QQuickWidget Class
The QQuickWidget class provides a widget for displaying a Qt Quick user interface. More...
| Header: | #include <QQuickWidget> |
| CMake: | find_package(Qt6 REQUIRED COMPONENTS QuickWidgets) target_link_libraries(mytarget PRIVATE Qt6::QuickWidgets) |
| qmake: | QT += quickwidgets |
| Inherits: | QWidget |
Public Types
| enum | ResizeMode { SizeViewToRootObject, SizeRootObjectToView } |
| enum | Status { Null, Ready, Loading, Error } |
Properties
- resizeMode : ResizeMode
- source : QUrl
- status : const Status
Public Functions
| QQuickWidget(QWidget *parent = nullptr) | |
| QQuickWidget(QQmlEngine *engine, QWidget *parent) | |
| QQuickWidget(const QUrl &source, QWidget *parent = nullptr) | |
| virtual | ~QQuickWidget() override |
| QQmlEngine * | engine() const |
| QList<QQmlError> | errors() const |
| QSurfaceFormat | format() const |
| QImage | grabFramebuffer() const |
| QSize | initialSize() const |
| QQuickWindow * | quickWindow() const |
| QQuickWidget::ResizeMode | resizeMode() const |
| QQmlContext * | rootContext() const |
| QQuickItem * | rootObject() const |
| void | setClearColor(const QColor &color) |
| void | setFormat(const QSurfaceFormat &format) |
| void | setResizeMode(QQuickWidget::ResizeMode) |
| QUrl | source() const |
| QQuickWidget::Status | status() const |
Public Slots
| void | setSource(const QUrl &url) |
Signals
| void | sceneGraphError(QQuickWindow::SceneGraphError error, const QString &message) |
| void | statusChanged(QQuickWidget::Status status) |
Reimplemented Protected Functions
| virtual void | dragEnterEvent(QDragEnterEvent *e) override |
| virtual void | dragLeaveEvent(QDragLeaveEvent *e) override |
| virtual void | dragMoveEvent(QDragMoveEvent *e) override |
| virtual void | dropEvent(QDropEvent *e) override |
| virtual bool | event(QEvent *e) override |
| virtual void | focusInEvent(QFocusEvent *event) override |
| virtual bool | focusNextPrevChild(bool next) override |
| virtual void | focusOutEvent(QFocusEvent *event) override |
| virtual void | hideEvent(QHideEvent *) override |
| virtual void | keyPressEvent(QKeyEvent *e) override |
| virtual void | keyReleaseEvent(QKeyEvent *e) override |
| virtual void | mouseDoubleClickEvent(QMouseEvent *e) override |
| virtual void | mouseMoveEvent(QMouseEvent *e) override |
| virtual void | mousePressEvent(QMouseEvent *e) override |
| virtual void | mouseReleaseEvent(QMouseEvent *e) override |
| virtual void | paintEvent(QPaintEvent *event) override |
| virtual void | showEvent(QShowEvent *) override |
| virtual void | wheelEvent(QWheelEvent *e) override |
Detailed Description
This is a convenience wrapper for QQuickWindow which will automatically load and display a QML scene when given the URL of the main source file. Alternatively, you can instantiate your own objects using QQmlComponent and place them in a manually set up QQuickWidget.
Typical usage:
QQuickWidget *view = new QQuickWidget; view->setSource(QUrl::fromLocalFile("myqmlfile.qml")); view->show();
To receive errors related to loading and executing QML with QQuickWidget, you can connect to the statusChanged() signal and monitor for QQuickWidget::Error. The errors are available via QQuickWidget::errors().
QQuickWidget also manages sizing of the view and root object. By default, the resizeMode is SizeViewToRootObject, which will load the component and resize it to the size of the view. Alternatively the resizeMode may be set to SizeRootObjectToView which will resize the view to the size of the root object.
Performance Considerations
QQuickWidget is an alternative to using QQuickView and QWidget::createWindowContainer(). The restrictions on stacking order do not apply, making QQuickWidget the more flexible alternative, behaving more like an ordinary widget.
However, the above mentioned advantages come at the expense of performance:
- Unlike QQuickWindow and QQuickView, QQuickWidget involves at least one additional render pass targeting an offscreen color buffer, typically a 2D texture, followed by drawing a texture quad. This means increased load especially for the fragment processing of the GPU.
