文档描述不够详细，有人测试结果如下：
- init does not cause layoutSubviews to be called (duh)
- addSubview causes layoutSubviews to be called on the view being added, the view it’s being added to (target view), and all the subviews of the target
- view setFrame intelligently calls layoutSubviews on the view having its frame set only if the size parameter of the frame is different
- scrolling a UIScrollView causes layoutSubviews to be called on the scrollView, and its superview
- rotating a device only calls layoutSubview on the parent view (the responding viewControllers primary view)
- Resizing a view will call layoutSubviews on its superview
什么时候layoutSubview会被调用
原文：http://www.cnblogs.com/walkerwang/p/3963450.html
