完全没问题啊,update()和repaint()是不同的,update()在文档里的说法就是Calling update() several times normally results in just one paintEvent() call.
它不会在你调用update()时马上画出线条,并显示,而repaint()就会马上画,这和Java中的处理是一样的,所以不用担心.
C++ GUI Programming with Qt 4, Second Edition里有讲:
If update() is called multiple times, Qt compresses the consecutive paint events into a single paint event to avoid flicker
应该没问题吧(猜的)