c++ - Pause Qt scene graph rendering based on Qt::ApplicationState

Question

I'm trying to pause the rendering in a Qt app on android when put into background. After the initial state change, the app is stuck as no updates reach the connection.

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);

My implementation stems from the suggestion for a related question.

Is there any chance to achieve this? Perhaps using a different approach?

Answer 1

等待大神答复