[Interest] QProgressDialog not showing & processEvents()

[David Faure]

On Monday, November 18, 2013 08:09:41 AM André Somers wrote:
> It is *not* called by setValue. That is a Good Thing(TM).

I strongly encourage reading code or documentation before making such 
answers...

  \warning If the progress dialog is modal
    (see QProgressDialog::QProgressDialog()),
    setValue() calls QApplication::processEvents(), so take care that
    this does not cause undesirable re-entrancy in your code. For example,
    don't use a QProgressDialog inside a paintEvent()!


void QProgressDialog::setValue(int progress)
{
    Q_D(QProgressDialog);
    if (progress == d->bar->value()
        || (d->bar->value() == -1 && progress == d->bar->maximum()))
        return;

    d->bar->setValue(progress);

    if (d->shown_once) {
        if (isModal())
            QApplication::processEvents();
    } else {
  [...]
}

This is not a bug however, it's the expected use case for QProgressDialog.
The usually-very-dangerous processEvents() is only midly dangerous because we 
are "protected by the progress dialog", i.e. the user cannot close the 
mainwindow, for instance. But of course, timers and sockets could still 
trigger code that deletes objects etc.

In any case - to come back to the initial question: did you call setValue(0) 
upfront?

I just pushed a review request for this to happen automatically, but it's not 
merged yet. https://codereview.qt-project.org/71619


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David Faure | david.faure at kdab.com | Managing Director KDAB France
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