How to process events during infinite loop in Worker thread

[armandomartins]

I've created a Worker object in Qt to process video input indefinitely, and then move it into a QThread to keep the UI thread going. The problem is, I designed it such that the video capture function runs in an infinite loop until interrupted by a flag, and the flag was supposed to be set by a slot in the Worker object, but since the Worker object is inside the infinite loop, it never processes this "quit" slot (or at least I think that's what's happening). I depend on an external library so substituting the video polling for another method is not really an option. Can someone please confirm that this is indeed the problem and suggest a solution? Here's the code:

class worker : public QObject{
    Q_OBJECT
public:
    worker(QObject* parent = NULL);
    ~worker(){}
    Q_SLOT void process();
    Q_SLOT void stop();
private:
    bool quit;
};

worker::worker(QObject *parent) : QObject(parent){
    quit = false;
}
void worker::process(){
    while(!quit){
        //this library call puts the thread to sleep until a frame is available
        WaitForVideoFrame();
    }
}
void worker::stop(){
    quit = true;
}

and then from the UI object I have:

MyWorker = new worker();
QThread* thread = new QThread;
MyWorker->moveToThread(thread);
connect(thread, SIGNAL(started()), MyWorker, SLOT(process()));
QPushButton* stop_button = new QPushButton(this);
connect(stop_button, SIGNAL(clicked(bool)), MyWorker, SLOT(stop()));
thread->start();

The problem here is that when I press the stop_button, nothing happens, the worker keeps running the loop. Is there maybe a function that I can call to yield processing time to the event loop from within the infinite loop? Or a better design/solution for this? Any suggestions are welcome.

[JKSH]

@armandomartins said in How to process events during infinite loop in Worker thread:

since the Worker object is inside the infinite loop, it never processes this "quit" slot (or at least I think that's what's happening).

Yes, that's exactly what's happening. Event loops (which process signals/slots) and infinite loops are incompatible with each other; you cannot have both in the same thread.

I depend on an external library so substituting the video polling for another method is not really an option. Can someone please confirm that this is indeed the problem and suggest a solution?

In this case:

1. Don't use a worker QObject + moveToThread(). Instead, subclass QThread and put your infinite loop inside your override of QThread::run().
2. From the main thread, call QThread::requestInterruption() when you want to quit. (You don't need to implement your own Quit function)
3. Replace while (!quit) {...} with while (!isInterruptionRequested()) {...}

This should also significantly simplify your code.

See:

• https://doc.qt.io/qt-5/qthread.html#requestInterruption
• https://doc.qt.io/qt-5/qthread.html#isInterruptionRequested