QQmlIncubationController Class

QQmlIncubationController instances drive the progress of QQmlIncubators. More...

Header: #include <QQmlIncubationController>
CMake: find_package(Qt6 REQUIRED COMPONENTS Qml)
target_link_libraries(mytarget PRIVATE Qt6::Qml)
qmake: QT += qml

Public Functions

QQmlEngine *engine() const
void incubateFor(int msecs)
void incubateWhile(std::atomic<bool> *flag, int msecs = 0)
int incubatingObjectCount() const

Protected Functions

virtual void incubatingObjectCountChanged(int incubatingObjectCount)

Detailed Description

In order to behave asynchronously and not introduce stutters or freezes in an application, the process of creating objects a QQmlIncubators must be driven only during the application's idle time. QQmlIncubationController allows the application to control exactly when, how often and for how long this processing occurs.

A QQmlIncubationController derived instance should be created and set on a QQmlEngine by calling the QQmlEngine::setIncubationController() method. Processing is then controlled by calling the QQmlIncubationController::incubateFor() or QQmlIncubationController::incubateWhile() methods as dictated by the application's requirements.

For example, this is an example of a incubation controller that will incubate for a maximum of 5 milliseconds out of every 16 milliseconds.

class PeriodicIncubationController : public QObject,
                                     public QQmlIncubationController
    PeriodicIncubationController() {

    void timerEvent(QTimerEvent *) override {

Although the example works, it is heavily simplified. Real world incubation controllers try and maximize the amount of idle time they consume while not disturbing the application. Using a static amount of 5 milliseconds like above may both leave idle time on the table in some frames and disturb the application in others.

QQuickWindow, QQuickView, and QQuickWidget all pre-create an incubation controller that spaces out incubation over multiple frames using a more intelligent algorithm. You rarely have to write your own.

Member Function Documentation


Create a new incubation controller.

QQmlEngine *QQmlIncubationController::engine() const

Return the QQmlEngine this incubation controller is set on, or 0 if it has not been set on any engine.

void QQmlIncubationController::incubateFor(int msecs)

Incubate objects for msecs, or until there are no more objects to incubate.

void QQmlIncubationController::incubateWhile(std::atomic<bool> *flag, int msecs = 0)

Incubate objects while the atomic bool pointed to by flag is true, or until there are no more objects to incubate, or up to msecs if msecs is not zero.

Generally this method is used in conjunction with a thread or a UNIX signal that sets the bool pointed to by flag to false when it wants incubation to be interrupted.

Note: flag is read using acquire memory ordering.

int QQmlIncubationController::incubatingObjectCount() const

Return the number of objects currently incubating.

[virtual protected] void QQmlIncubationController::incubatingObjectCountChanged(int incubatingObjectCount)

Called when the number of incubating objects changes. incubatingObjectCount is the new number of incubating objects.

The default implementation does nothing.

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