- Using QQuickWidget disables the threaded render loop on all platforms. This means that some of the benefits of threaded rendering, for example Animator classes and vsync driven animations, will not be available.
Note: Avoid calling winId() on a QQuickWidget. This function triggers the creation of a native window, resulting in reduced performance and possibly rendering glitches. The entire purpose of QQuickWidget is to render Quick scenes without a separate native window, hence making it a native widget should always be avoided.
Graphics API Support
QQuickWidget is functional with all the 3D graphics APIs supported by Qt Quick, as well as the software backend. Other backends, for example OpenVG, are not compatible however and attempting to construct a QQuickWidget will lead to problems.
Overriding the platform's default graphics API is done the same way as with QQuickWindow and QQuickView: either by calling QQuickWindow::setGraphicsApi() early on before constructing the first QQuickWidget, or by setting the QSG_RHI_BACKEND environment variable.
Note: One top-level window can only use one single graphics API for rendering. For example, attempting to place a QQuickWidget using Vulkan and a QOpenGLWidget in the widget hierarchy of the same top-level window, problems will occur and one of the widgets will not be rendering as expected.
Scene Graph and Context Persistency
QQuickWidget honors QQuickWindow::isPersistentSceneGraph(), meaning that applications can decide - by calling QQuickWindow::setPersistentSceneGraph() on the window returned from the quickWindow() function - to let scenegraph nodes and other Qt Quick scene related resources be released whenever the widget becomes hidden. By default persistency is enabled, just like with QQuickWindow.
When running with the OpenGL, QQuickWindow offers the possibility to disable persistent OpenGL contexts as well. This setting is currently ignored by QQuickWidget and the context is always persistent. The OpenGL context is thus not destroyed when hiding the widget. The context is destroyed only when the widget is destroyed or when the widget gets reparented into another top-level widget's child hierarchy. However, some applications, in particular those that have their own graphics resources due to performing custom OpenGL rendering in the Qt Quick scene, may wish to disable the latter since they may not be prepared to handle the loss of the context when moving a QQuickWidget into another window. Such applications can set the QCoreApplication::AA_ShareOpenGLContexts attribute. For a discussion on the details of resource initialization and cleanup, refer to the QOpenGLWidget documentation.
Note: QQuickWidget offers less fine-grained control over its internal OpenGL context than QOpenGLWidget, and there are subtle differences, most notably that disabling the persistent scene graph will lead to destroying the context on a window change regardless of the presence of QCoreApplication::AA_ShareOpenGLContexts.
Limitations
Putting other widgets underneath and making the QQuickWidget transparent will not lead to the expected results: the widgets underneath will not be visible. This is because in practice the QQuickWidget is drawn before all other regular, non-OpenGL widgets, and so see-through types of solutions are not feasible. Other type of layouts, like having widgets on top of the QQuickWidget, will function as expected.
When absolutely necessary, this limitation can be overcome by setting the Qt::WA_AlwaysStackOnTop attribute on the QQuickWidget. Be aware, however that this breaks stacking order. For example it will not be possible to have other widgets on top of the QQuickWidget, so it should only be used in situations where a semi-transparent QQuickWidget with other widgets visible underneath is required.
This limitation only applies when there are other widgets underneath the QQuickWidget inside the same window. Making the window semi-transparent, with other applications and the desktop visible in the background, is done in the traditional way: Set Qt::WA_TranslucentBackground on the top-level window, request an alpha channel, and change the Qt Quick Scenegraph's clear color to Qt::transparent via setClearColor().
Tab Key Handling
On press of the [TAB] key, the item inside the QQuickWidget gets focus. If this item can handle [TAB] key press, focus will change accordingly within the item, otherwise the next widget in the focus chain gets focus.
See also Exposing Attributes of C++ Types to QML, Qt Quick Widgets Example, and QQuickView.
Member Type Documentation
enum QQuickWidget::ResizeMode
This enum specifies how to resize the view.
| Constant | Value | Description |
|---|---|---|
QQuickWidget::SizeViewToRootObject | 0 | The view resizes with the root item in the QML. |
QQuickWidget::SizeRootObjectToView | 1 | The view will automatically resize the root item to the size of the view. |
enum QQuickWidget::Status
Specifies the loading status of the QQuickWidget.
| Constant | Value | Description |
|---|---|---|
QQuickWidget::Null | 0 | This QQuickWidget has no source set. |
QQuickWidget::Ready | 1 | This QQuickWidget has loaded and created the QML component. |
QQuickWidget::Loading | 2 | This QQuickWidget is loading network data. |
QQuickWidget::Error | 3 | One or more errors occurred. Call errors() to retrieve a list of errors. |
Property Documentation
resizeMode : ResizeMode
Determines whether the view should resize the window contents.
