[react] re-render

1. state changes
   1. 本级节点会 re-render
   2. 子节点1-普通component，2-pure-component(memoization技术)且props改变，则re-render；使用context的pure-co impotent，可以将context视作props

const GreetUser = React.memo(() => {
const user = React.useContext(UserContext);

// ...
});

// more-or-less equivalent to
const GreetUser = React.memo(({ user }) => {
⁠   // ...
}