Qt子控件调用update或repaint无效

当继承QAbstractItemView（如QHeaderView, QListView, QTableView）子类的时候，由于实现某个逻辑需要直接调用update或repaint时是不会触发paintEvent，因为Api文档里面说了：

Note: If you inherit QAbstractItemView and intend to update the contents of the viewport, you should use viewport->update() instead of update() as all painting operations take place on the viewport.

正确的做法是使用

viewport()->update();