refs的作用是什么，你在什么业务场景下使用过refs

class Test extends React.Component {
  constructor(props){
    super(props);
    this.state = {
      top: 0
    }
    this.handleWindowScroll = this.handleWindowScroll.bind(this);
  }

  handleWindowScroll() {
    this.setState({
      top: document.body.scrollTop
    })
  }

  componentDidMount() {
    window.addEventListener(‘scroll‘, this.handleWindowScroll);
  }

  componentWillUnmount() {
    window.removeEventListener(‘scroll‘, this.handleWindowScroll);
  }

  render() {
    return <div>{this.state.top}</div>
  }
}