搞懂React的state和生命周期函数(lifecycle methods)

State and Lifecycle

何谓state

A component needs state when some data associated with it changes over time. For example, a Checkbox component might need isChecked in its state,

• 组件内部涉及的数据会根据时间条件变化

• 随着数据的变化，要相应的更新DOM

• 此时：需要使用state去接收这些不断变化的数据，然后通过setState更新DOM

何谓Lifecycle methods

Lifecycle methods are custom functionality that gets executed during the different phases of a component. There are methods available when the component gets created and inserted into the DOM (mounting), when the component updates, and when the component gets unmounted or removed from the DOM

• 组件的生命周期即组件从被创建、渲染插入DOM，从DOM中移除所经历的阶段

• 每个阶段都有对应的函数，你可以去做相应的操作，比如下面demo中，componentDidMount函数在组件被渲染进DOM时调用，componentWillUnmount函数在组件从DOM中移除时调用

state VS props

The most important difference between state and props is that props are passed from a parent component, but state is managed by the component itself. A component cannot change its props, but it can change its state.

• props是由外部传入组件的，组件不能改变props
• state是在组件内部进行管理的，组件可以改变 state

何谓单向数据流

父组件可以向子组件传值，反之不行。比如下面DEMO中：父组件Clock将自己state中存的date的值传给了子组件FormattedDate

官网DEMO：

// 子组件
function FormattedDate(props) {
     
  return <h2>It is {
     props.date.toLocaleTimeString()}.</h2>;
}

class Clock extends React.Component {
     
  constructor(props) {
     
    super(props);
    this.state = {
     date: new Date()};
  }
 
   // 生命周期函数
  componentDidMount() {
       
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }

  componentWillUnmount() {
     
    clearInterval(this.timerID);
  }

  tick() {
     
    this.setState({
     
      date: new Date()
    });
  }

  render() {
     
    return (
      <div>
        <h1>Hello, world!</h1>
        <FormattedDate date={
     this.state.date} />
      </div>
    );
  }
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);

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