How can I make my QThread block and wait for a function called from the main thread to return a value?

Question

I am doing some work in a QThread reimplementation. Every now and then, I'd like to ask the user a Yes/No question, so I was planning on using QMessageBox::question(). Problem is, I can't call it from the thread. That's not a big one, I can emit a signal that connects to a slot in the main GUI thread which will display the message box, but I also need the custom thread to block and wait for the message box to be dismissed and retrieve the return value as well (here, a QMessageBox::StandardButton). How do I get around to doing that?

Will the following (pseudo-)code do the trick? 下面的（伪）代码会成功吗？

class MyThread
{
public:
    MyThread(QObject *parent)
    {
        connect(this, SIGNAL(inputRequired()), parent, SLOT(popMsgBox()), Qt::QueuedConnection);
    }

void MyThread::run()
{
    QMutex m;
    for (...)
    {
        if (something) 
        {
            m.lock();
            emit inputRequired();
            w.wait(&m);
            m.unlock();
        }

        if (MyGui->ans_ == Yes) do_something();
    }
}

signals:
    void inputRequired();

protected:
    QWaitCondition w;

};

void MyGui::popMsgBox()
{
    ans_ = QMessageBox::question(this, "Question", "Yes or no?", Yes | No);
    MyThread->w->wakeAll();
}

Answer 1

Simple answer - use a condition.

http://doc.qt.io/qt-5/qwaitcondition.html

Answer 2

If you are working with signals and slots anyway, you can also use the Qt::BlockingQueuedConnection connection type. This connection will wait until the slot (in another thread) is finished executing. Be careful not to deadlock though.