QProgressDialog doesn't update

DoubleC122

I am using the example from http://doc.qt.io/qt-5/qprogressdialog.html. The dialog will show only if I call exec(), but there will be displayed only an empty progress bar. I also use processEvents() inside the loop.

mrjj

Hi
The exact code ?

DoubleC122

This is it: QApplication a(argc, argv);

QProgressDialog progress("Copying files...", "Abort Copy", 0, 10);
progress.setWindowModality(Qt::WindowModal);

progress.exec();
for (int i = 1; i <= 10; i++) {

    QCoreApplication::processEvents();
    progress.setValue(i);
    if (progress.wasCanceled())
        break;
    std::cout<<"i: "<<i<<"\n";
}
progress.close();```

SGaist

Hi,

@DoubleC122 said in QProgressDialog doesn't update:

progress.exec();
for (int i = 1; i <= 10; i++) {

exec starts an event loop, so currently, your for loop is only executed after the dialog is closed.

DoubleC122

@SGaist I see. Well, otherwise my dialog won't show. Is there another way?

SGaist

Call show on the dialog.

Just in case, QProgressDialog's documentation offers some example on how to use it.

DoubleC122

Yes, but unfortunately, it won't appear when I call show().

SGaist

Something I didn't realise... Your loop is just too short and too fast for the dialog to appear.

DoubleC122

Hm, I thought it would appear anyway...Still, I changed the limit to something bigger, like 100 and 1000, and still won't show. Quite strange.

mrjj

@DoubleC122
Well its just still too fast as it do no real work.
It does show very fast. (flashing)

for test, you can do

#include <QThread>
...
 progress.show();
  for (int i = 1; i <= 13330; i++) {

    QCoreApplication::processEvents();
    progress.setValue(i);
    if (progress.wasCanceled())
    { break; }
    QThread::sleep(1); // bad in real code! but just to show it does work.
  }

  progress.close();

alt text

DoubleC122

@mrjj Yes, it works indeed. So I guess the dialog will appear only when there is a for loop big enough?

mrjj

@DoubleC122
yes, if it does real stuff like copy file or anything that takes time.
even with high for loop it just too fast to see :)
But if u do something real, it works. Thats what i used sleep to emulate.
but dont use sleep for anything production code as u sleep the app .)

DoubleC122

Got it. Thanks.