Detailed Description
The QEventLoop class manages the event queue.
It receives events from the window system and other sources. It
then sends them to QApplication for processing and delivery.
QEventLoop allows the application programmer to have more control
over event delivery. Programs that perform long operations can
call either processOneEvent() or processEvents() with various
ProcessEvent 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. The Motif Extension included with Qt
includes a reimplementation of QEventLoop for merging Qt and Motif
events together.
To use your own instance of QEventLoop or QEventLoop subclass create
it before you create the QApplication object.
See also Main Window and Related Classes and Event Classes.
Member Type Documentation
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.
( ButtonPress, KeyPress, etc. )
- QEventLoop::ExcludeSocketNotifiers - Do not process socket notifier
events.
- QEventLoop::WaitForMore - Wait for events if no pending events
are available.
See also processEvents().
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, const char * name = 0 )
Creates 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 QApplication::eventLoop(). To create your own event loop object create
it before you instantiate the QApplication object.
The parent and name arguments are 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 MacOS X, this function always returns 0 when the
GUI is enabled. On MacOS 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, and
returns the value that was set 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
exec() to start a 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 advanced 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.
bool QEventLoop::hasPendingEvents () const [virtual]
Returns TRUE if there is an event waiting, otherwise it returns FALSE.
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, which ever 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 will 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 will 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 need to
reimplement this method to tie a socket notifier into another
event loop. Reimplementations MUST call the base
implementation.
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 need to
reimplement this method to tie a socket notifier into another
event loop. Reimplementations MUST call the base
implementation.
void QEventLoop::wakeUp () [virtual]
Note: This function is thread-safe when Qt is built withthread support.
Wakes up the event loop.
See also awake().
This file is part of the Qt toolkit.
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