setNeedsLayout, layoutIfNeeded and layoutSubviews?之间的关系
setNeedsLayoutisaneasyone:itjustsetsaflagsomewhereintheUIViewthatmarksitasneedinglayout.ThatwillforcelayoutSubviewstobecalledontheviewbeforethenextredrawhappens.Notethatinmanycasesyoudon'tneedtocallth