聊聊React类组件中的setState()的同步异步（附面试题）

2023-10-14 11:35:34

一.setState()更新状态的两种写法

setState(updater, [callback]),
updater为返回stateChange对象的函数: (state, props) => stateChange
接收的state和props被保证为最新的
setState(stateChange, [callback])
stateChange为对象,
callback是可选的回调函数, 在状态更新且界面更新后才执行
总结:
对象方式是函数方式的简写方式
如果新状态不依赖于原状态 ===> 使用对象方式
如果新状态依赖于原状态 ===> 使用函数方式
如果需要在setState()后获取最新的状态数据, 在第二个callback函数中读取
例子:

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>01_setState()的使用</title>
</head>
<body>

<div id="example"></div>

<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>
<script src="https://cdn.bootcss.com/remarkable/1.7.1/remarkable.min.js"></script>

<script type="text/babel">

  class A extends React.Component {

    state = {
      count: 1
    }

    test1 = () => {
      this.setState(state => ({count: state.count + 1}))
      console.log('test1 setState()之后', this.state.count)
    }

    test2 = () => {
      /*const count = this.state.count + 1
      this.setState({
        count
      })*/
      this.setState({
        count: 3
      })
      console.log('test2 setState()之后', this.state.count)
    }

    test3 = () => {
      this.setState(state => ({count: state.count + 1}), () => { // 在状态更新且界面更新之后回调
        console.log('test3 setState callback()', this.state.count)
      })
    }

    render() {
      console.log('A render()')
      return (
        <div>
          <h1>A组件: {this.state.count}</h1>
          <button onClick={this.test1}>A 测试1</button>&nbsp;&nbsp;
          <button onClick={this.test2}>A 测试2</button>&nbsp;&nbsp;
          <button onClick={this.test3}>A 测试3</button>&nbsp;&nbsp;
        </div>
      )
    }
  }

  ReactDOM.render(<A/>, document.getElementById('example'))
</script>
</body>
</html>

当我们依次按下1、2、3按钮，我们会发现1按钮的事件监听函数运行时是先运行console.log('test1 setState()之后', this.state.count)这句代码然后在进行的render(),而在代码中this.setState(state => ({count: state.count + 1}))这句是在前的，由此我们可以推测~~setState()是异步的~~ ，同理2，3按钮也是。
值得一提的是，按钮3中this.setState(state => ({count: state.count + 1}), () => { // 在状态更新且界面更新之后回调 console.log('test3 setState callback()', this.state.count) }) 中有一个回调函数，在我们一般情况下是用其简写形式（对象形式），只有在需要setState()后获取最新的状态数据时才会用到函数形式的setState()。

二.setState()更新状态是异步还是同步的?

执行setState()的位置?
在react控制的回调函数中: 生命周期勾子 / react事件监听回调
非react控制的异步回调函数中: 定时器回调 / 原生事件监听回调 / promise回调 /…
异步 OR 同步?
react相关回调中: 异步
其它异步回调中: 同步
例子

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>02_setState()异步OR同步更新</title>
</head>
<body>

<div id="example"></div>

<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>

<script type="text/babel">
  class StateTest extends React.Component {

    state = {
      count: 0,
    }

    /*
     react事件监听回调中, setState()是异步更新状态
     */
    update1 = () => {
      console.log('update1 setState()之前', this.state.count)
      this.setState(state => ({count: state.count + 1}))
      console.log('update1 setState()之后', this.state.count)
    }

    /*
     react生命周期勾子中, setState()是异步更新状态
     */
    componentDidMount () {
      console.log('componentDidMount setState()之前', this.state.count)
      this.setState(state => ({count: state.count + 1}))
      console.log('componentDidMount setState()之后', this.state.count)
    }

