重要的钩子

1.render：初始化渲染或更新渲染调用
2.componentDidMount：开启监听，发送ajax请求
3.componentWillUnmount：收尾工作

生命周期（新）

1.初始化阶段:由ReactDOM.render()触发—初次渲染
(1). constructor
(2). getDerivedStateFromProps //从Props得到一个派生的状态，即state的值任何时候都取决与props，派生状态会导致代码冗余组件难以维护，罕见
(3). render()
(4). componentDidMount() ===>常用：开启定时器，发送网络请求，订阅消息

2.更新阶段:由组件内部this.setState()或父组件重新render触发
(1). getDerivedStateFromProps
(2). shouldComponentUpdate()
(3). render()
(4. getSnapshotBeforeUpdate //得到快照
(5). componentDidUpdate()

3.卸载组件:由ReactDOM.unmountComponentAtNode()触发
(1). componentWillUnmount() ===>常用：关闭定时器，取消订阅消息

      class Add extends React.Component{
            
            constructor(props){
                  console.log("1.constructor")
                  super(props)
                  this.state = { count:0 } 
            }

            state = { count:0 }
            
            add = ()=>{
                  const {count} = this.state
                  this.setState({ count:count+1 })
            }

            death =  ()=>{
                  ReactDOM.unmount(document.getElementById('test'))
            }
            
            //强制更新不改状态
            force = ()=>{
                  this.forceUpdate()
            }

            //从Props得到一个派生的状态，即state的值任何时候都取决与props，派生状态会导致代码冗余组件难以维护，罕见
            static getDerivedStateFormProps(props,state){
                  console.log("getDerivedStateFormProps");
                  console.log(props);
                  console.log(state);
                  return props
            }

            //在更新之前获取快照
            getSnapshotBeforeUpdate(prevProps,prevState){
                  console.log("getSnapshotBeforeUpdate");
                  return null
            }
            
            //组件挂载完毕的钩子
            componentDidMount(){
                  console.log('4.componentDidMount');
            }            
            
            //组件将要卸载的钩子
            componentWillUnmount(){
                  console.log('conponentWillUnmount');
            }

            //控制组件更新的"阀门"
            shouldComponentUpdate(){
                  console.log('shouldComponentUpdate');
                  return true
            }

            //组件更新完毕的钩子
            componentDidUpdate(prevProps,prevState,snapshotValue){
                  console.log('componentDidUpdate',preProps,prevState,snapshotValue);
            }

            render(){
                  console.log('3.render');
                  const { count } = this.state
                  return(
                        <div>
                              <h2>当前求和为</h2>
                              <button onClick="this.add">点我+1</button>
                              <button onClick="this.death">卸载组件</button>
                              <button onClick="this.force">不更改任何状态中的数据，强制更新一下</button>
                        </div>
                  )
            }
      }
      ReactDOM.render(<Count count="199" />,document.getElementById('test'))

getSnapshotBeforeUpdate

      class NewsList extends Reac.Component{
            state = {newsArr:[]}
            componentDidMount(){
                  setInterval(()=>{
                        const {newsArr} = this.state
                        const news = '新闻'+(newsArr.length+1)
                        this.setState({news,[news,...newsArr]})
                  },1000)
            }
            getSnapshotBeforeUpdate(){
                  return this.refs.list.scrolHeight
            }
            componentDidUpdate(preProps,preState,height){
                  this.refs.list.scrollTop += this.refs.list.scrolHeight-height
            }
            render(){
                  return(
                        <div class="list" ref="list">
                        {
                              this.state.newsArr.map((n,index)=>{
                                    return <div 
key={index} class="news">新闻{n}<div>
                              }
                        }
                              
                        <div>
                  )
            }
      }
      ReactDOM.render(<NewList />,document.getElementById('test'))