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变化了不是当前组件发现的，是上一级组件通知的（组件节点是树状结构，某个组件的state变化会带动所有子组件重新render，所以才有shouldcomponentupdate的优化点
