Qt paintEvent() unnormally triggered

Question

I'm a Qt beginner.

So I have my MainWindow with a QSlider and a QPixmap. I redefined the paintEvent( QPaintEvent* event ) and

connect( slider, SIGNAL(valueChanged(int)), this, SLOT(centerChange(int)) );

with a slot:

void MainWindow::centerChange(int value)
{
    center = value;
    update();
}

So I wanted repainte the Pixmap only if the slider's value is changed. But I notice that everytime when I make a mouse-in or a mouse-out to the slider, the repaint is triggered. Why is this happenning?

Thanks.

Answer 1

Paint event may be triggered at any time by the underlying Qt drawing system. You should not assume that paint event can be triggered only by you. You need to change the logic in your app.

Answer 2

The documentation says:

A paint event is a request to repaint all or part of a widget. It can happen for one of the following reasons:

• repaint() or update() was invoked,
• the widget was obscured and has now been uncovered, or
• many other reasons.

(emphasis mine). Specifically, the underlying OS could trigger a repaint event whenever it feels like.

Answer 3

Based on your requirements, don't reimplement paintEvent then. Just do your QPixmap updating when the slider value changed.