setneedslayout -> flag layout 需要改变 -> layoutifneed (立即改变) ->layoutsubviews
会重新布局子View
-----自动触发--------
bounds frame (position resize) addsubview
旋转 rotating
uiscrollow ( self and superview)
updateconstraint
text color image coregraph drawing
setneedsdisplay -> flag 需要重新draw -> 一般无需立即改变 -> draw rect
不会重写子View
setneedsupdateconstraint -> flag 需要更新 constraint -> updateconstraintifneed (立即改变) -> updateconstraints
active/deactive constraint
update constraint value priority
removefromsuperview
viewcontroller生命周期
instance
loadView
viewdidLoad
viewwillappear
viewwilllayoutsubview
viewdidlayoutsubview
viewdidappear
viewwilldisappear
viewdiddisappear
dealloc
