Question

I have a class, which is an abstraction of some device.

class Device  
{  
public:  
  ...  
  void Start();  
  void Stop();  
  void MsgLoop();

signals:
  void sMsgArrived();
}  

Start() and Stop() are called from GUI thread. Start() begins new thread, which runs MsgLoop(). It looks like this:

void MsgLoop()  
{
   forever {  
      if(SUCCESS == ReadMsg()) //synchronous, non-blocking
      {
        ProcessMsg(); //quite fast
        emit sMsgArrived(); //this signal is connected with a slot in GUI thread  
      }
   }
}

When Stop() is called, program should return from MsgLoop() and stop the thread. How can I implement this with QThread without subclassing it?

Answer 1

Generally you have to decide who will be responsible for managing the thread. Is it the Device or the main window? Or possibly some device manager. In your case the Device should probably manage its own thread, so if you don’t want to subclass it, use composition:

class Device : QObject
{
    Q_OBJECT
public:
    Device(QObject * parent = NULL);
    void Start();  
    void Stop();  

private slots:
    void MsgLoop();

signals:
    void sMsgArrived();

private:
    QThread thread;
    bool stopThread;
};


Device::Device(QObject * parent) : QObject(parent)
{
    moveToThread(&thread);
    connect(&thread, SIGNAL(started()), this, SLOT(MsgLoop()));
}

void Device::Start()
{
    stopThread = false;
    thread.start();
}

void Device::Stop()
{
    stopThread = true;
    thread.wait();      // if you want synchronous stop
}

void Device::MsgLoop()
{
  // your loop
  while(!stopThread)
    if(SUCCESS == ReadMsg())
    {
      ProcessMsg();
      emit sMsgArrived();
    }
  QThread::currentThread->quit();
}

NOTE: the thread stopping will only work if ReadMsg really is non-blocking. If you later decide to switch to blocking read (and that would probably be appropriate for most cases), you will have to figure out another way how to stop your thread.