How to pause rendering?

Attila Santo

I'm trying to prevent rendering while the application state is ApplicationSuspended but am stuck atm with my attempt.

QScopedPointer<QQuickWindow> window ... 

QSemaphore renderSemaphore(1);
bool releaseOnRender {};

QObject::connect(window.data(), &QQuickWindow::beforeRendering,
    qApp, [&] {
        releaseOnRender = renderSemaphore.tryAcquire(1, -1);
    }, Qt::DirectConnection);

QObject::connect(window.data(), &QQuickWindow::afterRendering,
    qApp, [&] {
        if (releaseOnRender) {
            renderSemaphore.release(1);
        }
    }, Qt::DirectConnection);

Qt::ApplicationState prevApplicationState = Qt::ApplicationState::ApplicationActive;

QObject::connect(qApp, &QGuiApplication::applicationStateChanged,
    qApp, [&] (Qt::ApplicationState state) {
        qDebug() << "Changed state to" << state;
        if (state != prevApplicationState) {
            if (state == Qt::ApplicationState::ApplicationSuspended) {
                renderSemaphore.acquire(1);
                qDebug() << "Freezing render";
            } else {
                renderSemaphore.release(1);
                qDebug() << "Releasing render";
            }
            prevApplicationState = state;
        }
    }, Qt::DirectConnection);

after initially being blocked in renderSemaphore.tryAcquire(1, -1);, QGuiApplication::applicationStateChanged isn't responding either (though it should be a different thread (shouldn't it?)

Am happy about any advice / idea on how to block, pause or stop rendering while being ApplicationSuspended

kshegunov

I can't tell you why it's not working exactly, but I'd suggest something along these lines:

QAtomicInteger<bool> pauseFlag(false);
QSemaphore pauseSemaphore;

QObject::connect(window.data(), &QQuickWindow::beforeRendering, window.data(), [] () {
    if (pauseFlag.loadAcquire())
        pauseSemaphore.acquire();
});

QObject::connect(qApp, &QGuiApplication::applicationStateChanged, qApp, [] (Qt::ApplicationState state) {
    switch (state)
    {
    case Qt::ApplicationState::ApplicationSuspended:
        pauseFlag.storeRelease(true);
        break;
    case Qt::ApplicationState::ApplicationActive:
        if (pauseFlag.testAndSetRelease(true, false))
            pauseSemaphore.release();
        break;
    }
});

PS. The pauseFlag.storeRelease(true); can probably safely be replaced by a relaxed store.

kshegunov

Something came to mind. You should check that the thread the signal is emitted from isn't the main thread, which'd explain why your code (and mine) deadlocks.