如何从子组件中读取值

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

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

class ChildComponent extends React.Component {
  handleClick() {
    const value = 'Hello from child';
    this.props.onChildClick(value);
  }

  render() {
    return <button onClick={this.handleClick.bind(this)}>Click me</button>;
  }
}

class ParentComponent extends React.Component {
  handleChildClick(value) {
    console.log(value);
  }

  render() {
    return <ChildComponent onChildClick={this.handleChildClick.bind(this)} />;
  }
}