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()));
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 QThreads.
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.
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(), and Timers.
Property Documentation
interval : int
This property holds the timeout interval in milliseconds.
The default value for this property is 0. A QTimer with a timeout interval of 0 will time out as soon as all the events in the window system's event queue have been processed.
Setting the interval of an active timer changes its timerId().
Access functions:
- int interval () const
- void setInterval ( int msec )
See also singleShot.
singleShot : bool
This property holds whether the timer is a single-shot timer.
A single-shot timer fires only once, non-single-shot timers fire every interval milliseconds.
Access functions:
- bool isSingleShot () const
- void setSingleShot ( bool singleShot )
See also interval and singleShot().
Member Function Documentation
QTimer::QTimer ( QObject * parent = 0 )
Constructs a timer with the given parent.
QTimer::~QTimer ()
Destroys the timer.
bool QTimer::isActive () const
Returns true if the timer is running (pending); otherwise returns false.
void QTimer::singleShot ( int msec, QObject * receiver, const char * member ) [static]
This static function calls a slot after a given time interval.
It is very convenient to use this function because you do not need to bother with a timerEvent or create a local QTimer object.
Example:
#include <QApplication>
#include <QTimer>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QTimer::singleShot(600000, &app, SLOT(quit()));
...
return app.exec();
}
This sample program automatically terminates after 10 minutes (600,000 milliseconds).
The receiver is the receiving object and the member is the slot. The time interval is msec milliseconds.
Note: This function is reentrant.
See also setSingleShot() and start().
void QTimer::start ( int msec ) [slot]
Starts or restarts the timer with a timeout interval of msec milliseconds.
void QTimer::start () [slot]
This is an overloaded member function, provided for convenience.
Starts or restarts the timer with the timeout specified in interval.
If singleShot is true, the timer will be activated only once.
void QTimer::stop () [slot]
Stops the timer.
See also start().
void QTimer::timeout () [signal]
This signal is emitted when the timer times out.
See also interval, start(), and stop().
int QTimer::timerId () const
Returns the ID of the timer if the timer is running; otherwise returns -1.
