CODEWITHSUNDEEP
c++multithreadingqt
Charrette
Charrette

Reputation: 720

QThread with slots and signals does not seem to create a new thread

I'm having some issues with Qt threading to allow the threaded part to update the GUI of my program. Seems like it's a known "problem" with Qt, so I found multiple tutorials, but I don't understand why my example here is not working.

I inherited from QThread as follow:

class CaptureThread: public QThread {
  Q_OBJECT

public:
  CaptureThread(const QObject *handler, QPushButton *start) {
    CaptureThread::connect(start, SIGNAL(clicked()), this, SLOT(capture_loop()));
    CaptureThread::connect(this, SIGNAL(sendPacket(Packet*)),
                           this, SLOT(receivePacket(Packet*)));
  }

signals:
  void sendPacket(Packet*);

public slots:
  void capture_loop() {
    Packet *packet;

    while (my_condition) {
      packet = Somewhere::getPacket();
      //getPacket is define somewhere and is working fine                                

      emit(sendPacket(packet));
      std::cout << "Sending packet!" << std::endl;
    }
  }
};

And here is the CaptureHandler:

class CaptureHandler: public QWidget {
  Q_OBJECT

public:
  CaptureHandler() {
    start = new QPushButton("Capture", this);

    thread = new CaptureThread(this, start);
    thread->start();
  }

public slots:
  void  receivePacket(Packet *packet) {
    std::cout << "Packet received!" << std::endl;
    /*                                                                                   
      Here playing with layout etc...                                                    
     */
  }

private:
  QPushButton   *start;
  CaptureThread *thread;
};

I think the signals and slots are ok, because it displays on the terminal

Sending packet!
Packet received!
Sending packet!
Packet received!
Sending packet!
Packet received!

But in the receivePacket slot, i'm trying to modify my GUI, and it does not work. The GUI just freeze, and all I can do is CTRL+C on terminal. So i think my capture_loop, which is an infinite loop for the moment, is blocking the program, which means my thread has not started.

But I called thread->start(). I even tried to start the thread in CaptureThread constructor, by calling this->start(), but the result is the same.

Any idea?

Upvotes: 0

Views: 1223

Answers (1)

Felix
Felix

Reputation: 7146

Your using QThread wrong. By just creating the thread, it will not execute on the thread. you will need to do it inside the QThread::run function (by overriding it), since thats the only one that will run on the new thread. Please notice, that as soon as you return from this function, the thread will exit.

If you want to use your own loop inside the QThread::run function (instead using Qts default event loop), the thread won't be able to receive signals inside the run-function!

Here an example on how to use QThread:

class CaptureThread: public QThread {
  Q_OBJECT

public:
  CaptureThread(const QObject *handler, QPushButton *start) {
     //calling "start" will automatically run the `run` function on the new thread
     CaptureThread::connect(start, SIGNAL(clicked()), this, SLOT(start()));
    //use queued connection, this way the slot will be executed on the handlers thread
    CaptureThread::connect(this, SIGNAL(sendPacket(Packet*)),
                           handler, SLOT(receivePacket(Packet*)), Qt::QueuedConnection);
  }

signals:
  void sendPacket(Packet*);

protected:
  void run() {
    Packet *packet;

    while (my_condition) {
      packet = Somewhere::getPacket();
      //getPacket is define somewhere and is working fine                                

      emit sendPacket(packet) ;//emit is not a function
      qDebug() << "Sending packet!";//you can use qDebug
    }
  }
};

Upvotes: 2

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