If this property is set to SizeViewToRootObject (the default), the view resizes to the size of the root item in the QML.
If this property is set to SizeRootObjectToView, the view will automatically resize the root item to the size of the view.
Regardless of this property, the sizeHint of the view is the initial size of the root item. Note though that since QML may load dynamically, that size may change.
Access functions:
| QQuickWidget::ResizeMode | resizeMode() const |
| void | setResizeMode(QQuickWidget::ResizeMode) |
See also initialSize().
source : QUrl
This property holds the URL of the source of the QML component.
Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.
Note: Setting a source URL will result in the QML component being instantiated, even if the URL is unchanged from the current value.
Access functions:
[read-only] status : const Status
The component's current status.
Access functions:
| QQuickWidget::Status | status() const |
Notifier signal:
| void | statusChanged(QQuickWidget::Status status) |
Member Function Documentation
[explicit] QQuickWidget::QQuickWidget(QWidget *parent = nullptr)
Constructs a QQuickWidget with a default QML engine as a child of parent.
The default value of parent is nullptr.
QQuickWidget::QQuickWidget(QQmlEngine *engine, QWidget *parent)
Constructs a QQuickWidget with the given QML engine as a child of parent.
Note: The QQuickWidget does not take ownership of the given engine object; it is the caller's responsibility to destroy the engine. If the engine is deleted before the view, status() will return QQuickWidget::Error.
[explicit] QQuickWidget::QQuickWidget(const QUrl &source, QWidget *parent = nullptr)
Constructs a QQuickWidget with a default QML engine and the given QML source as a child of parent.
The default value of parent is nullptr.
[override virtual noexcept] QQuickWidget::~QQuickWidget()
Destroys the QQuickWidget.
[override virtual protected] void QQuickWidget::dragEnterEvent(QDragEnterEvent *e)
Reimplements: QWidget::dragEnterEvent(QDragEnterEvent *event).
[override virtual protected] void QQuickWidget::dragLeaveEvent(QDragLeaveEvent *e)
Reimplements: QWidget::dragLeaveEvent(QDragLeaveEvent *event).
[override virtual protected] void QQuickWidget::dragMoveEvent(QDragMoveEvent *e)
Reimplements: QWidget::dragMoveEvent(QDragMoveEvent *event).
[override virtual protected] void QQuickWidget::dropEvent(QDropEvent *e)
Reimplements: QWidget::dropEvent(QDropEvent *event).
QQmlEngine *QQuickWidget::engine() const
Returns a pointer to the QQmlEngine used for instantiating QML Components.
QList<QQmlError> QQuickWidget::errors() const
Return the list of errors that occurred during the last compile or create operation. When the status is not Error, an empty list is returned.
See also status.
[override virtual protected] bool QQuickWidget::event(QEvent *e)
Reimplements: QWidget::event(QEvent *event).
[override virtual protected] void QQuickWidget::focusInEvent(QFocusEvent *event)
Reimplements: QWidget::focusInEvent(QFocusEvent *event).
[override virtual protected] bool QQuickWidget::focusNextPrevChild(bool next)
Reimplements: QWidget::focusNextPrevChild(bool next).
[override virtual protected] void QQuickWidget::focusOutEvent(QFocusEvent *event)
Reimplements: QWidget::focusOutEvent(QFocusEvent *event).
QSurfaceFormat QQuickWidget::format() const
Returns the actual surface format.
If the widget has not yet been shown, the requested format is returned.
See also setFormat().
QImage QQuickWidget::grabFramebuffer() const
Renders a frame and reads it back into an image.
Note: This is a potentially expensive operation.
[override virtual protected] void QQuickWidget::hideEvent(QHideEvent *)
Reimplements: QWidget::hideEvent(QHideEvent *event).
QSize QQuickWidget::initialSize() const
Returns the initial size of the root object.
If resizeMode is SizeRootObjectToView, the root object will be resized to the size of the view. This function returns the size of the root object before it was resized.
