setState
1. it mutate value asynchronously
2. the second parameter can be a function
Props:
1.As soon as component's props change, children component will update.
2. props變化了不是當前組件發(fā)現(xiàn)的，是上一級組件通知的（組件節(jié)點是樹狀結(jié)構(gòu)，某個組件的state變化會帶動所有子組件重新render，所以才有shouldcomponentupdate的優(yōu)化點
