QPaintDevice: Cannot destroy paint device that is being painted

在paintEvent中，使用QPainter * 繪製圖像出現此問題。解決：

一、改成不使用QPainter指針。

二、添上begin(), end()

QPainter * painter = new QPainter();
painter->begin (this);//!!!
painter->drawLine (...);
painter->end ();//!!!