每次调用paintEvent()函数，整个界面就会被擦除然后再重绘……但我的数据量很多……这样频繁地擦除以及频繁地刷新就达不到实时性
Qt 4 always erases regions that require painting. The exception to this rule is if the widget sets the Qt::WA_OpaquePaintEvent or Qt::WA_NoSystemBackground attributes. If either one of those attributes is set and the window system does not make use of subwidget alpha composition (currently X11 and Windows, but this may change), then the region is not erased.