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So I have been using QtConcurrent::run for some time and its fantastic. But now I need the function to return an object. Therefore I use the pseudo code

QFutureWatcher<MyObject> fw;
QFuture<MyObject> t1 = QtConcurrent::run(&thOb, &MythreadObjFunc::getList, ConSettings, form, query);
fw.setFuture(t1);
// Both .results() and waitForFinished() block
fw.waitForFinished();
MyObject entries = t1.result();

Then I iterate through the myObject. The issue is that this is blocking e.g. my main GUI is not responsive. And this was the whole reason I started using QtConcurrent::run

Therefore, what is the recommended way to have my GUI execute a QtConcurrent::run and get the object back but not block? I thought of signals and slots where the signal would be from the QtConcurrent::run but this would mean that it would be from a different thread and I read thats not recommended.

Thanks for your time.

DannyK
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2 Answers2

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You should never use any waitForFinished functions in the GUI thread. Instead, connect a slot to the future watcher's finished signal. See this answer for an example.

Community
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Kuba hasn't forgotten Monica
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1

From QtConcurrent::run() you can't emit any signal. Runnable function is not a QObject. That is the first thing.

The other thing is that QFutureWatcher::waitForFinished() blocks the execution until the thread ends its execution. This is the intended behaviour. If you have to wait for your function to finish, why do you even launch it on separate thread? It makes no sense.

The easiest solution would be to make your function a member of QObject-inherited class, move the instance to the other thread, launch the calculations and emit done() signal. Qt's signal and slot system is thread-safe and it is the perfect way to use it. There is a outstanding documentation provided by Qt that covers this subject more than enough. You should start reading here: http://qt-project.org/doc/qt-4.8/threads.html

Garrappachc
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