QQmlComponent Class

 import QtQuick 2.0

 Item {
     width: 200
     height: 200
 }

 QQmlEngine *engine = new QQmlEngine;
 QQmlComponent component(engine, QUrl::fromLocalFile("main.qml"));

 QObject *myObject = component.create();
 QQuickItem *item = qobject_cast<QQuickItem*>(myObject);
 int width = item->width();  // width = 200

 void MyCppItem::init()
 {
     QQmlEngine *engine = qmlEngine(this);
     // Or:
     // QQmlEngine *engine = qmlContext(this)->engine();
     QQmlComponent component(engine, QUrl::fromLocalFile("MyItem.qml"));
     QQuickItem *childItem = qobject_cast<QQuickItem*>(component.create());
     childItem->setParentItem(this);
 }

 MyApplication::MyApplication()
 {
     // ...
     component = new QQmlComponent(engine, QUrl("http://www.example.com/main.qml"));
     if (component->isLoading()) {
         QObject::connect(component, &QQmlComponent::statusChanged,
                          this, &MyApplication::continueLoading);
     } else {
         continueLoading();
     }
 }

 void MyApplication::continueLoading()
 {
     if (component->isError()) {
         qWarning() << component->errors();
     } else {
         QObject *myObject = component->create();
     }
 }

Member Function Documentation

QQmlComponent::QQmlComponent(QQmlEngine *engine, QObject *parent = nullptr)

Create a QQmlComponent with no data and give it the specified engine and parent. Set the data with setData().

QQmlComponent::QQmlComponent(QQmlEngine *engine, const QString &fileName, QObject *parent = nullptr)

Create a QQmlComponent from the given fileName and give it the specified parent and engine.

See also loadUrl().

QQmlComponent::QQmlComponent(QQmlEngine *engine, const QUrl &url, QObject *parent = nullptr)

Create a QQmlComponent from the given url and give it the specified parent and engine.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

Relative paths will be resolved against QQmlEngine::baseUrl(), which is the current working directory unless specified.

See also loadUrl().

QQmlComponent::QQmlComponent(QQmlEngine *engine, const QString &fileName, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)

Create a QQmlComponent from the given fileName and give it the specified parent and engine. If mode is Asynchronous, the component will be loaded and compiled asynchronously.

See also loadUrl().

QQmlComponent::QQmlComponent(QQmlEngine *engine, const QUrl &url, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)

Create a QQmlComponent from the given url and give it the specified parent and engine. If mode is Asynchronous, the component will be loaded and compiled asynchronously.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

Relative paths will be resolved against QQmlEngine::baseUrl(), which is the current working directory unless specified.

See also loadUrl().

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

Create a QQmlComponent from the given uri and typeName and give it the specified parent and engine. If possible, the component will be loaded synchronously.

This is an overloaded function.

This function was introduced in Qt 6.5.

See also loadFromModule().

[explicit, since 6.5] QQmlComponent::QQmlComponent(QQmlEngine *engine, QAnyStringView uri, QAnyStringView typeName, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)

Create a QQmlComponent from the given uri and typeName and give it the specified parent and engine. If mode is Asynchronous, the component will be loaded and compiled asynchronously.

This is an overloaded function.

This function was introduced in Qt 6.5.

See also loadFromModule().

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

Destruct the QQmlComponent.

[virtual] QObject *QQmlComponent::beginCreate(QQmlContext *context)

Create an object instance from this component, within the specified context. Returns nullptr if creation failed.

When QQmlComponent constructs an instance, it occurs in three steps:

1. The object hierarchy is created, and constant values are assigned.
2. Property bindings are evaluated for the first time.
3. If applicable, QQmlParserStatus::componentComplete() is called on objects.

QQmlComponent::beginCreate() differs from QQmlComponent::create() in that it only performs step 1. QQmlComponent::completeCreate() must be called to complete steps 2 and 3.

This breaking point is sometimes useful when using attached properties to communicate information to an instantiated component, as it allows their initial values to be configured before property bindings take effect.

The ownership of the returned object instance is transferred to the caller.

See also completeCreate() and QQmlEngine::ObjectOwnership.

[virtual] void QQmlComponent::completeCreate()

This method provides advanced control over component instance creation. In general, programmers should use QQmlComponent::create() to create a component.

This function completes the component creation begun with QQmlComponent::beginCreate() and must be called afterwards.

See also beginCreate().

[virtual] QObject *QQmlComponent::create(QQmlContext *context = nullptr)

Create an object instance from this component, within the specified context. Returns nullptr if creation failed.

If context is nullptr (the default), it will create the instance in the root context of the engine.

The ownership of the returned object instance is transferred to the caller.

