C++ 将信号连接到lambda时Qt:：QueuedConnection和Qt:：DirectConnection之间的差异 让我们考虑两种情况：代码< QObjs > SeordObjult >代码> > SeordObjult< /C>和接收对象< /代码>类型接收对象< /代码>。code>senderObject已在工作线程上创建，而receiverObject已在主线程上创建。现在让我们来看看代码> SeordObjult有一个信号一些改变了< /代码>，让我们考虑下面的代码片段：

代码片段1:

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  ReceiverObject receiverObject = new ReceiverObject();
  //in this case we know for a fact that receiverObject's 
  //onSomethingsChanged() will be called on the main thread
  QObject::connect(senderObject, &SenderObject::somethingsChanged, 
  receiverObject, &ReceiverObject::onSomethingsChanged, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  //where will qDebug() << "Hello, world" be executed? 
  //On the main thread again since the lambda lives on the main thread?
  QObject::connect(senderObject, &SenderObject::somethingsChanged, 
  []{qDebug << "Hello, world";});
  t.start();
  return a.exec();
}

现在让我们考虑下面的代码片段：

代码片段2:

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  ReceiverObject receiverObject = new ReceiverObject();
  //in this case we know for a fact that receiverObject's 
  //onSomethingsChanged() will be called on the main thread
  QObject::connect(senderObject, &SenderObject::somethingsChanged, 
  receiverObject, &ReceiverObject::onSomethingsChanged, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  //where will qDebug() << "Hello, world" be executed? 
  //On the main thread again since the lambda lives on the main thread?
  QObject::connect(senderObject, &SenderObject::somethingsChanged, 
  []{qDebug << "Hello, world";});
  t.start();
  return a.exec();
}

intmain（intargc，char*argv[]）{
qcorea应用程序（argc、argv）；
qt线程；
SenderObject SenderObject=新的SenderObject（）；
senderObject.moveToThread（&t）；
//qDebug（）将在何处这是我们的类
SenderObject:

class Sender : public QObject {
  Q_OBJECT
 public:
  explicit Sender() : QObject() {
    _timer.setInterval(1000);
    connect(&_timer, &QTimer::timeout, this,
            [this] { emit somethingsChanged(); });
    _timer.start();
  }

 signals:
  void somethingsChanged();

 private:
  QTimer _timer;
};

由于不存在采用lambda和
Qt
连接类型参数的
QObject:：connect
重载，因此Snippet2将在发送方线程（工作线程）中执行

为了强制lambda在主线程上执行，这里有一个解决方法：

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  QObject::connect(senderObject, &SenderObject::somethingsChanged, new QObject,
  []{qDebug << "Hello, world";}, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

或者，更好的解决方案是使用
qApp
，它是应用程序的一个实例和一个单例，可以在任何地方使用

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  QObject::connect(senderObject, &SenderObject::somethingsChanged, qApp,
  []{qDebug << "Hello, world";}, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

intmain（intargc，char*argv[]）{
qcorea应用程序（argc、argv）；
qt线程；
SenderObject SenderObject=新的SenderObject（）；
senderObject.moveToThread（&t）；
QObject:：connect（senderObject和senderObject:：somethingsChanged、qApp、，
[]{qDebug这是我们的类
SenderObject:

class Sender : public QObject {
  Q_OBJECT
 public:
  explicit Sender() : QObject() {
    _timer.setInterval(1000);
    connect(&_timer, &QTimer::timeout, this,
            [this] { emit somethingsChanged(); });
    _timer.start();
  }

 signals:
  void somethingsChanged();

 private:
  QTimer _timer;
};

由于不存在采用lambda和
Qt
连接类型参数的
QObject:：connect
重载，因此Snippet2将在发送方线程（工作线程）中执行

为了强制lambda在主线程上执行，这里有一个解决方法：

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  QObject::connect(senderObject, &SenderObject::somethingsChanged, new QObject,
  []{qDebug << "Hello, world";}, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

或者，更好的解决方案是使用
qApp
，它是应用程序的一个实例和一个单例，可以在任何地方使用

int main(int argc, char *argv[]) {
  QCoreApplication a(argc, argv);
  QThread t;
  SenderObject senderObject = new SenderObject();
  senderObject.moveToThread(&t);
  QObject::connect(senderObject, &SenderObject::somethingsChanged, qApp,
  []{qDebug << "Hello, world";}, Qt::QueuedConnection);
  t.start();
  return a.exec();
}

intmain（intargc，char*argv[]）{
qcorea应用程序（argc、argv）；
qt线程；
SenderObject SenderObject=新的SenderObject（）；
senderObject.moveToThread（&t）；
QObject:：connect（senderObject和senderObject:：somethingsChanged、qApp、，
[]{qDebug