react学习——15react生命周期（新）

一、生命周期图新
在这里插入图片描述

二、生命周期三个阶段（新）

1. 初始化阶段：由ReactDOM.render()触发—初次渲染

1.	constructor()
2.	getDerivedStateFromProps 
3.	render()
4.	componentDidMount()

2. 更新阶段：由组件内部this.setSate()或父组件重新render触发

1.	getDerivedStateFromProps
2.	shouldComponentUpdate()
3.	render()
4.	getSnapshotBeforeUpdate（）
5.	componentDidUpdate()

3. 卸载组件: 由ReactDOM.unmountComponentAtNode()触发

1.componentWillUnmount()

4、重要的勾子

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：做一些收尾工作, 如: 清理定时器

5、即将废弃的勾子

1.	componentWillMount
2.	componentWillReceiveProps
3.	componentWillUpdate

代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <!--    移动端适配-->
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>3_react生命周期(新）.html</title>
</head>
<body>
<!--准备一个容器-->
<div id="root"></div>
<!--引入react核心库-->
<script type="text/javascript" src="../js/17.0.1/react.development.js"></script>
<script type="text/javascript" src="../js/17.0.1/react-dom.development.js"></script>
<!--引入babel，用于jsx的转换jsx-->
<script type="text/javascript" src="../js/17.0.1/babel.min.js"></script>
<script type="text/babel">/*此处一定要写babel*/
    //创建组件
class Demo extends React.Component{
    //构造器
    constructor(props){
        console.log("count--constructor")
        super(props)
        this.state={
            count: 1
        }
    }
    //组件挂载完毕
    componentDidMount(){
        console.log("count--componentDidMount")
    }


    //组件挂载完毕
    componentDidMount(){
        console.log("count--componentDidMount")
    }
    //组将将要被卸载
    UNSAFE_componentWillUnmount(){
        console.log("count--componentWillUnmount")

    }
    //控制组件更新的阀门
    shouldComponentUpdate(nextProps, nextState, nextContext) {
        console.log("count--shouldComponentUpdate")
        return true
    }
    //组件更新完毕
    componentDidUpdate(prevProps, prevState, snapshot) {
        console.log("count--componentDidUpdate",prevProps,prevState,snapshot)
    }
    //更新之前获取快照
    getSnapshotBeforeUpdate(prevProps, prevState) {
        console.log("count--getSnapshotBeforeUpdate")
        //return null
        return 'getSna'
    }
    static getDerivedStateFromProps(nextProps, prevState) {
        console.log("count--getDerivedStateFromProps",nextProps,prevState)
        return null
    }
    death=()=>{
        ReactDOM.unmountComponentAtNode(document.getElementById('root'))
    }
    add=()=>{
        this.setState({
            count:this.state.count+1
        })
    }
    //强制更新
    force=()=>{
        this.forceUpdate()
    }
    // 调用时机：初始化渲染和更新之后
    render(){
        console.log("count--render")
        const {count} = this.state
        return(
            <div>
                <h1 >当前求和为：{count}</h1>
                <button onClick={this.add}>点我加1</button>
                <button onClick={this.death}>卸载组件</button>
                <button onClick={this.force}>不更改任何数据中的状态，强制更新一下</button>
            </div>
        )
    }
}
        ReactDOM.render(<Demo count={199}/>,document.getElementById('root'))
</script>
</body>
</html>

6、getSnapshotBeforeUpdate场景

代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <!--    移动端适配-->
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>4_getSnapshotBeforeUpdate使用场景.html</title>
    <style>
        .list{
            width: 200px;
            height: 150px;
            background-color: skyblue;
            overflow: auto;
        }
        .news{
            height: 30px;
        }
    </style>
</head>
<body>
<!--准备一个容器-->
<div id="root"></div>
<!--引入react核心库-->
<script type="text/javascript" src="../js/17.0.1/react.development.js"></script>
<script type="text/javascript" src="../js/17.0.1/react-dom.development.js"></script>
<!--引入babel，用于jsx的转换jsx-->
<script type="text/javascript" src="../js/17.0.1/babel.min.js"></script>
<script type="text/babel">/*此处一定要写babel*/
    //创建组件
class Demo extends React.Component{
    state= {
        newsArr:[]
    }
    componentDidMount(){
        setInterval(()=>{
            //获取老数据
            const {newsArr} = this.state
            //模拟一条新闻
            const news = "新闻" + (newsArr.length+1)
            this.setState({
                newsArr:[news,...newsArr]
            })
        },1000)
    }
    getSnapshotBeforeUpdate(prevProps, prevState) {
        return this.refs.list.scrollHeight

    }
    componentDidUpdate(prevProps, prevState,height) {
        this.refs.list.scrollTop += this.refs.list.scrollHeight - height
    }
    render(){
        return(
            <div class="list" ref="list">
                {
                    this.state.newsArr.map((item,index)=>{
                        return <div class="news" key={index}>{item}</div>
                    })
                }
            </div>
        )
    }
}
        ReactDOM.render(<Demo />,document.getElementById('root'))
</script>
</body>
</html>