Rate limit of callbacks when control is modified - c++

Question

Let's say I have a slider control and my user is sliding it back and forth really fast.
Is it possible to limit the rate at which QML calls the "new value available" C++ callback?

Answer 1

If you want to completely avoid the value being updated while the slider is dragged, you can use the updateValueWhileDragging property in Qt Quick Controls 1, and the live property in Qt Quick Controls 2.
In Qt Quick Controls 2, the slider controls have a valueAt() function which can be called to check the value at any time.
If you're writing your own slider in QML, you could limit the change signal emission using a Timer, for example:
property int value
readonly property int actualValue: // some calculation...
Timer {
running: slider.pressed
interval: 200
repeat: true
onTriggered: slider.value = slider.actualValue
}

Answer 2

Here's a generic C++-side solution that works with any QObject:
// https://github.com/KubaO/stackoverflown/tree/master/questions/qml-rate-limter-42284163
#include <QtCore>
class PropertyRateLimiter : public QObject {
Q_OBJECT
qint64 msecsPeriod{500};
const QByteArray property;
bool dirty{};
QVariant value;
QElapsedTimer time;
QBasicTimer timer;
QMetaMethod slot = metaObject()->method(metaObject()->indexOfSlot("onChange()"));
void signal() {
if (time.isValid()) time.restart(); else time.start();
if (dirty)
emit valueChanged(value, parent(), property);
else
timer.stop();
dirty = false;
}
Q_SLOT void onChange() {
dirty = true;
value = parent()->property(property);
auto elapsed = time.isValid() ? time.elapsed() : 0;
if (!time.isValid() || elapsed >= msecsPeriod)
signal();
else
if (!timer.isActive())
timer.start(msecsPeriod - elapsed, this);
}
void timerEvent(QTimerEvent *event) override {
if (timer.timerId() == event->timerId())
signal();
}
public:
PropertyRateLimiter(const char * propertyName, QObject * parent) :
QObject{parent}, property{propertyName}
{
auto mo = parent->metaObject();
auto property = mo->property(mo->indexOfProperty(this->property));
if (!property.hasNotifySignal())
return;
connect(parent, property.notifySignal(), this, slot);
}
void setPeriod(int period) { msecsPeriod = period; }
Q_SIGNAL void valueChanged(const QVariant &, QObject *, const QByteArray & name);
};
#include "main.moc"
And a test harness for it:
#include <QtQuick>
const char qmlData[] =
R"__end(
import QtQuick 2.6
import QtQuick.Controls 2.0
ApplicationWindow {
minimumWidth: 300
minimumHeight: 250
visible: true
Column {
anchors.fill: parent
Slider { objectName: "slider" }
Label { objectName: "label" }
}
}
)__end";
int main(int argc, char ** argv) {
QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app{argc, argv};
QQmlApplicationEngine engine;
engine.loadData(QByteArray::fromRawData(qmlData, sizeof(qmlData)-1));
auto window = engine.rootObjects().first();
auto slider = window->findChild<QObject*>("slider");
auto label = window->findChild<QObject*>("label");
PropertyRateLimiter limiter("position", slider);
QObject::connect(&limiter, &PropertyRateLimiter::valueChanged, [&](const QVariant & val){
label->setProperty("text", val);
});
return app.exec();
}