QFutureWatcher not calling connected slot

Question

I have the following code which implements QtConcurrent::run() with QFutureWatcher to start the fetch() function which runs a shell process. Upon completion, I want to call the writeDesc function, but it never gets called.

void MyClass::on_fetchButton_clicked()
{
    QFuture<void> fetcher;
    QFutureWatcher<void> watcher;
    connect(&watcher, SIGNAL(finished()), this, SLOT(writeDesc()));
    fetcher = QtConcurrent::run(this, &MyClass::fetch);
    watcher.setFuture(fetcher);
}

void MyClass::fetch()
{
    [...]
    qDebug()<<"Thread done";
}

void MyClass::writeDesc()
{
    qDebug()<<"Slot called";
    [...]
}

fetch() works fine but the program shows only the debug message Thread done and not Slot called. Why isn't writeDesc being called?

Reminder: questions with problems caused by an automatic variable getting destroyed at the end of a function are subject to closure. They are in the "simple typo" category. OTOH, it's true that Qt could offer a qWarning in destructors of some classes that are especially prone to such misuse. — Kuba hasn't forgotten Monica, Mar 22 '14 at 14:29

score 6 · Accepted Answer · answered Mar 22 '14 at 14:11

The problem is that the watcher object is destroyed when leaving the MyClass::on_fetchButton_clicked() method. A destroyed object can not emit something. Try to allocate and connect the watcher in the constructor of your class. Don't forget to destroy in the destructor.

QFutureWatcher not calling connected slot

1 Answers1