QQmlComponent Class▲

  • Header: QQmlComponent

  • Since: Qt 5.0

  • CMake:

    find_package(Qt6 REQUIRED COMPONENTS Qml)

    target_link_libraries(mytarget PRIVATE Qt6::Qml)

  • qmake: QT += qml

  • Inherited By:

  • Instantiated By: qml-qtqml-component.xml

  • Inherits: QObject

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:

 
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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:

 
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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:

 
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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.

 
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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.

Constant

Value

Description

QQmlComponent::PreferSynchronous

0

Prefer loading/compiling the component immediately, blocking the thread. This is not always possible; for example, remote URLs will always load asynchronously.

QQmlComponent::Asynchronous

1

Load/compile the component in a background thread.

enum QQmlComponent::Status▲

Specifies the loading status of the QQmlComponent.

Constant

Value

Description

QQmlComponent::Null

0

This QQmlComponent has no data. Call loadUrl() or setData() to add QML content.

QQmlComponent::Ready

1

This QQmlComponent is ready and create() may be called.

QQmlComponent::Loading

2

This QQmlComponent is loading network data.

QQmlComponent::Error

3

An 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:

  • progress() const

Notifier signal:

[read-only] status : const Status▲

The component's current status.

Access functions:

  • status() const

Notifier signal:

[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:

  • 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▲

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▲

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▲

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▲

See also