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# QTimer Class Reference
[com.trolltech.qt.core module]

The `QTimer` class provides repetitive and single-shot timers. More...

Inherits `QObject`.

## Detailed Description

The `QTimer` class provides repetitive and single-shot timers.

The `QTimer` class provides a high-level programming interface for timers. To use it, create a `QTimer`, connect its `timeout` signal to the appropriate slots, and call `start`. From then on it will emit the `timeout` signal at constant intervals.

Example for a one second (1000 millisecond) timer (from the Analog Clock example):

            QTimer *timer = new QTimer(this);
            connect(timer, SIGNAL(timeout()), this, SLOT(update()));
            timer->start(1000);

From then on, the `update()` slot is called every second.

You can set a timer to time out only once by calling `setSingleShot`(true). You can also use the static QTimer::singleShot() function to call a slot after a specified interval:

            QTimer::singleShot(200, this, SLOT(updateCaption()));

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 QThread::exec(). Qt uses the 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.

As a special case, a `QTimer` with a timeout of 0 will time out as soon as all the events in the window system's event queue have been processed. This can be used to do heavy work while providing a snappy user interface:

            QTimer *timer = new QTimer(this);
            connect(timer, SIGNAL(timeout()), this, SLOT(processOneThing()));
            timer->start();

`processOneThing()` will from then on be called repeatedly. It should be written in such a way that it always returns quickly (typically after processing one data item) so that Qt can deliver events to widgets and stop the timer as soon as it has done all its work. This is the traditional way of implementing heavy work in GUI applications; multithreading is now becoming available on more and more platforms, and we expect that zero-millisecond QTimers will gradually be replaced by `QThread`s.

Note that `QTimer`'s accuracy depends on the underlying operating system and hardware. Most platforms support an accuracy of 1 millisecond, but Windows 98 supports only 55. If Qt is unable to deliver the requested number of timer clicks, it will silently discard some.

An alternative to using `QTimer` is to call QObject::startTimer() for your object and reimplement the QObject::timerEvent() event handler in your class (which must inherit `QObject`). The disadvantage is that `timerEvent` does not support such high-level features as single-shot timers or signals.

Another alternative to using `QTimer` is to use `QBasicTimer`. It is typically less cumbersome than using QObject::startTimer() directly. See Timers for an overview of all three approaches.

Some operating systems limit the number of timers that may be used; Qt tries to work around these limitations.

See also `QBasicTimer`, `QTimerEvent`, `QObject::timerEvent`, Timers, Analog Clock Example, and Wiggly Example.