QProgressBar not showing progress?

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As @rjh and @Georg have pointed out, there are essentially two different options:

  1. Force processing of events using QApplication::processEvents(), OR
  2. Create a thread that emits signals that can be used to update the progress bar

If you’re doing any non-trivial processing, I’d recommend moving the processing to a thread.

The most important thing to know about threads is that except for the main GUI thread (which you don’t start nor create), you can never update the GUI directly from within a thread.

The last parameter of QObject::connect() is a Qt::ConnectionType enum that by default takes into consideration whether threads are involved.

Thus, you should be able to create a simple subclass of QThread that does the processing:

class DataProcessingThread : public QThread
 {

 public:
     void run();
 signals:
     void percentageComplete(int);
 };

 void MyThread::run()
 {
    while(data.hasMoreItems())
    {
      doSomeProcessing(data.nextItem())
      emit percentageCompleted(computePercentageCompleted());
    }
 }

And then somewhere in your GUI code:

DataProcessingThread dataProcessor(/*data*/);
connect(dataProcessor, SIGNAL(percentageCompleted(int)), progressBar, SLOT(setValue(int));
dataProcessor.start();

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