Change interval timeouts of QTimer with slider - c++

Question

timer = new QTimer(this);
timer->setInterval(50);
QPushButton *start = new QPushButton("Start/Stop", this);
start->setText("Start/Stop");
layout->addWidget(start);
connect(start, &QPushButton::clicked, this, [this]() {
if (!timer->isActive()) {
timer->start();
} else {
timer->stop();
}
});
connect(timer, &QTimer::timeout, this, [&slider, sliderDisplay]() {
//increment slider value until at max, then reset to min
}
How can I use another slider to change the interval of timeouts emitted by my QTimer? I've tried using setInterval but it seems that I can't change the interval once it is set. Is the best way to do this just delete this QTimer and create a new QTimer with the specified interval every time?

Answer 1

OP has claimed that
How can I use another slider to change the interval of timeouts emitted by my QTimer? I've tried using setInterval but it seems that I can't change the interval once it is set.
I wondered a bit about that claim because I could've sworn that you can change the interval at any time.
The only restriction, I would consider: the new set interval might not be considered before the next timeout happens. (Usually, I use single-shot timers or timers with small intervals so that you wouldn't notice the difference.)
While playing with my demo, I got the impression that a call of QTimer::setInterval() restarts the interval.
Unfortunately, the doc. of QTimer::setInterval() doesn't mention this behavior explicitly, except:
Setting the interval of an active timer changes its timerId().
Thanks to #Scopchanov who had the look into source code (I was too lazy to).
QTimer::setInterval():
void QTimer::setInterval(int msec)
{
inter = msec;
if (id != INV_TIMER) { // create new timer
QObject::killTimer(id); // restart timer
id = QObject::startTimer(msec, Qt::TimerType(type));
}
}
So, in fact, setting the interval while the timer is running, it kills the currently running timer and restarts a new one.
My MCVE for demonstration – testQTimer.cc:
// Qt header:
#include <QtWidgets>
// main application
int main(int argc, char **argv)
{
qDebug() << "Qt Version:" << QT_VERSION_STR;
QApplication app(argc, argv);
// setup GUI
QWidget qWinMain;
qWinMain.setWindowTitle("Test QTimer");
QFormLayout qForm;
QSlider qSliderInterval(Qt::Horizontal);
qSliderInterval.setRange(100, 1000);
qForm.addRow("Interval: ", &qSliderInterval);
QSlider qSliderStep(Qt::Horizontal);
qSliderStep.setRange(1, 10);
qForm.addRow("Step: ", &qSliderStep);
QPushButton qBtnStartStop("Start / Stop");
qForm.addRow("Timer:", &qBtnStartStop);
QSlider qSliderAnim(Qt::Horizontal);
qForm.addRow("Animation:", &qSliderAnim);
qSliderAnim.setRange(0, 100);
qWinMain.setLayout(&qForm);
qWinMain.show();
// setup timer
QTimer qTimer;
qTimer.setInterval(qSliderInterval.value());
// install signal handlers
QObject::connect(&qSliderInterval, &QSlider::valueChanged,
[&](int value) { qTimer.setInterval(value); });
QObject::connect(&qBtnStartStop, &QPushButton::clicked,
[&]() {
if (qTimer.isActive()) qTimer.stop();
else qTimer.start();
});
QObject::connect(&qTimer, &QTimer::timeout,
[&]() {
int value = qSliderAnim.value() + qSliderStep.value();
if (value > qSliderAnim.maximum()) {
value -= qSliderAnim.maximum() - qSliderAnim.minimum();
}
qSliderAnim.setValue(value);
});
// runtime loop
return app.exec();
}
Output:
Qt Version: 5.13.0