QThread moveToThread不起作用

Question

I've a main thread for the GUI, in which is run the MainWindow object, in its constructor I create a new worker object and a QThread object and the i move the worker to the thread, the problem is that when printing their ids they are the same:

MainWindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    std::cout<<"MAIN_ID "<< QThread::currentThreadId()<<std::endl;
    QThread *t_pc = new QThread;
    worker *pc_w;
    pc_w = new pc_worker();
    pc_w->moveToThread(t_pc);
    t_pc->start();
    pc_w->initialize();
    // ...
}

worker.cpp

worker::worker(QObject *parent) : QObject(parent) {

}

void worker::initialize() {
    std::cout << "INITIALIZE " << QThread::currentThreadId() << std::endl;
}

I get:

MAIN_ID 0x7f4009ccb780
INITIALIZE 0x7f4009ccb780

What's wrong?

Answer 1

Answer: moveToThread does work, just not in the way you expected.

Looks like after calling pc_w->moveToThread(t_pc) you expected all member functions of pc_w to be called in t_pc now. However, that is not what moveToThread() does.

The purpose of moveToThread() is to change QObject "thread affinity" or in other words thread where an object lives. But on the basic level everything it gives you is just the guarantee that all the object`s slots connected to any signal via Qt::QueuedConnection will be invoked (run) in that particular thread.

Member functions still run in the thread you invoke them from. In your case, you call initialize() from the GUI thread, so QThread::currentThreadId() gives you the id of that thread.

I really recommend to read official doc on thread affinity and this article on thread event loops.

QThread* thread = new QThread;
Worker* worker = new Worker;

// Uses Qt::AutoConnection (default)
// which will be transalted into Qt::QueuedConnection
QObject::connect(thread, &QThread::started, worker, &Worker::initialize);

std::cout<<"MAIN_ID "<< QThread::currentThreadId()<<std::endl;

worker->moveToThread(thread);
thread->start();

Output:

MAIN_ID 0000000000003E5C
INITIALIZE 0000000000003DAC

---

The solution trivelt proposed artificially puts "call initialize() " event into the thread event loop reaching the same effect. However, it does not perform any compile-time checks ("initialize" is specified as a string).

Answer 2

Try to use invokeMethod instead of diretly calling pc_w->initialize() , eg:

QMetaObject::invokeMethod(pc_w, "initialize", Qt::QueuedConnection);

Your initialize() method should be SLOT or SIGNAL in this case.