QAbstractEventDispatcher Class

The QAbstractEventDispatcher class provides an interface to manage Qt's event queue. More...

Header: #include <QAbstractEventDispatcher>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QObject

Public Types

struct TimerInfo

Public Functions

QAbstractEventDispatcher(QObject *parent = nullptr)
virtual ~QAbstractEventDispatcher()
bool filterNativeEvent(const QByteArray &eventType, void *message, qintptr *result)
void installNativeEventFilter(QAbstractNativeEventFilter *filterObj)
virtual void interrupt() = 0
virtual bool processEvents(QEventLoop::ProcessEventsFlags flags) = 0
virtual void registerSocketNotifier(QSocketNotifier *notifier) = 0
int registerTimer(qint64 interval, Qt::TimerType timerType, QObject *object)
virtual void registerTimer(int timerId, qint64 interval, Qt::TimerType timerType, QObject *object) = 0
virtual QList<QAbstractEventDispatcher::TimerInfo> registeredTimers(QObject *object) const = 0
virtual int remainingTime(int timerId) = 0
void removeNativeEventFilter(QAbstractNativeEventFilter *filter)
virtual void unregisterSocketNotifier(QSocketNotifier *notifier) = 0
virtual bool unregisterTimer(int timerId) = 0
virtual bool unregisterTimers(QObject *object) = 0
virtual void wakeUp() = 0


void aboutToBlock()
void awake()

Static Public Members

QAbstractEventDispatcher *instance(QThread *thread = nullptr)

Detailed Description

An event dispatcher receives events from the window system and other sources. It then sends them to the QCoreApplication or QApplication instance for processing and delivery. QAbstractEventDispatcher provides fine-grained control over event delivery.

For simple control of event processing use QCoreApplication::processEvents().

For finer control of the application's event loop, call instance() and call functions on the QAbstractEventDispatcher object that is returned. If you want to use your own instance of QAbstractEventDispatcher or of a QAbstractEventDispatcher subclass, you must install it with QCoreApplication::setEventDispatcher() or QThread::setEventDispatcher() before a default event dispatcher has been installed.

The main event loop is started by calling QCoreApplication::exec(), and stopped by calling QCoreApplication::exit(). Local event loops can be created using QEventLoop.

Programs that perform long operations can call processEvents() with a bitwise OR combination of various QEventLoop::ProcessEventsFlag values to control which events should be delivered.

QAbstractEventDispatcher also allows the integration of an external event loop with the Qt event loop.

See also QEventLoop, QCoreApplication, and QThread.

Member Function Documentation

[explicit] QAbstractEventDispatcher::QAbstractEventDispatcher(QObject *parent = nullptr)

Constructs a new event dispatcher with the given parent.

[virtual noexcept] QAbstractEventDispatcher::~QAbstractEventDispatcher()

Destroys the event dispatcher.

[signal] void QAbstractEventDispatcher::aboutToBlock()

This signal is emitted before the event loop calls a function that could block.

See also awake().

[signal] void QAbstractEventDispatcher::awake()

This signal is emitted after the event loop returns from a function that could block.

See also wakeUp() and aboutToBlock().

bool QAbstractEventDispatcher::filterNativeEvent(const QByteArray &eventType, void *message, qintptr *result)

Sends message through the event filters that were set by installNativeEventFilter(). This function returns true as soon as an event filter returns true, and false otherwise to indicate that the processing of the event should continue.

Subclasses of QAbstractEventDispatcher must call this function for all messages received from the system to ensure compatibility with any extensions that may be used in the application. The type of event eventType is specific to the platform plugin chosen at run-time, and can be used to cast message to the right type. The result pointer is only used on Windows, and corresponds to the LRESULT pointer.

Note that the type of message is platform dependent. See QAbstractNativeEventFilter for details.

See also installNativeEventFilter() and QAbstractNativeEventFilter::nativeEventFilter().

void QAbstractEventDispatcher::installNativeEventFilter(QAbstractNativeEventFilter *filterObj)

Installs an event filter filterObj for all native events received by the application.

The event filter filterObj receives events via its nativeEventFilter() function, which is called for all events received by all threads.

