React 类式组件生命周期

1. 首次加载时

1) contructor

类式组件的构造器，可在构造器中初始化组件状态( state )

2) getDerivedStateFromProps

state的值在任何时候都取决于props( props变化时更新state )，接收两个参数props、state

3) render

渲染函数，返回vnode

4) componentDidMount

组件第一次被渲染到DOM中，可在此钩子中设置定时器，调用AJAX

2. 更新时

1) getDerivedStateFromProps

2) shouldComponentUpdate

根据return的布尔值决定是否更新组件

3) render

4) getSnapshotBeforeUpdate

获取更新前的快照，其返回值默认作为componentDidUpdate第三个参数

5) componentDidUpdate

组件更新之后，其可接收三个参数未更新前的props、未更新前的state、getSnapshotBeforeUpdate的返回值

3. 卸载时

1) componentWillUnmount

组件即将卸载之前调用的钩子，可在此阶段销毁定时器

4. Demo

import { Component, Fragment } from 'react';
import { render, unmountComponentAtNode } from 'react-dom';

class Demo extends Component {
  constructor(props) {
    super(props);
    this.state = { title: 'This is a initial demo.' };
    console.log('constructor');
  }

  static getDerivedStateFromProps(props, state) {
    console.log('getDerivedStateFromProps');
    return { subTitle: props.subTitle };
  }

  componentDidMount() {
    console.log('componentDidMount');
  }

  getSnapshotBeforeUpdate() {
    console.log('getSnapshotBeforeUpdate');
    return { snapshotRes: 'getSnapshotBeforeUpdate' };
  }

  shouldComponentUpdate() {
    console.log('shouldComponentUpdate');
    return true;
  }

  componentDidUpdate(preProps,preState,snapshotValue) {
    console.log('componentDidUpdate');
  }

  componentWillUnmount() {
    console.log('componentWillUnmount');
  }

  /**
   * 使用箭头函数，防止找不到this
   */
  updateComponent = () => {
    this.forceUpdate();
  }

  unmountComponent = () => {
    unmountComponentAtNode(document.querySelector('#app'));
  }

  render() {
    console.log('render');
    const { title, subTitle } = this.state;
    return (
      <Fragment>
        <h1>{ title }</h1>
        <h2>{ suTitle }</h2>
        <button onClick={ this.updateComponent }>Force update</button>
        <button onClick={ this.unmountComponent }>Unmount component</button>
      </Fragment>
    )
  }
}

render(<Demo suTitle="Alison" />, document.querySelector('#app'));