Question

I would like to know if a QTimer will keep on counting once it has timed out.

For example, let’s say I have a QTimer that times out every 500ms. Let’s say when this particular QTimer times out, my program happens to be in some function that I have made (not in the event loop) and it takes 10ms for the program to get from my function, to the event loop. Does that mean that the next time the QTimer times out will be at 1010ms?

If this is the case, is there a way to get around it?

Thanks.

Edit:

Answer 1

I made some test using multiple timers but with timerEvent (QObject::setTimer()).

The thing is, of course if you have multiple timers that interfere at some point (tick exactly in the same time) all your doStuffThatTake10ms code will be ‘queued’ … But the absolute precision of timers should remain over time. Here some code to try it out.

#ifndef MULTITIMER_H
#define MULTITIMER_H

#include <QObject>
#include <QTime>

class MultiTimer : public QObject
{
    Q_OBJECT
public:
    explicit MultiTimer(QObject *parent = 0);
    void timerEvent(QTimerEvent *event);

private:
    int timerId[4];
    int interval[4];
    int count[4];
    QTime absoluteTimer;
};

#endif // MULTITIMER_H

Implementation .cpp

#include "MultiTimer.h"
#include <QTimerEvent>
#include <QTime>
#include <QDebug>

MultiTimer::MultiTimer(QObject *parent) :
    QObject(parent)
{
    interval[0] = 500;
    interval[1] = 1000;
    interval[2] = 1500;
    interval[3] = 2000;

    for( int i = 0; i < 4; i++)
        timerId[i] = startTimer(interval[i]);

    for( int i = 0; i < 4; i++)
        count[i] = 0;

    absoluteTimer.start();
}

void MultiTimer::timerEvent(QTimerEvent *event)
{
    int id = -1;
    for( int i = 0; i < 4; i++){
        if( event->timerId() == timerId[i]){
            id = i;
            count[id]++;
            break;
        }
    }

    if( id != -1) {
        qDebug() << "timer" << id
                 << "interval" << interval[id]
                 << "count" << count[id]
                 << "total" << count[id]*interval[id]
                 << "reference" << absoluteTimer.elapsed();

        usleep(10000);
    }
}

To try it out create a = QApllication() and a MultiTimer object and fire a.exec().

Result after 1 minute on linux

timer 1 interval 1000 count 59 total 59000 reference 59010 
timer 0 interval 500 count 119 total 59500 reference 59500 
timer 0 interval 500 count 120 total 60000 reference 60000 
timer 1 interval 1000 count 60 total 60000 reference 60010 
timer 3 interval 2000 count 30 total 60000 reference 60021 
timer 2 interval 1500 count 40 total 60000 reference 60031 
timer 0 interval 500 count 121 total 60500 reference 60500 

As you can see the 121th tick of timer 0 is right on time at 60500ms … but at 60000ms all timers collide creating delayed execution.