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# QEventLoop Class Reference

The QEventLoop class manages Qt's event queue, excluding GUI-related events. More...

    #include <QEventLoop>

Inherits QObject.

Inherited by QGuiEventLoop.

- List of all members, including inherited members

### Public Types

- enum ProcessEvents { AllEvents, ExcludeUserInput, ExcludeSocketNotifiers, WaitForMore }
- typedef ProcessEventsFlags

### Writable Properties

- 1 writable property inherited from QObject

### Public Functions

- QEventLoop ( QObject * parent = 0 )
- ~QEventLoop ()
- int activateSocketNotifiers ()
- int activateTimers ()
- virtual int enterLoop ()
- virtual int exec ()
- virtual void exit ( int retcode = 0 )
- virtual void exitLoop ()
- virtual void flush ()
- virtual bool hasPendingEvents () const
- virtual int loopLevel () const
- void processEvents ( ProcessEventsFlags flags, int maxTime )
- virtual bool processEvents ( ProcessEventsFlags flags )
- virtual void registerSocketNotifier ( QSocketNotifier * notifier )
- EventFilter setEventFilter ( EventFilter filter )
- ProcessEventHandler setProcessEventHandler ( ProcessEventHandler handler )
- void setSocketNotifierPending ( QSocketNotifier * notifier )
- int timeToWait () const
- virtual void unregisterSocketNotifier ( QSocketNotifier * notifier )
- virtual void wakeUp ()

- 24 public functions inherited from QObject

### Public Slots

- 1 public slot inherited from QObject

### Signals

- void aboutToBlock ()
- void awake ()

- 1 signal inherited from QObject

### Static Public Members

- QEventLoop * instance ( QThread * thread = 0 )

- 4 static public members inherited from QObject

### Protected Functions

- 9 protected functions inherited from QObject

## Detailed Description

The QEventLoop class manages Qt's event queue, excluding GUI-related events.

It receives events from the window system and other sources. It then sends them to QApplication for processing and delivery. QEventLoop provides fine-grained control over event delivery.

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

For finer control of the application's event loop call QApplication::eventLoop() and call functions on the QEventLoop object that is returned. If you want to use your own instance of QEventLoop, QGuiEventLoop, or a QEventLoop subclass, you must create your instance before you create the QApplication object.

The event loop is started by calling exec(), and stopped by calling exit().

Programs that perform long operations can call processEvents() with various `ProcessEvents` values OR'ed together to control which events should be delivered.

QEventLoop also allows the integration of an external event loop with the Qt event loop. For example, the Motif Extension included with Qt includes a reimplementation of QEventLoop that merges Qt and Motif events together.

See also QGuiEventLoop.

## Member Type Documentation

### enum QEventLoop::ProcessEvents

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

`QEventLoop::AllEvents` | - All events are processed
`QEventLoop::ExcludeUserInput` | - Do not process user input events, such as ButtonPress and KeyPress.
`QEventLoop::ExcludeSocketNotifiers` | - Do not process socket notifier events.
`QEventLoop::WaitForMore` | - Wait for events if no pending events are available.

See also processEvents().

### typedef QEventLoop::ProcessEventsFlags

A `typedef` to allow various ProcessEvents values to be OR'ed together.

See also ProcessEvents.

## Member Function Documentation

### QEventLoop::QEventLoop ( QObject * parent = 0 )

Constructs a QEventLoop object. This object becomes the global event loop object. There can only be one event loop object. The QEventLoop is usually constructed by calling QCoreApplication::eventLoop(). If you want to create your own event loop object you must create it before you instantiate the QApplication object.

The parent argument is passed on to the QObject constructor.

### QEventLoop::~QEventLoop ()

Destructs the QEventLoop object.

### void QEventLoop::aboutToBlock () `[signal]`

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

See also awake().

### int QEventLoop::activateSocketNotifiers ()

Activates all pending socket notifiers and returns the number of socket notifiers that were activated.

### int QEventLoop::activateTimers ()

Activates all Qt timers and returns the number of timers that were activated.

QEventLoop subclasses that do their own timer handling need to call this after the time returned by timeToWait() has elapsed.