[override virtual protected] void QQuickWidget::keyPressEvent(QKeyEvent *e)
Reimplements: QWidget::keyPressEvent(QKeyEvent *event).
[override virtual protected] void QQuickWidget::keyReleaseEvent(QKeyEvent *e)
Reimplements: QWidget::keyReleaseEvent(QKeyEvent *event).
[override virtual protected] void QQuickWidget::mouseDoubleClickEvent(QMouseEvent *e)
Reimplements: QWidget::mouseDoubleClickEvent(QMouseEvent *event).
[override virtual protected] void QQuickWidget::mouseMoveEvent(QMouseEvent *e)
Reimplements: QWidget::mouseMoveEvent(QMouseEvent *event).
[override virtual protected] void QQuickWidget::mousePressEvent(QMouseEvent *e)
Reimplements: QWidget::mousePressEvent(QMouseEvent *event).
[override virtual protected] void QQuickWidget::mouseReleaseEvent(QMouseEvent *e)
Reimplements: QWidget::mouseReleaseEvent(QMouseEvent *event).
[override virtual protected] void QQuickWidget::paintEvent(QPaintEvent *event)
Reimplements: QWidget::paintEvent(QPaintEvent *event).
QQuickWindow *QQuickWidget::quickWindow() const
Returns the offscreen QQuickWindow which is used by this widget to drive the Qt Quick rendering. This is useful if you want to use QQuickWindow APIs that are not currently exposed by QQuickWidget, for instance connecting to the QQuickWindow::beforeRendering() signal in order to draw native OpenGL content below Qt Quick's own rendering.
Warning: Use the return value of this function with caution. In particular, do not ever attempt to show the QQuickWindow, and be very careful when using other QWindow-only APIs.
Warning: The offscreen window may be deleted (and recreated) during the life time of the QQuickWidget, particularly when the widget is moved to another QQuickWindow. If you need to know when the window has been replaced, connect to its destroyed() signal.
QQmlContext *QQuickWidget::rootContext() const
This function returns the root of the context hierarchy. Each QML component is instantiated in a QQmlContext. QQmlContext's are essential for passing data to QML components. In QML, contexts are arranged hierarchically and this hierarchy is managed by the QQmlEngine.
QQuickItem *QQuickWidget::rootObject() const
Returns the view's root item. Can be null when setSource() has not been called, if it was called with broken QtQuick code or while the QtQuick contents are being created.
[signal] void QQuickWidget::sceneGraphError(QQuickWindow::SceneGraphError error, const QString &message)
This signal is emitted when an error occurred during scene graph initialization.
Applications should connect to this signal if they wish to handle errors, like OpenGL context creation failures, in a custom way. When no slot is connected to the signal, the behavior will be different: Quick will print the message, or show a message box, and terminate the application.
This signal will be emitted from the GUI thread.
See also QQuickWindow::sceneGraphError().
void QQuickWidget::setClearColor(const QColor &color)
Sets the clear color. By default this is an opaque color.
To get a semi-transparent QQuickWidget, call this function with color set to Qt::transparent, set the Qt::WA_TranslucentBackground widget attribute on the top-level window, and request an alpha channel via setFormat().
See also QQuickWindow::setColor().
void QQuickWidget::setFormat(const QSurfaceFormat &format)
Sets the surface format for the context and offscreen surface used by this widget.
Call this function when there is a need to request a context for a given OpenGL version or profile. The sizes for depth, stencil and alpha buffers are taken care of automatically and there is no need to request those explicitly.
See also QWindow::setFormat(), QWindow::format(), and format().
[slot] void QQuickWidget::setSource(const QUrl &url)
Sets the source to the url, loads the QML component and instantiates it.
Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.
Calling this method multiple times with the same URL will result in the QML component being reinstantiated.
Note: Setter function for property source.
See also source().
[override virtual protected] void QQuickWidget::showEvent(QShowEvent *)
Reimplements: QWidget::showEvent(QShowEvent *event).
QUrl QQuickWidget::source() const
Returns the source URL, if set.
Note: Getter function for property source.
See also setSource().
[signal] void QQuickWidget::statusChanged(QQuickWidget::Status status)
This signal is emitted when the component's current status changes.
Note: Notifier signal for property status.
[override virtual protected] void QQuickWidget::wheelEvent(QWheelEvent *e)
Reimplements: QWidget::wheelEvent(QWheelEvent *event).