多线程 – 为什么信号/插槽不能与多个线程一起工作？

class A : public QObject{

  Q_OBJECT

signals:
  void a_sig();

public:
  A(){ }

public slots:
  void begin(){
    QObject::connect(&_timer,SIGNAL(timeout()),this,SIGNAL(a_sig()));
    _timer.start(1000); 
  }

private:
  QTimer _timer;
};


class B : public QObject{

  Q_OBJECT

public:
  B(){ value = 0; }

public slots:
  void b_slot(){

    ++value;
    QFile file("out.txt");
    file.open(QIODevice::WriteOnly);
    QTextStream out(&file);
    out << value << "n";
    file.close();
  }

private:
  int value;
};

int main(int argc,char **argv){

  QCoreApplication app(argc,argv);

  A a;
  B b;
  QThread aThread;
  QThread bThread;

  QObject::connect(&aThread,SIGNAL(started()),&a,SLOT(begin()));
  QObject::connect(&a,SIGNAL(a_sig()),&b,SLOT(b_slot()));

  a.moveToThread(&aThread);
  b.moveToThread(&bThread);

  aThread.start();
  bThread.start();

  return app.exec();
}

我试图理解为什么没有调用b_slot().任何人都可以解释发生了什么,以及为什么没有调用b_slot()？