Note: This function is only useful on systems where `select()` is used to block the eventloop. On Windows, this function always returns 0. On Mac OS X, this function always returns 0 when the GUI is enabled. On Mac OS X, this function returns the documented value when the GUI is disabled.

### void QEventLoop::awake () `[signal]`

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

See also wakeUp() and aboutToBlock().

### int QEventLoop::enterLoop () `[virtual]`

This function enters the main event loop (recursively). Do not call it unless you really know what you are doing.

### int QEventLoop::exec () `[virtual]`

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

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 call 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 QApplication::quit(), exit(), and processEvents().

### void QEventLoop::exit ( int retcode = 0 ) `[virtual]`

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 retcode.

By convention, a retcode 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 QApplication::quit() and exec().

### void QEventLoop::exitLoop () `[virtual]`

This function exits from a recursive call to the main event loop. Do not call it unless you really know what you are doing.

### void QEventLoop::flush () `[virtual]`

Flushes the event queue. This normally returns almost immediately. Does nothing on platforms other than X11.

### bool QEventLoop::hasPendingEvents () const `[virtual]`

Returns true if there is an event waiting; otherwise returns false.

### QEventLoop * QEventLoop::instance ( QThread * thread = 0 ) `[static]`

Returns a pointer to the event loop object for the specified thread. If thread is zero, the current thread is used. If no event loop exists for the specified thread, this function returns 0.

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

See also QApplication::eventLoop().

### int QEventLoop::loopLevel () const `[virtual]`

Returns the current loop level.

### void QEventLoop::processEvents ( 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 `ExcludeUserInput` flag.

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

Note: Specifying the `WaitForMore` flag makes no sense and will be ignored.

### bool QEventLoop::processEvents ( ProcessEventsFlags flags ) `[virtual]`

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Processes pending events that match flags until there are no more events to process.

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 `ExcludeUserInput` flag.

If the `WaitForMore` 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 `WaitForMore` 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.

This function returns true if an event was processed; otherwise it returns false.

See also ProcessEvents and hasPendingEvents().

### void QEventLoop::registerSocketNotifier ( QSocketNotifier * notifier ) `[virtual]`

Registers notifier with the event loop. Subclasses must reimplement this method to tie a socket notifier into another event loop. Reimplementations <b>must</b> call the base implementation.

### EventFilter QEventLoop::setEventFilter ( EventFilter filter )

Sets the event filter filter. Returns a pointer to the filter function previously defined.

The event filter is a function that is called for every message received. This does not include messages to objects that are not handled by Qt.

The function can return true to stop the event to be processed by Qt, or false to continue with the standard event processing.

Only one filter can be defined, but the filter can use the return value to call the previously set event filter. By default, no filter is set (ie. the function returns 0).

### ProcessEventHandler QEventLoop::setProcessEventHandler ( ProcessEventHandler handler )

Sets the process event handler handler. Returns a pointer to the handler previously defined.

The process event handler is a function that receives all messages taken from the system event loop before the event is dispatched to the respective target. This includes messages that are not sent to Qt objects.

The function can return true to prevent the message from being dispatched, or false to pass the message back to the standard event processing.

Only one handler can be defined, but the handler can use the return value to call the previously set event handler. By default, no handler is set (ie. the function returns 0).

### void QEventLoop::setSocketNotifierPending ( QSocketNotifier * notifier )

Marks notifier as pending. The socket notifier will be activated the next time activateSocketNotifiers() is called.

### int QEventLoop::timeToWait () const

Returns the number of milliseconds that Qt needs to handle its timers or -1 if there are no timers running.

QEventLoop subclasses that do their own timer handling need to use this to make sure that Qt's timers continue to work.

Note: This function is only useful on systems where `select()` is used to block the eventloop. On Windows, this function always returns -1. On MacOS X, this function always returns -1 when the GUI is enabled. On MacOS X, this function returns the documented value when the GUI is disabled.

### void QEventLoop::unregisterSocketNotifier ( QSocketNotifier * notifier ) `[virtual]`

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

### void QEventLoop::wakeUp () `[virtual]`

Wakes up the event loop.

Note: function is thread-safe.

See also awake().