MoveToThread clarification

[Thuan Seah Tan]

I read some article on the Qt dev blog which suggest the proper way to do multithread stuff is to use moveToThread instead of subclassing QThread.

I have this class that does some serious work which i like to run on a separate thread.

So basically the code look something like this:

.H file
class Data : QObject
{
public:
    void update();
};
 
class Worker : QObject
{
public:
    void setData( Data* data ) { mData = data; }
 
signals:
    void workCompleted();
 
public slots:
    void doWork();
 
private:
    Data* mData;
};
class Master : QObject
{
    Master() { mWorker = new Worker; mWorkerThread = new QThread; }
    void askWorkerToWork();
 
private:
    Worker* mWorker;
    QThread* mWorkerThread;
};
 
.CPP file
void Data::update()
{
    UpdateEvent e("Updated");
    QCoreApplication::sendEvent(this, &e);
}
 
void Worker::doWork()
{
    // run some computation
   this->moveToThread(QCoreApplication::instance()->thread());
   mData->update();
   emit workCompleted();
}
 
void Master::askWorkerToWork()
{
    Data* data = new Data;
    mWorker->setData(data);
    mWorker->moveToThread(mWorkerThread);
    connect(mWorkerThread, SIGNAL(started()), mWorker, SLOT(doWork()), Qt::UniqueConnection);
    connect(mWorker, SIGNAL(workCompleted()), mWorkerThread, SLOT(quit()), Qt::UniqueConnection);
    mWorkerThread->start();
}

.H file
class Data : QObject
{
public:
    void update();
};

class Worker : QObject
{
public:
    void setData( Data* data ) { mData = data; }

signals:
    void workCompleted();

public slots:
    void doWork();

private:
    Data* mData;
};
class Master : QObject
{
    Master() { mWorker = new Worker; mWorkerThread = new QThread; }
    void askWorkerToWork();

private:
    Worker* mWorker;
    QThread* mWorkerThread;
};

.CPP file
void Data::update()
{
    UpdateEvent e("Updated");
    QCoreApplication::sendEvent(this, &e);
}

void Worker::doWork()
{
    // run some computation
   this->moveToThread(QCoreApplication::instance()->thread());
   mData->update();
   emit workCompleted();
}

void Master::askWorkerToWork()
{
    Data* data = new Data;
    mWorker->setData(data);
    mWorker->moveToThread(mWorkerThread);
    connect(mWorkerThread, SIGNAL(started()), mWorker, SLOT(doWork()), Qt::UniqueConnection);
    connect(mWorker, SIGNAL(workCompleted()), mWorkerThread, SLOT(quit()), Qt::UniqueConnection);
    mWorkerThread->start();
}

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I would like to know given the above setup does Qt automatically move the mWorker back to thread which Master object is created when the thread quits? Or do I have to do a moveToThread as shown in the code manually?

Does the moveToThread function actually move the object to the target thread immediately? Qt complains that I cannot call the function QCoreApplication::sendEvent in the Data object because it is created on a different thread.

[wysota]

Qt doesn't move any objects anywhere on its own. Using sendEvent() on an object that lives in a different thread doesn't make any sense. Use postEvent() instead.