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.
}

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

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

class MyClass : public QObject {
  Q_OBJECT

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

signals:
  void mySignalToMainThread();

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

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;
}

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_;
};

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;
}

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();
}

__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