reactjs - 为什么不能根据 Prop react 设置初始状态

Question

我有一个 es6 react 组件，我希望状态的初始值取决于传递的 prop 的初始值，但它的值始终为 false:

AttachStateToProps 组件

<AttachStateToProps VALUE=false />

AttachStateToProps 组件:

class AttachStateToProps extends React.Component {
  state = {
    stateValue: this.props.VALUE,
  }
  render() {
    console.log('Value of Prop - ', this.props.VALUE)
    console.log('Value of State - ', this.state.stateValue)

  return null
  }
}

每次更改 Prop VALUE 的值时，我都会得到:

`Value of Prop - false` // this changes whenever I change prop value in 
   <AttachStateToProps />

和

`Value of State - false` // this does not change accordingly.

我图 it could be something to do with state/setState being async及以上 getinitialState 但我不明白为什么。

Answer 1

从构造函数中的 props 或作为类属性初始化状态，不会在 prop 更改时更新状态。然而，react 会检测到 prop 的变化，并重新渲染组件。

示例:

class AttachStateToProps extends React.Component {
  state = {
    stateValue: this.props.VALUE,
  }
  render() {
    console.log('Value of Prop - ', this.props.VALUE)
    console.log('Value of State - ', this.state.stateValue)

  return null
  }
}

const renderWithVal = (val) => ReactDOM.render(
  <AttachStateToProps VALUE={val} />,
  demo
);

renderWithVal(5);
renderWithVal(15);
renderWithVal(115);

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="demo"></div>

要在 prop 更改时更新状态，您需要使用组件的 lifecycle method .

使用 React ^16.3，您可以使用静态 getDerivedStateFromProps() 从 props 更新状态的方法(并初始化它):

static getDerivedStateFromProps(nextProps) {    
  return {
    stateValue: nextProps.VALUE,
  }
}

class AttachStateToProps extends React.Component {
  state = {};

  static getDerivedStateFromProps(nextProps) {    
    return {
      stateValue: nextProps.VALUE,
    }
  }
      
  render() {
    console.log('Value of Prop - ', this.props.VALUE)
    console.log('Value of State - ', this.state.stateValue)

  return null
  }
}

const renderWithVal = (val) => ReactDOM.render(
  <AttachStateToProps VALUE={val} />,
  demo
);

renderWithVal(5);
renderWithVal(15);
renderWithVal(115);

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="demo"></div>

对于 16.3 之前的 React 版本，您可以使用 componentWillReceiveProps() .

注意:componentWillReceiveProps 已弃用，但会一直工作到版本 17。

componentWillReceiveProps(nextProps, prevState) {
  this.setState({
    stateValue: nextProps.VALUE,
  })
}