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;
    }