
`(since 6.4)` void | objectCreationFailed(const QUrl &url)

## Detailed Description

This class combines a QQmlEngine and QQmlComponent to provide a convenient way to load a single QML file. It also exposes some central application functionality to QML, which a C++/QML hybrid application would normally control from C++.

It can be used like so:

     #include <QGuiApplication>
     #include <QQmlApplicationEngine>

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

Unlike QQuickView, QQmlApplicationEngine does not automatically create a root window. If you are using visual items from Qt Quick, you will need to place them inside of a Window.

You can also use QCoreApplication with QQmlApplicationEngine, if you are not using any QML modules which require a QGuiApplication (such as `QtQuick`).

List of configuration changes from a default QQmlEngine:

- Connecting Qt.quit() to QCoreApplication::quit()
- Automatically loads translation files from an i18n directory adjacent to the main QML file.
- Translations are reloaded when the `QJSEngine::uiLanguage` / `Qt.uiLanguage` property is changed.
- Automatically sets an incubation controller if the scene contains a QQuickWindow.
- Automatically sets a `QQmlFileSelector` as the url interceptor, applying file selectors to all QML files and assets.

The engine behavior can be further tweaked by using the inherited methods from QQmlEngine.

Note: Translation files must have a qml_ prefix in order to be recognized, e.g. qml_ja_JP.qm.

Note: Placing translation files relative to the main QML file involves adding a RESOURCE_PREFIX to the relevant qt_add_translations call. This needs to include the resource prefix of the main file's QML module (/qt/qml by default) and the module URI. For example, to provide translation files for a module called "Translated":

     qt_add_translations(App
         RESOURCE_PREFIX /qt/qml/Translated/i18n
         TS_FILE_BASE qml
         TS_FILE_DIR i18n
     )

## 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 QString &filePath, QObject *parent = nullptr)

Create a new QQmlApplicationEngine and loads the QML file at the given filePath, which must be a local file or qrc 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.

### 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.

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

Destroys the QQmlApplicationEngine and all QML objects it loaded.

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

Loads the root QML file located at filePath. filePath must be a path to a local file or a path to a file in the resource file system. 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.

Note: This slot is overloaded. To connect to this slot:

     // Connect using qOverload:
     connect(sender, &SenderClass::signal,
             qmlApplicationEngine, qOverload<const QString &>(&QQmlApplicationEngine::load));

     // Or using a lambda as wrapper:
     connect(sender, &SenderClass::signal,
             qmlApplicationEngine, [receiver = qmlApplicationEngine](const QString &filePath) { receiver->load(filePath); });

For more examples and approaches, see connecting to overloaded slots.

### `[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.

Note: This slot is overloaded. To connect to this slot:

     // Connect using qOverload:
     connect(sender, &SenderClass::signal,
             qmlApplicationEngine, qOverload<const QUrl &>(&QQmlApplicationEngine::load));

     // Or using a lambda as wrapper:
     connect(sender, &SenderClass::signal,
             qmlApplicationEngine, [receiver = qmlApplicationEngine](const QUrl &url) { receiver->load(url); });

For more examples and approaches, see connecting to overloaded slots.

### `[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");

Note: The module identified by uri is searched in the import path, in the same way as if you were doing `import uri` inside a QML file. If the module cannot be located there, this function will fail.

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.

Note: If the path to the component was provided as a QString containing a relative path, the url will contain a fully resolved path to the file.

### `[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();

Note: If the path to the component was provided as a QString containing a relative path, the url will contain a fully resolved path to the file.

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.

Note: In Qt versions prior to 5.9, this function is marked as non-`const`.

### `[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.