If the object being created from this component is a visual item, it must have a visual parent, which can be set by calling QQuickItem::setParentItem(). See Concepts - Visual Parent in Qt Quick for more details.

See also QQmlEngine::ObjectOwnership.

void QQmlComponent::create(QQmlIncubator &incubator, QQmlContext *context = nullptr, QQmlContext *forContext = nullptr)

Create an object instance from this component using the provided incubator. context specifies the context within which to create the object instance.

If context is nullptr (by default), it will create the instance in the engine's root context.

forContext specifies a context that this object creation depends upon. If the forContext is being created asynchronously, and the QQmlIncubator::IncubationMode is QQmlIncubator::AsynchronousIfNested, this object will also be created asynchronously. If forContext is nullptr (by default), the context will be used for this decision.

The created object and its creation status are available via the incubator.

See also QQmlIncubator.

QObject *QQmlComponent::createWithInitialProperties(const QVariantMap &initialProperties, QQmlContext *context = nullptr)

Create an object instance of this component, within the specified context, and initialize its top-level properties with initialProperties.

If any of the initialProperties cannot be set, a warning is issued. If there are unset required properties, the object creation fails and returns nullptr, in which case isError() will return true.

See also QQmlComponent::create.

QQmlContext *QQmlComponent::creationContext() const

Returns the QQmlContext the component was created in. This is only valid for components created directly from QML.

QQmlEngine *QQmlComponent::engine() const

Returns the QQmlEngine of this component.

QList<QQmlError> QQmlComponent::errors() const

Returns the list of errors that occurred during the last compile or create operation. An empty list is returned if isError() is not set.

[since 6.5] bool QQmlComponent::isBound() const

Returns true if the component was created in a QML files that specifies pragma ComponentBehavior: Bound, otherwise returns false.

This function was introduced in Qt 6.5.

bool QQmlComponent::isError() const

Returns true if status() == QQmlComponent::Error.

bool QQmlComponent::isLoading() const

Returns true if status() == QQmlComponent::Loading.

bool QQmlComponent::isNull() const

Returns true if status() == QQmlComponent::Null.

bool QQmlComponent::isReady() const

Returns true if status() == QQmlComponent::Ready.

[slot, since 6.5] void QQmlComponent::loadFromModule(QAnyStringView uri, QAnyStringView typeName, QQmlComponent::CompilationMode mode = PreferSynchronous)

Load the QQmlComponent for typeName in the module uri. If the type is implemented via a QML file, mode is used to load it. Types backed by C++ are always loaded synchronously.

 QQmlEngine engine;
 QQmlComponent component(&engine);
 component.loadFromModule("QtQuick", "Item");
 // once the component is ready
 std::unique_ptr<QObject> item(component.create());
 Q_ASSERT(item->metaObject() == &QQuickItem::staticMetaObject);

This function was introduced in Qt 6.5.

See also loadUrl().

[slot] void QQmlComponent::loadUrl(const QUrl &url)

Load the QQmlComponent from the provided url.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

Relative paths will be resolved against QQmlEngine::baseUrl(), which is the current working directory unless specified.

[slot] void QQmlComponent::loadUrl(const QUrl &url, QQmlComponent::CompilationMode mode)

Load the QQmlComponent from the provided url. If mode is Asynchronous, the component will be loaded and compiled asynchronously.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

Relative paths will be resolved against QQmlEngine::baseUrl(), which is the current working directory unless specified.

[signal] void QQmlComponent::progressChanged(qreal progress)

Emitted whenever the component's loading progress changes. progress will be the current progress between 0.0 (nothing loaded) and 1.0 (finished).

[slot] void QQmlComponent::setData(const QByteArray &data, const QUrl &url)

Sets the QQmlComponent to use the given QML data. If url is provided, it is used to set the component name and to provide a base path for items resolved by this component.

void QQmlComponent::setInitialProperties(QObject *object, const QVariantMap &properties)

Set top-level properties of the object that was created from a QQmlComponent.

This method provides advanced control over component instance creation. In general, programmers should use QQmlComponent::createWithInitialProperties to create an object instance from a component.

Use this method after beginCreate and before completeCreate has been called. If a provided property does not exist, a warning is issued.

This method does not allow setting initial nested properties directly. Instead, setting an initial value for value type properties with nested properties can be achieved by creating that value type, assigning its nested property and then passing the value type as an initial property of the object to be constructed.

For example, in order to set fond.bold, you can create a QFont, set its weight to bold and then pass the font as an initial property.

[signal] void QQmlComponent::statusChanged(QQmlComponent::Status status)

Emitted whenever the component's status changes. status will be the new status.