    /*
    定时器回调 / 原生事件监听回调 / promise回调 /...
     */
    update2 = () => {
      setTimeout(() => {
        console.log('setTimeout setState()之前', this.state.count)
        this.setState(state => ({count: state.count + 1}))
        console.log('setTimeout setState()之后', this.state.count)
      })
    }
    update3 = () => {
      const h2 = this.refs.count
      h2.onclick = () => {
        console.log('onclick setState()之前', this.state.count)
        this.setState(state => ({count: state.count + 1}))
        console.log('onclick setState()之后', this.state.count)
      }
    }
    update4 = () => {
      Promise.resolve().then(value => {
        console.log('Promise setState()之前', this.state.count)
        this.setState(state => ({count: state.count + 1}))
        console.log('Promise setState()之后', this.state.count)
      })
    }


    update5 = () => {
      console.log('onclick setState()之前', this.state.count)
      this.setState(state => ({count: state.count + 1}))
      console.log('onclick setState()之后', this.state.count)
      console.log('onclick setState()之前2', this.state.count)
      this.setState(state => ({count: state.count + 1}))
      console.log('onclick setState()之后2', this.state.count)
    }

    update6 = () => {
      console.log('onclick setState()之前', this.state.count)
      this.setState({count: this.state.count + 1})
      console.log('onclick setState()之后', this.state.count)
      console.log('onclick setState()之前2', this.state.count)
      this.setState({count: this.state.count + 1})
      console.log('onclick setState()之后2', this.state.count)
    }

    update7 = () => {
      console.log('onclick setState()之前', this.state.count)
      this.setState({count: this.state.count + 1})
      console.log('onclick setState()之后', this.state.count)

      console.log('onclick setState()之前2', this.state.count)
      this.setState(state => ({count: state.count + 1}))
      console.log('onclick setState()之后2', this.state.count)
    }


    render() {
      const {count} = this.state
      console.log('render()', count)
      return (
        <div>
          <h2 ref='count'>{count}</h2>
          <button onClick={this.update1}>更新1</button> ---
          <button onClick={this.update2}>更新2</button> &nbsp;
          <button onClick={this.update3}>更新3</button> &nbsp;
          <button onClick={this.update4}>更新4</button> ---
          <button onClick={this.update5}>更新5</button> &nbsp;
          <button onClick={this.update6}>更新6</button> &nbsp;
          <button onClick={this.update7}>更新7</button> &nbsp;
        </div>
      )
    }
  }

  ReactDOM.render(<StateTest/>, document.getElementById('example')) // 渲染组件标签, 内部会调用组件标签对象的render()虚拟DOM

</script>
</body>
</html>

结论我已经放在开头了，但是在更新6的时候我们发现我们明明代码里写了两次setState()，而最终结果却只更新了一次，而更新5同样是写了两次setState()却是两次作用后的结果，这是为什么呢？

三.关于异步的setState()

多次调用, 如何处理?
setState({}): 合并更新一次状态, 只调用一次render()更新界面 —状态更新和界面更新都合并了
setState(fn): 更新多次状态, 但只调用一次render()更新界面 —状态更新没有合并, 但界面更新合并了
如何得到异步更新后的状态数据?
在setState()的callback回调函数中

四.面试题

注释里箭头左侧为次序，右侧为打印出的值

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>03_setState()面试题</title>
</head>
<body>

<div id="example"></div>

<script type="text/javascript" src="./js/react.development.js"></script>
<script type="text/javascript" src="./js/react-dom.development.js"></script>
<script type="text/javascript" src="./js/babel.min.js"></script>

<script type="text/babel">
  class StateTest extends React.Component {

    state = {
      count: 0,
    }

    componentDidMount() {
      this.setState({count: this.state.count + 1})
      this.setState({count: this.state.count + 1})
      console.log(this.state.count) // 2 ==> 0

      this.setState(state => ({count: state.count + 1}))
      this.setState(state => ({count: state.count + 1}))
      console.log(this.state.count) // 3 ==> 0

      setTimeout(() => {
        this.setState({count: this.state.count + 1})
        console.log('timeout', this.state.count) // 10 ==> 6

        this.setState({count: this.state.count + 1})
        console.log('timeout', this.state.count) // 12 ==> 7
      }, 0)

      Promise.resolve().then(value => {
        this.setState({count: this.state.count + 1})
        console.log('promise', this.state.count)  // 6 ==>4

        this.setState({count: this.state.count + 1})
        console.log('promise', this.state.count) // 8 ==> 5
      })
    }

    render() {
      const count = this.state.count
      console.log('render', count)  // 1 ==> 0   4 ==>3   5 ==>4  7 ==>5  9 ==>6  11 ==>7
      return (
        <div>
          <p>{count}</p>
        </div>
      )
    }
  }

  ReactDOM.render(<StateTest/>, document.getElementById('example'))

</script>
</body>
</html>

码农公寓

一.setState()更新状态的两种写法

二.setState()更新状态是异步还是同步的?

三.关于异步的setState()

四.面试题

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