react生命周期（旧）

1.代码

import React, { Component } from "react";
import ReactDOM from "react-dom";
import "./styles.css";

function fetchData() {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      resolve('获取ajax数据');
    }, 2000);
  });
}

class Welcome extends Component {
  constructor(props) {
    console.log("constructor", props);
    super(props);
    this.state = {
      color: props.color
    };
  }

  componentWillMount() {
    console.log("componentWillMount");
    // this.setState({color: "yellow"}); 1.在componentWillMount中setState

    // setTimeout(() => {                       /***************************************************/
    //   this.setState({ color: "green" });     /******2.在componentWillMount中延时setState**********/
    // }, 1000);                                /***************************************************/
  }

  componentDidMount() {
    console.log("componentDidMount");
   // this.setState({ color: "yellow" }); 3.在componentDidMount中setState
  }

  componentWillReceiveProps(nextProps, nextState) {
    console.log("componentWillReceiveProps", nextProps, nextState);
    if (nextProps.color !== this.state.color) {
      this.setState({ color: nextProps.color });
    }
  }

  shouldComponentUpdate(nextProps, nextState) {
    console.log("shouldComponentUpdate", nextProps, nextState);
    return true;
  }

  componentWillUpdate(nextProps, nextState) {
    console.log("componentWillUpdate", nextProps, nextState);
  }

  componentDidUpdate(prevProps, prevState) {
    console.log("componentDidUpdate", prevProps, prevState);
  }

  changeColor = () => {
    this.setState({color: "#ddd"})
  }
  render() {
    console.log("render", this.state.color);
    return (
      

        
react 旧的生命周期
        
{this.state.color}
        内部change color
      
    );
  }
}

class App extends Component {
  state = {
    currentColor: "red",
    colorIndex: 0
  };

  changeColor = () => {
    let colors = ["red", "yellow", "blue", "green", "pink"];
    let { colorIndex } = this.state;
    colorIndex = ++colorIndex % 5;
    this.setState({
      currentColor: colors[colorIndex],
      colorIndex
    });
  };

  render() {
    let { currentColor } = this.state;
    return (
      

        
        

        外部change color
      
    );
  }
}

const rootElement = document.getElementById("root");
ReactDOM.render(, rootElement);

2. 输出结果

1.直接输出

constructor {color: "red"}
componentWillMount
render red
componentDidMount

2.componentWillMount中改变state

• ①在componentWillMount中改变state，即将代码注释1处打开，setState先执行，改变完了state，然后才执行render，可见在这方法里同步地设置状态将不会触发重渲。

constructor {color: "red"}
componentWillMount
render yellow
componentDidMount

• ②在componentWillMount中延时改变state，即将代码注释2处打开， 因为延时，render在setState之前执行，执行完了，会触发一次重新渲染（render）。可见在这方法里异步地设置状态会触发重渲。

constructor {color: "red"}
componentWillMount
render red
componentDidMount
shouldComponentUpdate {color: "red"} {color: "green"}
componentWillUpdate {color: "red"} {color: "green"}
render green
componentDidUpdate {color: "red"} {color: "red"}

• ③在componentWillMount中使用async，await（发送ajax），将componentWillMount改成如下形式

  async componentWillMount() {
    console.log("componentWillMount");
    let result = await delay()
    this.setState({ color: "green" });
  }

输出结果如下，也会触发一次重新渲染（render），what the Fuck！！！
我一直以为不会重新渲染，那为什么ajax请求不建议放在componentWillMount中，网上搜了一大堆，都说不能触发重新render，莫不是智障？？？？？

constructor {color: "red"}
componentWillMount
render red
componentDidMount
result 获取ajax数据
shouldComponentUpdate {color: "red"} {color: "green"}
componentWillUpdate {color: "red"} {color: "green"}
render green
componentDidUpdate {color: "red"} {color: "red"}

3.componentDidMount中改变state

• ①在componentDidMount中改变state，即将代码注释3处打开，可见componentDidMount会重新触发一次render，注意这里没有ajax的异步请求，直接setState就会重新渲染！！！

constructor {color: "red"}
componentWillMount
render red
componentDidMount
shouldComponentUpdate {color: "red"} {color: "yellow"}
componentWillUpdate {color: "red"} {color: "yellow"}
render yellow
componentDidUpdate {color: "red"} {color: "red"}

4.改变父组件传入值，从而改变子组件state

点击父组件改变颜色按钮，改变父组件传入的props的值，查看结果
componentWillReceiveProps => shouldComponentUpdate => componentWillUpdate => render => componentDidUpdate

componentWillReceiveProps {color: "yellow"} {}
shouldComponentUpdate {color: "yellow"} {color: "yellow"}
componentWillUpdate {color: "yellow"} {color: "yellow"}
render yellow
componentDidUpdate {color: "red"} {color: "yellow"}

5.改变子组件state值

点击子组件改变颜色按钮，改变子组件state值，查看结果
shouldComponentUpdate => componentWillUpdate => render => componentDidUpdate

shouldComponentUpdate {color: "red"} {color: "#ddd"}
componentWillUpdate {color: "red"} {color: "#ddd"}
render #ddd
componentDidUpdate {color: "red"} {color: "yellow"}

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