UIView的layoutSubviews、layoutIfNeeded、setNeedsLayout区别和联系
关键词layoutSubviews,layoutIfNeeded,setNeedsLayoutlayoutSubviews官方描述ThedefaultimplementationofthismethoddoesnothingoniOS5.1andearlier.Otherwise,thedefaultimplementationusesanyconstraintsyouhavesettodeter