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QTimer Class Reference

The QTimer class provides timer signals and single-shot timers. More...

#include <qtimer.h>

Inherits QObject.

List of all member functions.

Public Members


Static Public Members

Detailed Description

The QTimer class provides timer signals and single-shot timers.

It uses timer events internally to provide a more versatile timer. QTimer is very easy to use, create a QTimer, call start() to start it and connect its timeout() to the appropriate slots, then when the time is up it will emit timeout().

Note that a QTimer object is destroyed automatically when its parent object is destroyed.


    QTimer *timer = new QTimer( myObject );
    connect( timer, SIGNAL(timeout()),
             myObject, SLOT(timerDone()) );
    timer->start( 2000, TRUE );                 // 2 seconds single-shot

As a special case, a QTimer with timeout 0 times 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 *t = new QTimer( myObject );
    connect( t, SIGNAL(timeout()), SLOT(processOneThing()) );
    t->start( 0, FALSE );

myObject->processOneThing() will be called repeatedly and should return 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; multi-threading is now becoming available on more and more platforms and we expect that null events will eventually be replaced by threading.

Note that QTimer's accuracy depends on the underlying operating system and hardware. Most platforms support an accuracy of 20ms; some provide more. 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 of course inherit QObject). The disadvantage is that timerEvent() does not support such high-level features as single-shot timers or signals.

Examples: forever/forever.cpp

Member Function Documentation

QTimer::QTimer ( QObject * parent=0, const char * name=0 )

Constructs a timer with a parent and a name.

Notice that the destructor of the parent object will destroy this timer object.

QTimer::~QTimer ()

Destructs the timer.

void QTimer::changeInterval ( int msec )

Changes the timeout interval to msec milliseconds.

If the timer signal is pending, it will be stopped and restarted, otherwise it will be started.

See also start() and isActive().

bool QTimer::event ( QEvent * e ) [virtual protected]

Reimplemented for internal reasons; the API is not affected.

Reimplemented from QObject.

bool QTimer::isActive () const

Returns TRUE if the timer is running (pending), or FALSE is the timer is idle.

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 to create a local QTimer object.


    #include <qapplication.h>
    #include <qtimer.h>

    int main( int argc, char **argv )
        QApplication a( argc, argv );
        QTimer::singleShot( 10*60*1000, &a, SLOT(quit()) );
            ... // create and show your widgets
        return a.exec();

This sample program automatically terminates after 10 minutes (i.e. 600000 milliseconds).

int QTimer::start ( int msec, bool sshot = FALSE )

Starts the timer with a msecs milliseconds timeout.

If sshot is TRUE, the timer will be activated only once, otherwise it will continue until it is stopped.

Any pending timer will be stopped.

See also stop(), changeInterval() and isActive().

Examples: forever/forever.cpp

void QTimer::stop ()

Stops the timer.

See also start().

void QTimer::timeout () [signal]

This signal is emitted when the timer is activated.

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Qt version 2.3.10