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