Wait till all the Qt concurrent threads finished

Question

I am running 5 threads as below

for(int i=0; i< 5 ; i++
{
   QtConcurrent::run()
}

I want to wait here till all the threads get finished.

void QFutureSynchronizer::waitForFinished ()  is hanging my main GUI.

So I want to use some thing like this QEventLoop::exec();

But how to exit out of this exec() once all the results available?

Edited: - I did some thing like this, that works for me

constructor()
{
   m_noOfThreadsFinished = 0;
   m_totalThreads = 5;

    for(int i=0; i< 5 ; i++
    {
       QFuture<void> l_future =  QtConcurrent::run();
       QPointer< QFutureWatcher<void> >  l_futurewatcher = new  QFutureWatcher<void>();
        connect(l_futurewatcher, SIGNAL(finished()), this, SLOT(FinishedThread()) );
        l_futurewatcher->setFuture(l_future);    
    }

    if(eventLoop != NUL)
          delete eventLoop;
    eventLoop = new QEventLoop(); // QPointer<QEventLoop> eventLoop; is class member

     //start event loop here, so that GUI wont block
     eventLoop->ecec(); 

    //do things after all threads finished
}

void FinishedThread() //slot 
{
    QFutureWatcher<void>* l_futurewatcher = static_cast< QFutureWatcher<void>* > (sender());
    l_futurewatcher->deleteLater();

   if( (++m_noOfThreadsFinished == m_totalThreads) && !m_processCancelled)
   {
        emit finishedreading();
   }
}

void FinishedAllThreads() //slot for finishedreading 
{
    killLocalEventLoop();
}

void killLocalEventLoop()
{
  //QPointer automatically make eventLoop to NULL, when it got deleted 
  eventLoop->quit();
  eventLoop->deleteLater();
}

NOTE : you people can ask why cann't I do the things in FinishedThread() slot when I get to know all threads are finished, project I am working in forcing me to do the things in constructor only when all threads finished(This may not be situation for you then you can do things in FinishedThread() slot).

How about start creating and wainting threads finished at separate thread with own(non-GUI) event loop? — t3ft3l--i, Jun 30 '15 at 09:41
I dont want to use low level threads here, I just looking for how can I come out of exec() once I done with all the threads ?? — NDestiny, Jun 30 '15 at 09:46
I found that QFutureWatcher hangs my main gui - at least if it is inside a button click handler. (The title bar had "(Not responding" on it). I used a progress dialog to hide this, but that isn't appropriate in all cases. — Michael Vincent, Jun 30 '15 at 11:04
@Durga do you have a [MCVE](http://stackoverflow.com/help/mcve) which reproduces the issue you have with `QFutureWatcher`? — m.s., Jun 30 '15 at 13:28
@m.s. if you remove the progress dialog from the tutorial here: http://www.bogotobogo.com/Qt/Qt5_QtConcurrent_QFutureWatcher_QProgressDialog_map.php you will see the problem with the application not responding when using QFutureWatcher inside a button click. — Michael Vincent, Jun 30 '15 at 14:29
@Durga I haven't treid it, but I wonder if one solution would be to move the calling code into another non-gui thread, then call the 5 threads from that thread. A single signal back to the gui from the intermediate thread could be sent when all other threads are complete. — Michael Vincent, Jun 30 '15 at 14:33
@MichaelVincent why would one use `waitForFinished`? of course this will block the GUI. `QFutureWatcher` provides a `finished`signal, use this! — m.s., Jun 30 '15 at 14:35

score 2 · Answer 1 · edited May 23 '17 at 10:31

2

The solution is quite simple: don't ever use any waitForXxx methods in Qt. They are never necessary.

Instead, you should be connecting to the QFutureWatcher::finished() signal. See, for example, this answer.

For some other reason I can not use QFutureWatcher() to get to know all the finished results.

There's no such reason.

edited May 23 '17 at 10:31

Community

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answered Jun 30 '15 at 20:14

Kuba hasn't forgotten Monica

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Wait till all the Qt concurrent threads finished

1 Answers1