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QEventLoop Class

The QEventLoop class provides a means of entering and leaving an event loop. More...

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

Public Types

enum ProcessEventsFlag { AllEvents, ExcludeUserInputEvents, ExcludeSocketNotifiers, WaitForMoreEvents }
flags ProcessEventsFlags

Public Functions

QEventLoop(QObject *parent = nullptr)
virtual ~QEventLoop()
int exec(QEventLoop::ProcessEventsFlags flags = AllEvents)
bool isRunning() const
bool processEvents(QEventLoop::ProcessEventsFlags flags = AllEvents)
void processEvents(QEventLoop::ProcessEventsFlags flags, int maxTime)
void wakeUp()

Reimplemented Public Functions

virtual bool event(QEvent *event) override

Public Slots

void exit(int returnCode = 0)
void quit()

Detailed Description

At any time, you can create a QEventLoop object and call exec() on it to start a local event loop. From within the event loop, calling exit() will force exec() to return.

See also QAbstractEventDispatcher.

Member Type Documentation

enum QEventLoop::ProcessEventsFlag
flags QEventLoop::ProcessEventsFlags

This enum controls the types of events processed by the processEvents() functions.

Constant Value Description
QEventLoop::AllEvents 0x00 All events. Note that DeferredDelete events are processed specially. See QObject::deleteLater() for more details.
QEventLoop::ExcludeUserInputEvents 0x01 Do not process user input events, such as ButtonPress and KeyPress. Note that the events are not discarded; they will be delivered the next time processEvents() is called without the ExcludeUserInputEvents flag.
QEventLoop::ExcludeSocketNotifiers 0x02 Do not process socket notifier events. Note that the events are not discarded; they will be delivered the next time processEvents() is called without the ExcludeSocketNotifiers flag.
QEventLoop::WaitForMoreEvents 0x04 Wait for events if no pending events are available.

The ProcessEventsFlags type is a typedef for QFlags<ProcessEventsFlag>. It stores an OR combination of ProcessEventsFlag values.

See also processEvents().

Member Function Documentation

QEventLoop::QEventLoop(QObject *parent = nullptr)

Constructs an event loop object with the given parent.

[slot] void QEventLoop::exit(int returnCode = 0)

Tells the event loop to exit with a return code.

After this function has been called, the event loop returns from the call to exec(). The exec() function returns returnCode.

By convention, a returnCode of 0 means success, and any non-zero value indicates an error.

Note that unlike the C library function of the same name, this function does return to the caller -- it is event processing that stops.

See also QCoreApplication::quit(), quit(), and exec().

[slot] void QEventLoop::quit()

Tells the event loop to exit normally.

Same as exit(0).

See also QCoreApplication::quit() and exit().

[virtual] QEventLoop::~QEventLoop()

Destroys the event loop object.

[override virtual] bool QEventLoop::event(QEvent *event)

Reimplements: QObject::event(QEvent *e).

int QEventLoop::exec(QEventLoop::ProcessEventsFlags flags = AllEvents)

Enters the main event loop and waits until exit() is called. Returns the value that was passed to exit().

If flags are specified, only events of the types allowed by the flags will be processed.

It is necessary to call this function to start event handling. The main event loop receives events from the window system and dispatches these to the application widgets.

Generally speaking, no user interaction can take place before calling exec(). As a special case, modal widgets like QMessageBox can be used before calling exec(), because modal widgets use their own local event loop.

To make your application perform idle processing (i.e. executing a special function whenever there are no pending events), use a QTimer with 0 timeout. More sophisticated idle processing schemes can be achieved using processEvents().

See also QCoreApplication::quit(), exit(), and processEvents().

bool QEventLoop::isRunning() const

Returns true if the event loop is running; otherwise returns false. The event loop is considered running from the time when exec() is called until exit() is called.

See also exec() and exit().

bool QEventLoop::processEvents(QEventLoop::ProcessEventsFlags flags = AllEvents)

Processes some pending events that match flags. Returns true if pending events were handled; 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; i.e. by using the ExcludeUserInputEvents flag.

This function is simply a wrapper for QAbstractEventDispatcher::processEvents(). See the documentation for that function for details.

void QEventLoop::processEvents(QEventLoop::ProcessEventsFlags flags, int maxTime)

Process pending events that match flags for a maximum of maxTime milliseconds, or until there are no more events to process, whichever is shorter. This function is especially useful if you have a long running operation and want to show its progress without allowing user input, i.e. by using the ExcludeUserInputEvents flag.


  • This function does not process events continuously; it returns after all available events are processed.
  • Specifying the WaitForMoreEvents flag makes no sense and will be ignored.

void QEventLoop::wakeUp()

Wakes up the event loop.

See also QAbstractEventDispatcher::wakeUp().

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