Question

我正在将一个对象实例传递给一个opaque模块，该模块在我的对象上执行SLOT方法（回调）。调用模块位于不同的线程上。我需要在主线程上执行方法。我以为我可以使用信号和插槽执行此操作，但信号方法仍然在调用程序线程上执行。我不认为我应该使用moveToThread（）将我的对象移动到调用者线程 - 是不是有一种向我的应用程序主线程发出信号的机制？

MyClass::MyClass(...)
{
   OtherClass::getInstance()->setCallback(this);
   connect(this, SIGNAL(mySignalToMainThread()), this, SLOT(doThisOnMainThread()));
}

// Public slot, called by OtherClass on its own thread.
void MyClass::someCallback() 
{
   emit mySignalToMainThread();
}

void MyClass::doThisOnMainThread()
{
   // AHHH! I am still on callers thread.
}

Answer 1

起初我认为除了MyClass所在的线程上下文之外，你的信号发射可能是一个问题。但是，Qt thread documentation states：

...你可以安全地从你的QThread :: run（）发出信号实现，因为信号发射是线程安全的。

这几乎取消了这个想法。而且，您使用的是Qt::AutoConnection，for which the documentation states：

如果信号是从与接收不同的线程发出的对象，信号排队，表现为Qt :: QueuedConnection。否则，直接调用插槽，表现为 Qt的:: DirectConnection。连接类型是在确定时确定的发出信号。

最后一点特别重要。如果你要写这段代码：

void MyClass::someCallback() 
{
   Q_ASSERT(QThread::currentThread() != this->thread());
   emit mySignalToMainThread();
}

void MyClass::doThisOnMainThread()
{
   Q_ASSERT(QThread::currentThread() == this->thread());
}

我希望没有断言失败，但你建议你有一个。我不得不得出结论，Qt文档是错误的，或者这个问题比你提到的更多。

Answer 2

这对我有用。也许可以将您的实施与我下面的实施进行比较，看看它可能有何不同。我试图尽可能地遵循你的惯例，猜测缺少的细节：

<强> my_class.h

class MyClass : public QObject {
  Q_OBJECT

public:
  explicit MyClass(QObject *parent = 0);

signals:
  void mySignalToMainThread();

public slots:
  void someCallback();
  void doThisOnMainThread();
};

<强> my_class.cpp

MyClass::MyClass(QObject *parent) :
  QObject(parent) {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
  OtherClass::getInstance().setCallback(this);
  connect(this, SIGNAL(mySignalToMainThread()), SLOT(doThisOnMainThread()));
}

void MyClass::someCallback() {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
  emit mySignalToMainThread();
}

void MyClass::doThisOnMainThread() {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
}

<强> other_class.h

class OtherClass : public QObject {
  Q_OBJECT

public:
  static OtherClass& getInstance();
  void setCallback(MyClass *cb);

public slots:
  void doCallback();

private:
  explicit OtherClass(QObject *parent = 0);

  MyClass *cb_;
};

<强> other_class.cpp

OtherClass::OtherClass(QObject *parent) : QObject(parent) {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
}

OtherClass& OtherClass::getInstance() {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
  static OtherClass singleton;
  return singleton;
}

void OtherClass::doCallback() {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
  cb_->someCallback();
}

void OtherClass::setCallback(MyClass *cb) {
  std::cout << Q_FUNC_INFO <<  QThread::currentThreadId() << std::endl;
  cb_ = cb;
}

<强>的main.cpp

int main (int argc, char **argv) {
  QApplication app(argc, argv);

  MyClass c;

  QThread other;
  OtherClass::getInstance().moveToThread(&other);
  other.connect(&other, SIGNAL(started()),
                &OtherClass::getInstance(), SLOT(doCallback()));
  other.start();

  QMainWindow w;
  w.show();

  return app.exec();
}

示例输出：

__thiscall MyClass::MyClass(class QObject *)00001108
class OtherClass &__cdecl OtherClass::getInstance(void)00001108
__thiscall OtherClass::OtherClass(class QObject *)00001108
void __thiscall OtherClass::setCallback(class MyClass *)00001108
class OtherClass &__cdecl OtherClass::getInstance(void)00001108
class OtherClass &__cdecl OtherClass::getInstance(void)00001108
void __thiscall OtherClass::doCallback(void)000015AC
void __thiscall MyClass::someCallback(void)000015AC
void __thiscall MyClass::doThisOnMainThread(void)00001108

如何在主线程上执行回调

2 个答案: