Debug functions to assert which phase the render thread and main thread are in

Jarrod

In more complex applications it would make code easier to reason about and guard against regressions to be able to do things like this in debug builds:

Q_ASSERT(QSG::isGuiBlocked());
Q_ASSERT(QSG::isRenderThread());

Q_ASSERT(QSG::isUpdatingPaintNodes());
Q_ASSERT(QQuickItem::isUpdating());
Q_ASSERT(QQuickItem::isUpdatingPolish());

Regarding thread detection, currently AFAIK we can only robustly detect the Qt UI thread, not the render thread. I have this:

inline bool isQtUI() { return QCoreApplication::instance() && QThread::currentThread() == QCoreApplication::instance()->thread(); }

I think there should also be a standard function for this in the Qt API, at least in debug.

Just wanted to double check if there is anything like these functions already before I consider adding my own solution. I would especially like to be able to robustly assert that the render thread is current.

Jarrod

I did this as a workable but not ideal solution to checking if we are on the render thread...

#define QT_REQUIRE_RENDER_THREAD() Q_ASSERT(util::thread::isQtRender())

constexpr auto kRenderThreadName = "Qt Render Thread";

inline bool isQtRender() {
    Q_ASSERT(QThread::currentThread()); 
    return QThread::currentThread()->objectName() == kRenderThreadName;
}

class NameRenderThreadJob : public QRunnable
{
public:
    void run() override
    {
        QThread::currentThread()->setObjectName(kRenderThreadName);
    }
};

And add this to the constructor of our QQuickWindow / QQuickView subclasses, which thankfully in our project are used in all cases:

scheduleRenderJob(new util::thread::NameRenderThreadJob(), QQuickWindow::BeforeSynchronizingStage);

So just manually naming the Qt Render Thread QObject and crossing my fingers that none of the asserts will be checked before the name is set.

Jarrod

I think NoStage is actually a better choice than BeforeSynchronizingStage for this:
https://doc.qt.io/qt-6/qquickwindow.html#scheduleRenderJob

Update: No, use BeforeSynchronizingStage.

If stage is NoStage, job will be run at the earliest opportunity whenever the render thread is not busy rendering a frame.

So I thought perhaps NoStage would cause the render job to run earlier than with BeforeSynchronizingStage. But in my tests this was not the case. Probably this is key: "whenever the render thread is not busy".

Jarrod

Better solution using metaObject():

    constexpr auto kQtRenderThreadClassName = "QSGRenderThread";

    bool util::thread::isQtRender()
    {
        Q_ASSERT(QThread::currentThread()); 
        QString threadName { QThread::currentThread()->metaObject()->className() };
        return threadName == kQtRenderThreadClassName;
    }