The nativeEventFilter() function should return true if the event should be filtered, (in this case, stopped). It should return false to allow normal Qt processing to continue: the native event can then be translated into a QEvent and handled by the standard Qt event filtering, e.g. QObject::installEventFilter().

If multiple event filters are installed, the filter that was installed last is activated first.

Note: The filter function set here receives native messages, that is, MSG or XEvent structs.

For maximum portability, you should always try to use QEvent objects and QObject::installEventFilter() whenever possible.

See also QObject::installEventFilter().

[static] QAbstractEventDispatcher *QAbstractEventDispatcher::instance(QThread *thread = nullptr)

Returns a pointer to the event dispatcher object for the specified thread. If thread is nullptr, the current thread is used. If no event dispatcher exists for the specified thread, this function returns nullptr.

Note: If Qt is built without thread support, the thread argument is ignored.

[pure virtual] void QAbstractEventDispatcher::interrupt()

Interrupts event dispatching. The event dispatcher will return from processEvents() as soon as possible.

[pure virtual] bool QAbstractEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)

Processes pending events that match flags until there are no more events to process. Returns true if an event was processed; otherwise returns false.

This function is especially useful if you have a long running operation, and want to show its progress without allowing user input by using the QEventLoop::ExcludeUserInputEvents flag.

If the QEventLoop::WaitForMoreEvents flag is set in flags, the behavior of this function is as follows:

  • If events are available, this function returns after processing them.
  • If no events are available, this function will wait until more are available and return after processing newly available events.

If the QEventLoop::WaitForMoreEvents flag is not set in flags, and no events are available, this function will return immediately.

Note: This function does not process events continuously; it returns after all available events are processed.

[pure virtual] void QAbstractEventDispatcher::registerSocketNotifier(QSocketNotifier *notifier)

Registers notifier with the event loop. Subclasses must implement this method to tie a socket notifier into another event loop.

int QAbstractEventDispatcher::registerTimer(qint64 interval, Qt::TimerType timerType, QObject *object)

Registers a timer with the specified interval and timerType for the given object and returns the timer id.

[pure virtual] void QAbstractEventDispatcher::registerTimer(int timerId, qint64 interval, Qt::TimerType timerType, QObject *object)

Register a timer with the specified timerId, interval, and timerType for the given object.

[pure virtual] QList<QAbstractEventDispatcher::TimerInfo> QAbstractEventDispatcher::registeredTimers(QObject *object) const

Returns a list of registered timers for object. The TimerInfo struct has timerId, interval, and timerType members.

See also Qt::TimerType.

[pure virtual] int QAbstractEventDispatcher::remainingTime(int timerId)

Returns the remaining time in milliseconds with the given timerId. If the timer is inactive, the returned value will be -1. If the timer is overdue, the returned value will be 0.

See also Qt::TimerType.

void QAbstractEventDispatcher::removeNativeEventFilter(QAbstractNativeEventFilter *filter)

Removes the event filter filter from this object. The request is ignored if such an event filter has not been installed.

All event filters for this object are automatically removed when this object is destroyed.

It is always safe to remove an event filter, even during event filter filter activation (that is, even from within the nativeEventFilter() function).

See also installNativeEventFilter() and QAbstractNativeEventFilter.

[pure virtual] void QAbstractEventDispatcher::unregisterSocketNotifier(QSocketNotifier *notifier)

Unregisters notifier from the event dispatcher. Subclasses must reimplement this method to tie a socket notifier into another event loop. Reimplementations must call the base implementation.

[pure virtual] bool QAbstractEventDispatcher::unregisterTimer(int timerId)

Unregisters the timer with the given timerId. Returns true if successful; otherwise returns false.

See also registerTimer() and unregisterTimers().

[pure virtual] bool QAbstractEventDispatcher::unregisterTimers(QObject *object)

Unregisters all the timers associated with the given object. Returns true if all timers were successful removed; otherwise returns false.

See also unregisterTimer() and registeredTimers().

[pure virtual] void QAbstractEventDispatcher::wakeUp()

Wakes up the event loop.

Note: This function is thread-safe.

See also awake().

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