Launching QThreads from a loop

[lynchkp]

Hello,

I have an algorithm which runs on image data, which I have implemented so that it occurs on a separate QThread to prevent locking the GUI. This works perfectly when I am running it with one image. Instead, I'd like to run a loop over multiple images, but only one thread executing at a time... something along the lines of...

// Loop over all files.
    int i = 0;
    while (i < selected_files.size()) {
 
        QThread* thread = new QThread;
        Worker *worker = new Worker(filenames,s2);
        worker->moveToThread(thread);
        connect(thread, SIGNAL(started()), worker, SLOT(process()));
        connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        thread->start();
 
        i+=step;
    }

// Loop over all files.
    int i = 0;
    while (i < selected_files.size()) {

        QThread* thread = new QThread;
        Worker *worker = new Worker(filenames,s2);
        worker->moveToThread(thread);
        connect(thread, SIGNAL(started()), worker, SLOT(process()));
        connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        thread->start();

        i+=step;
    }

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But it currently runs all the threads at the same time. I want this to operate one thread at a time. I know this is probably very basic, but I am still learning threaded programming!

Thanks!

[ChrisW67]

Then only start one thread that processes a queue of files rather than one thread for each entry in the queue.

[amleto]

someclass::slot_make_next_thread()
{
  // need some saved variables so we know which 'filenames' and which 's2' to pass on.
  make_thread(filenames, s2);
}
 
someclass::make_thread(filenames, s2)
{
        QThread* thread = new QThread;
        Worker *worker = new Worker(filenames,s2);
        worker->moveToThread(thread);
        connect(thread, SIGNAL(started()), worker, SLOT(process()));
        connect(thread, SIGNAL(finished()), this, SLOT(slot_make_next_thread()));
        connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        thread->start();
}

someclass::slot_make_next_thread()
{
  // need some saved variables so we know which 'filenames' and which 's2' to pass on.
  make_thread(filenames, s2);
}

someclass::make_thread(filenames, s2)
{
        QThread* thread = new QThread;
        Worker *worker = new Worker(filenames,s2);
        worker->moveToThread(thread);
        connect(thread, SIGNAL(started()), worker, SLOT(process()));
        connect(thread, SIGNAL(finished()), this, SLOT(slot_make_next_thread()));
        connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        thread->start();
}

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[wysota]

Another solution:

QImage algorithm(QImage img) {
 // ...
}
 
QList<QImage> imagesToProcess = ...;
QThreadPool::globalInstance()->setMaxThreadCount(1); // if you want to force to have only one thread at a time
QFuture<QImage> future = QtConcurrent::mapped(imagesToProcess, algorithm);
QFutureWatcher<QImage> *watcher = new QFutureWatcher<QImage>(this);
watcher->setFuture(future);
connect(watcher, SIGNAL(finished()), ..., ...);

QImage algorithm(QImage img) {
 // ...
}

QList<QImage> imagesToProcess = ...;
QThreadPool::globalInstance()->setMaxThreadCount(1); // if you want to force to have only one thread at a time
QFuture<QImage> future = QtConcurrent::mapped(imagesToProcess, algorithm);
QFutureWatcher<QImage> *watcher = new QFutureWatcher<QImage>(this);
watcher->setFuture(future);
connect(watcher, SIGNAL(finished()), ..., ...);

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