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QQmlComponent Class

The QQmlComponent class encapsulates a QML component definition. More...

Header: #include <QQmlComponent>
CMake: find_package(Qt6 REQUIRED COMPONENTS Qml)
target_link_libraries(mytarget PRIVATE Qt6::Qml)
qmake: QT += qml
In QML: Component
Inherits: QObject

Public Types

enum CompilationMode { PreferSynchronous, Asynchronous }
enum Status { Null, Ready, Loading, Error }

Properties

Public Functions

QQmlComponent(QQmlEngine *engine, QObject *parent = nullptr)
QQmlComponent(QQmlEngine *engine, const QString &fileName, QObject *parent = nullptr)
QQmlComponent(QQmlEngine *engine, const QUrl &url, QObject *parent = nullptr)
QQmlComponent(QQmlEngine *engine, const QString &fileName, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)
QQmlComponent(QQmlEngine *engine, const QUrl &url, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)
(since 6.5) QQmlComponent(QQmlEngine *engine, QAnyStringView uri, QAnyStringView typeName, QObject *parent = nullptr)
(since 6.5) QQmlComponent(QQmlEngine *engine, QAnyStringView uri, QAnyStringView typeName, QQmlComponent::CompilationMode mode, QObject *parent = nullptr)
virtual ~QQmlComponent() override
virtual QObject *beginCreate(QQmlContext *context)
virtual void completeCreate()
virtual QObject *create(QQmlContext *context = nullptr)
void create(QQmlIncubator &incubator, QQmlContext *context = nullptr, QQmlContext *forContext = nullptr)
QObject *createWithInitialProperties(const QVariantMap &initialProperties, QQmlContext *context = nullptr)
QQmlContext *creationContext() const
QQmlEngine *engine() const
QList<QQmlError> errors() const
(since 6.5) bool isBound() const
bool isError() const
bool isLoading() const
bool isNull() const
bool isReady() const
qreal progress() const
void setInitialProperties(QObject *object, const QVariantMap &properties)
QQmlComponent::Status status() const
QUrl url() const

Public Slots

(since 6.5) void loadFromModule(QAnyStringView uri, QAnyStringView typeName, QQmlComponent::CompilationMode mode = PreferSynchronous)
void loadUrl(const QUrl &url)
void loadUrl(const QUrl &url, QQmlComponent::CompilationMode mode)
void setData(const QByteArray &data, const QUrl &url)

Signals

void progressChanged(qreal progress)
void statusChanged(QQmlComponent::Status status)

Detailed Description

Components are reusable, encapsulated QML types with well-defined interfaces.

A QQmlComponent instance can be created from a QML file. For example, if there is a main.qml file like this:

import QtQuick 2.0

Item {
    width: 200
    height: 200
}

The following code loads this QML file as a component, creates an instance of this component using create(), and then queries the Item's width value:

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

To create instances of a component in code where a QQmlEngine instance is not available, you can use qmlContext() or qmlEngine(). For example, in the scenario below, child items are being created within a QQuickItem subclass:

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);
}

Note that these functions will return null when called inside the constructor of a QObject subclass, as the instance will not yet have a context nor engine.

Network Components

If the URL passed to QQmlComponent is a network resource, or if the QML document references a network resource, the QQmlComponent has to fetch the network data before it is able to create objects. In this case, the QQmlComponent will have a Loading status. An application will have to wait until the component is Ready before calling QQmlComponent::create().

The following example shows how to load a QML file from a network resource. After creating the QQmlComponent, it tests whether the component is loading. If it is, it connects to the QQmlComponent::statusChanged() signal and otherwise calls the continueLoading() method directly. Note that QQmlComponent::isLoading() may be false for a network component if the component has been cached and is ready immediately.

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 Type Documentation

enum QQmlComponent::CompilationMode

Specifies whether the QQmlComponent should load the component immediately, or asynchonously.

ConstantValueDescription
QQmlComponent::PreferSynchronous0Prefer loading/compiling the component immediately, blocking the thread. This is not always possible; for example, remote URLs will always load asynchronously.
QQmlComponent::Asynchronous1Load/compile the component in a background thread.

enum QQmlComponent::Status

Specifies the loading status of the QQmlComponent.

ConstantValueDescription
QQmlComponent::Null0This QQmlComponent has no data. Call loadUrl() or setData() to add QML content.
QQmlComponent::Ready1This QQmlComponent is ready and create() may be called.
QQmlComponent::Loading2This QQmlComponent is loading network data.
QQmlComponent::Error3An error has occurred. Call errors() to retrieve a list of errors.

Property Documentation

[read-only] progress : const qreal

The progress of loading the component, from 0.0 (nothing loaded) to 1.0 (finished).

Access functions:

qreal progress() const

Notifier signal:

void progressChanged(qreal progress)

[read-only] status : const Status

The component's current status.

Access functions:

QQmlComponent::Status status() const

Notifier signal:

void statusChanged(QQmlComponent::Status status)

[read-only] url : const QUrl

The component URL. This is the URL passed to either the constructor, or the loadUrl(), or setData() methods.

Access functions:

QUrl url() const

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.

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

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.

Note: The categorization of bindings into constant values and actual bindings is intentionally unspecified and may change between versions of Qt and depending on whether and how you are using qmlcachegen. You should not rely on any particular binding to be evaluated either before or after beginCreate() returns. For example a constant expression like MyType.EnumValue may be recognized as such at compile time or deferred to be executed as binding. The same holds for constant expressions like -(5) or "a" + " constant string".

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).

Note: Notifier signal for property progress.

[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.

Note: Notifier signal for property status.

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