Signal from Main Thread not reaching slot in Second Thread Qt 5

Question

I am writing a program that captures an Image from the Camera every REFRESH_RATE milliseconds to send it to some calculation algorithm. I decided to time the launch of this algorithm using a QThread, but the algorithm is running on the main thread, because I need objects created before in this one. Anyway, this works well. The trouble begins when I try to shut the second thread down... Here's the code.

Camera:

class Camera : public QMainWindow
{
    Q_OBJECT

    public:
        Camera(QWidget *parent = 0){

             AlgoQThread = new QThread;
             myAlgoThreadWorker = new AlgoThreadWorker( (Camera *) this );
             myAlgoThreadWorker->moveToThread(AlgoQThread);

             //Launch The thread
             connect(AlgoQThread, SIGNAL(started()), myAlgoThreadWorker, SLOT(process())); 

             connect(myAlgoThreadWorker, SIGNAL(finished()), AlgoQThread, SLOT(quit())); 
             // The line meant to stop the thread at next loop
             connect(this, SIGNAL(stopAlgoThread()), myAlgoThreadWorker, SLOT(stopThread()));

             connect(myAlgoThreadWorker, SIGNAL(finished()), myAlgoThreadWorker, SLOT(deleteLater()));
             connect(AlgoQThread, SIGNAL(finished()), AlgoQThread, SLOT(deleteLater()));

             AlgoQThread->start();
         }

     private slots:
         void errorString(QString error);

         //This function is triggered by a signal sent by the method called in AlgoThreadWoker::process()
         void some_algorithm(){
             ...
             emit stopAlgoThread();
         }

     signals:
         void stopAlgoThread();

     private:
         QThread * AlgoQThread;
         AlgoThreadWorker * myAlgoThreadWorker;
};

algoThreadWorker:

class AlgoThreadWorker : public QObject
{
    Q_OBJECT

    public:
        AlgoThreadWorker(Camera * context){
            parentCamera = context;
        }
        Camera* parentCamera;

    public slots:
        void process(){
            while(1){

                QMutexLocker locker(&m_mutex);
                if (t_stop) break;

                parentCamera->isCapturingImage = true;

                //This triggers the some_algorithm() using a signal sent by imageCapture from main Thread
                parentCamera->imageCapture->capture();

                //Wait REFRESH_RATE millisecondes
                Sleep(REFRESH_RATE);

            }

            //Ends the thread
            emit finished();
        }

    private slots:
        void stopThread(){
            QMutexLocker locker(&m_mutex);
            t_stop = true;
        };

    signals:
        void finished();
        void error(QString);

    private:
        bool t_stop;
        QMutex m_mutex;
    };

And well, as you may have foresee, it doesn't work. I can launch some_algorithm() with no problems but I can't end the thread. The stopThread() slot isn't even launched, I've tested that already.

Oh, it's a mistake; it's not like that in the actual code, sorry. — Remy San, Jun 29 '14 at 16:41
Can you try putting `qApp->processEvents()` somewhere in the while loop of `process()` method ? — Murat Şeker, Jun 29 '14 at 16:57
Where do you get the sleep function from? Windows? Or [QThread](http://qt-project.org/doc/qt-5/qthread.html)? If the latter is the case, it sleeps whole seconds instead of milliseconds. — Simon Warta, Jun 29 '14 at 20:10
No I did a stupid mistake; I left a comment somewhere on the code; your solution @MuratŞeker with the qApp->processEvents() works well! ’stopThread()’ is finally executed! Sorry for the time loss, and thanks! — Remy San, Jun 29 '14 at 20:13
Nice. @Rems, I figured out a bunch of things with signals and slots while looking for a solution that I'll probably implement soon in my own project ;) — Simon Warta, Jun 29 '14 at 20:28
@MuratŞeker Can you repeat your solution in an answer? That's an important hint that might help other people as well. — Simon Warta, Jun 29 '14 at 20:29

score 3 · Accepted Answer · answered Jun 29 '14 at 20:51

Since you are busy waiting in your while loop, the event loop never gets a chance to process the received signals.

qApp->processEvents() gives the control to your event loop in order to process the awaiting jobs.

Note that generally you do not have to call it yourself, Qt does it for you. In your case, it is neccessary because you have an endless loop which prevents Qt from doing its job.

Solution :

void process(){

    while(1){
        qApp->processEvents();
        ^^^^^^^^^^^^^^^^^^^^^^

        .....
    }

    //Ends the thread
    emit finished();
}

score 0 · Answer 2 · answered Jun 29 '14 at 18:52

0

stopThread() is private and thus only accessible from AlgoThreadWorker.

answered Jun 29 '14 at 18:52

Simon Warta

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`stopThread()` is accesible from the signal slot mechanism, even though it is private. – Murat Şeker Jun 29 '14 at 19:04
It didn't changed anything to switch to public slot anyway: stopThread() is still never triggered. – Remy San Jun 29 '14 at 19:11

score 0 · Answer 3 · edited May 23 '17 at 11:57

Qt's event loop already provides for safe cross-thread slot call and event delivery. Your mistakes are mostly to do with incorrectly reimplementing what Qt already provides:

Things are already thread-safe. Drop the mutex.
thread->quit() works and will exit from the event loop running in the given thread. In a QThread, the thread will then finish,.
Don't make your code non-portable for no reason (Sleep is Windows-specific). If you wish do things periodically, simply run a timer (QBasicTimer or QTimer).

This answer provides a complete example.

Signal from Main Thread not reaching slot in Second Thread Qt 5

3 Answers3