Qt创建者：Qwidget的paintEvent

Question

I have some problems. Hope anyone can help me. I have a Qwidget1 and Qwidget2. Qwidget1 have a widget that promote to Qwidget2. Both Qwidget1 and Qwidget2 have paintEvent. I have writed "qDebug()<< "Update"; " in paint event of Qwidget1. When I run project, I see a word "Update" has been printed a lot of times. So why Qwidget1 execute paint event a lot of times. How can I fix it, just execute paint event when show Qwidget1 at the first time and when I call update.

Answer 1

This is expected behavior. Your code works like it should. From Qt documentation:

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.

Answer 2

There can be any number of situations when a window or its part becomes invalidated and has to be repainted. Such situations include, but are not limited to:

• window size change (including minimizing / maximizing / restoring the window);
• mouse pointer passing over a widget - it may or may not trigger repaint;
• other window moving over the window in question.

When it happens, Windows will send the WM_PAINT message to the application. You could check whether or not the number of WM_PAINT messages received matches the number of paintEvent calls, but I doubt Qt adds any significant overhead.