react--生命周期

react16.3开始对生命周期做了一些改动，本项目是16.8所以就用新版生命周期来说明，先上图

新建文件Mount.js

import React from 'react';

export default class Mount extends React.Component {

    state = {
        value: ''
    };

static getDerivedStateFromProps(props, state){
    if(props.value || state.value===''){
        console.log('getDerivedStateFromProps');
        return {
            value: 'getDerivedStateFromProps'
        }
    }
    return true
}

shouldComponentUpdate(props, state){
    console.log('shouldComponentUpdate')
    if(state.value === 'getDerivedStateFromProps'){
        console.log('shouldComponentUpdate')
        this.setState({
            value: 'shouldComponentUpdate'
        })
    }
    return true
}

componentDidMount(){
    console.log('componentDidMount');
    // if(this.state.value === 'getDerivedStateFromProps'){
    //     this.setState({
    //         value: 'componentDidMount'
    //     })
    // }
}

 render(){ console.log('render'); return( <div>我是生命周期组件{this.state.value}</div> ) } }

import React from 'react';
import {Switch, Route} from "react-router-dom";

import Home2 from '../pages/Home2';
import OnePage from '../pages/OnePage';
import TwoPage from '../pages/TwoPage';
import This from '../pages/This';
import Mount from '../pages/Mount';

const Routers = (
    <Switch>
        <Route path="/" exact component={Home2} />
        <Route path="/onePage" component={OnePage} />
        <Route path="/twoPage/:id" component={TwoPage} />
        <Route path="/this" component={This} />
        <Route path="/mount"  component={Mount} />
    </Switch>
);

export default Routers

启动项目切换url  http://localhost:3000/#/mount   

F12打开控制台，打印顺序为getDerivedStateFromProps =》render=》componentDidMount，和第一张图片显示的一样，这是默认加载顺序，下面是更新顺序

我们把componentDidMount里的注释打开

componentDidMount(){
    console.log('componentDidMount');
    if(this.state.value === 'getDerivedStateFromProps'){
        this.setState({
            value: 'componentDidMount'
        })
    }
}
再看浏览器打印

先走getDerivedStateFromProps  =》render=》 componentDidMount  正常流程走完之后，触发shouldComponentUpdate注意这里20行是进入这个生命周期但if条件不成立，
然后触发render结束，这里可以确定componentDidMount在每次更新只执行一次，我们再看如果手动让shouldComponentUpdate的if条件成立会怎么执行呢？

import React from 'react';

export default class Mount extends React.Component {

    state = {
        value: ''
    };

    static getDerivedStateFromProps(props, state){
        if(props.value || state.value===''){
            console.log('getDerivedStateFromProps');
            return {
                value: 'getDerivedStateFromProps'
            }
        }
        return true
    }

    shouldComponentUpdate(props, state){
        console.log('shouldComponentUpdate')
        if(state.value === 'componentDidMount'){
            console.log('shouldComponentUpdate')
            this.setState({
                value: 'shouldComponentUpdate'
            })
        }
        return true
    }

    componentDidMount(){
        console.log('componentDidMount');
        if(this.state.value === 'getDerivedStateFromProps'){
            this.setState({
                value: 'componentDidMount'
            })
        }
    }

    render(){
        console.log('render');
        return(
            <div>我是生命周期组件{this.props.value}{this.state.value}</div>
        )
    }
}

执行顺序为

getDerivedStateFromProps  =》render=》 componentDidMount =》shouldComponentUpdate注意这里if条件成立所以打印了22行代码，然后render，
但是render之后又打印了shouldComponentUpdate说明程序又进入了shouldComponentUpdate生命周期，然后if条件没有成立，继续走render，
所以shouldComponentUpdate这个生命周期必须要加if条件 否则会死循环 永远不会停止造成浏览器崩溃！！！