All I can find is "whenever the widget needs to be painted."
When is that, specifically?
When you call updateGL() on your widget (or update()), or just Qt decides to redraw your widget. Reasons why Qt might choose to ask for a redraw include:
- your widget gets resized
- your widget is hidden and shown again
- your widget is minimized and then restored
- something else is put in front of your widget and then moved away
- the Moon is in the third quarter
- a distant butterfly had flapped its wings
- ...
In short, you have very little control about when Qt asks for a repaint. Just be sure to paint fast! :-)
来源：https://stackoverflow.com/questions/17710514/when-is-qglwidgets-paintgl-called