How to use Qt timers in your application.
QObject , the base class of all Qt objects, provides the basic timer support in Qt. With
startTimer() , you start a timer with an interval in milliseconds as argument. The function returns a unique integer timer ID. The timer will now fire at regular intervals until you explicitly call
killTimer() with the timer ID.
For this mechanism to work, the application must run in an event loop. You start an event loop with
exec() . When a timer fires, the application sends a
QTimerEvent , and the flow of control leaves the event loop until the timer event is processed. This implies that a timer cannot fire while your application is busy doing something else. In other words: the accuracy of timers depends on the granularity of your application.
In multithreaded applications, you can use the timer mechanism in any thread that has an event loop. To start an event loop from a non-GUI thread, use
exec() . Qt uses the object’s
thread affinity to determine which thread will deliver the
QTimerEvent . Because of this, you must start and stop all timers in the object’s thread; it is not possible to start timers for objects in another thread.
The upper limit for the interval value is determined by the number of milliseconds that can be specified in a signed integer (in practice, this is a period of just over 24 days). The accuracy depends on the underlying operating system. Windows 2000 has 15 millisecond accuracy; other systems that we have tested can handle 1 millisecond intervals.
The main API for the timer functionality is
QTimer . That class provides regular timers that emit a signal when the timer fires, and inherits
QObject so that it fits well into the ownership structure of most Qt programs. The normal way of using it is like this:
timer = QTimer(self) connect(timer, QTimer.timeout, self, Foo.updateCaption) timer.start(1000)
QTimer object is made into a child of
this object so that, when
this object is deleted, the timer is deleted too. Next, its
timeout() signal is connected to the slot that will do the work, it is started with a value of 1000 milliseconds, indicating that it will time out every second.
QTimer also provides a static function for single-shot timers. For example:
QTimer.singleShot(200, self, Foo.updateCaption)
200 milliseconds (0.2 seconds) after this line of code is executed, the
updateCaption() slot will be called.
QTimer to work, you must have an event loop in your application; that is, you must call
exec() somewhere. Timer events will be delivered only while the event loop is running.
In multithreaded applications, you can use
QTimer in any thread that has an event loop. To start an event loop from a non-GUI thread, use
exec() . Qt uses the timer’s
thread affinity to determine which thread will emit the
timeout() signal. Because of this, you must start and stop the timer in its thread; it is not possible to start a timer from another thread.
The Analog Clock example shows how to use
QTimer to redraw a widget at regular intervals. From
def __init__(self, parent): QWidget.__init__(self, parent) timer = QTimer(self) connect(timer, QTimer.timeout, self, QOverload<>.of(AnalogClock.update)) timer.start(1000) ...
QTimer will call the
update() slot to refresh the clock’s display.
If you already have a
QObject subclass and want an easy optimization, you can use
QBasicTimer instead of
QTimer . With
QBasicTimer , you must reimplement
timerEvent() in your
QObject subclass and handle the timeout there. The Wiggly example shows how to use
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