如何在React中从子组件中获取值？

// 父组件
class ParentComponent extends React.Component {
  render() {
    const value = 'Hello World';
    return <ChildComponent value={value} />;
  }
}

// 子组件
class ChildComponent extends React.Component {
  render() {
    const { value } = this.props;
    return <div>{value}</div>;
  }
}

// 创建上下文
const MyContext = React.createContext();

// 父组件
class ParentComponent extends React.Component {
  render() {
    const value = 'Hello World';
    return (
      <MyContext.Provider value={value}>
        <ChildComponent />
      </MyContext.Provider>
    );
  }
}

// 子组件
class ChildComponent extends React.Component {
  static contextType = MyContext;
  
  render() {
    const value = this.context;
    return <div>{value}</div>;
  }
}

// 父组件
class ParentComponent extends React.Component {
  constructor(props) {
    super(props);
    this.childRef = React.createRef();
  }
  
  handleClick() {
    const value = this.childRef.current.getValue();
    console.log(value);
  }
  
  render() {
    return (
      <div>
        <ChildComponent ref={this.childRef} />
        <button onClick={this.handleClick.bind(this)}>获取子组件的值</button>
      </div>
    );
  }
}

// 子组件
class ChildComponent extends React.Component {
  getValue() {
    return 'Hello World';
  }
  
  render() {
    return <div>Child Component</div>;
  }
}