How to stop looping thread

Question

I want to stop a looping thread when a signal was emitted so here is my code

void  MyThread::stopWatchingThread()
{
    qDebug()<<"MyThread::stopWatchingThread()";    
    Keep_running=false;
    qDebug()<<"MyThread::stopWatchingThread Keep_running"<<Keep_running;
    ...
}

void MyThread::run()
{
  qDebug()<<"MyThread::run()";
  qDebug()<<"MyThread::run Keep_running"<<Keep_running;  
  while(Keep_running)
    {
     ...
     }
  qDebug()<<"MyThread::run Keep_running"<<Keep_running;
  Keep_running=false;
  qDebug()<<"MyThread::run Keep_running"<<Keep_running;
}

void Watcher::Init()
{
    WatchingThread=new MyThread(this->L_RootToWatch);
    connect(this,SIGNAL(stopmonotiring()),WatchingThread, SLOT(stopWatchingThread()));
...
}
void Watcher::StartWatching()
{
    WatchingThread->start();
}

void Watcher::StopWatching()
{
    emit stopmonotiring();        
}

So every thing goes all right but my problem is that Keep_running never get false value in MyThread::run() after emitting stopWatchingThread and so while loop for ever. What did I miss ? any help will be appreciated.

Answer 1

Don't create threaded classes explicitly in Qt. Instead, create a worker object, move that object to a QThread , then call start() on the QThread . Here's a quick example:

class Worker : public QObject
{
  Q_OBJECT
public:
  Worker( QObject * parent = 0 )
    : QObject( parent )
  {}

public slots:
  void doWork( ... )
  { 
    // do work here
  }

  void stopMonitoring()
  { 
    emit finished();
  }

signals:
  void finished();
};

int main()
{
  Worker * w = new Worker();
  QThread * thread = new QThread();
  QObject::connect( w, SIGNAL(finished()), thread, SLOT(quit())
  QObject::connect( w, SIGNAL(finished()), w, SLOT(deleteLater())
  QObject::connect( thread, SIGNAL(finished()), thread, SLOT(deleteLater())
  w->moveToThread( thread );
  thread->start();

  // some other object emits a signal connected to the 'doWork()' slot.
}

I omitted some of the standard QApplication boiler-plate, but you have that already if you're using Qt. This should get you started.

Answer 2

As your run() method is blocking and the event loop never entered, the slot stopWatchingThread will never be called. You must call exec() and not block the event loop by a spinning loop in run(). Either that, or have the watcher thread call stopWatchingThread directly instead of using a signal/slot connection. I'd go for the latter. keepRunning will be accessed from multiple threads then, so you have to protect it using a QMutex, QReadWriteLock or QAtomic. (Start with QMutex, it's easiest).

Answer 3

如果在线程中使用事件循环，只需将quit()信号发布到线程对象。

Answer 4

Maybe your C++ compiler optimizes away the read operation on Keep_running . Try declaring it as volatile , which tells the compiler that this variable might change "unexpectedly", eg from other threads or hardware interrupts.