QTimer

Remarks#

QTimer can also be used to request a function to run as soon as the event loop has processed all the other pending events. To do this, use an interval of 0 ms.

// option 1: Set the interval to 0 explicitly.
QTimer *timer = new QTimer;
timer->setInterval( 0 );
timer->start();

// option 2: Passing 0 with the start call will set the interval as well.
QTimer *timer = new QTimer;
timer->start( 0 );

// option 3: use QTimer::singleShot with interval 0
QTimer::singleShot(0, [](){
    // do something
});

Simple example

The following example shows how to use a QTimer to call a slot every 1 second.

In the example, we use a QProgressBar to update its value and check the timer is working properly.

main.cpp

#include <QApplication>

#include "timer.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    Timer timer;
    timer.show();

    return app.exec();
}

timer.h

#ifndef TIMER_H
#define TIMER_H

#include <QWidget>

class QProgressBar;

class Timer : public QWidget
{
    Q_OBJECT

public:
    Timer(QWidget *parent = 0);

public slots:
    void updateProgress();

private:
    QProgressBar *progressBar;
};

#endif

timer.cpp

#include <QLayout>
#include <QProgressBar>
#include <QTimer>

#include "timer.h"

Timer::Timer(QWidget *parent)
    : QWidget(parent)
{
    QHBoxLayout *layout = new QHBoxLayout();

    progressBar = new QProgressBar();
    progressBar->setMinimum(0);
    progressBar->setMaximum(100);

    layout->addWidget(progressBar);
    setLayout(layout);

    QTimer *timer = new QTimer(this);
    connect(timer, &QTimer::timeout, this, &Timer::updateProgress);
    timer->start(1000);

    setWindowTitle(tr("Timer"));
    resize(200, 200);
}

void Timer::updateProgress()
{
    progressBar->setValue(progressBar->value()+1);
}

timer.pro

QT += widgets

HEADERS = \
          timer.h
SOURCES = \
           main.cpp \
           timer.cpp

Singleshot Timer with Lambda function as slot

If a singleshot timer is required, it is quiet handy to have the slot as lambda function right in the place where the timer is declared:

QTimer::singleShot(1000, []() { /*Code here*/ } );

Due to this Bug (QTBUG-26406), this is way is only possible since Qt5.4.

In earlier Qt5 versions it has to be done with more boiler plate code:

  QTimer *timer = new QTimer(this);
  timer->setSingleShot(true);

  connect(timer, &QTimer::timeout, [=]() {
    /*Code here*/
    timer->deleteLater();
  } );

Using QTimer to run code on main thread

void DispatchToMainThread(std::function<void()> callback)
{
    // any thread
    QTimer* timer = new QTimer();
    timer->moveToThread(qApp->thread());
    timer->setSingleShot(true);
    QObject::connect(timer, &QTimer::timeout, [=]()
    {
        // main thread
        callback();
        timer->deleteLater();
    });
    QMetaObject::invokeMethod(timer, "start", Qt::QueuedConnection, Q_ARG(int, 0));
}

This is useful when you need to update a UI element from a thread. Keep in mind lifetime of anything the callback references.

DispatchToMainThread([]
{
    // main thread
    // do UI work here
});

Same code could be adapted to run code on any thread that runs Qt event loop, thus implementing a simple dispatch mechanism.

Basic Usage

QTimer add the functionality to have a specific function/slot called after a certain interval (repeatedly or just once).

The QTimer thus allows a GUI application to “check” things regularly or handle timeouts without having to manually start an extra thread for this and be careful about race conditions, because the timer will be handled in the main-event loop.

A timer can simply be used like this:

QTimer* timer = new QTimer(parent); //create timer with optional parent object
connect(timer,&QTimer::timeout,[this](){ checkProgress(); }); //some function to check something
timer->start(1000); //start with a 1s interval

The timer triggers the timeout signal when the time is over and this will be called in the main-event loop.

QTimer::singleShot simple usage

The QTimer::singleShot is used to call a slot/lambda asynchronously after n ms.

The basic syntax is :

QTimer::singleShot(myTime, myObject, SLOT(myMethodInMyObject()));

with myTime the time in ms, myObject the object which contain the method and myMethodInMyObject the slot to call

So for example if you want to have a timer who write a debug line “hello !” every 5 seconds:

.cpp

void MyObject::startHelloWave()
{
    QTimer::singleShot(5 * 1000, this, SLOT(helloWave()));
}

void MyObject::helloWave()
{
    qDebug() << "hello !";
    QTimer::singleShot(5 * 1000, this, SLOT(helloWave()));
}

.hh

class MyObject : public QObject {
    Q_OBJECT
    ...
    void startHelloWave();

private slots:
    void helloWave();
    ...
};