我想用document.execCommand("copy")實(shí)現(xiàn)點(diǎn)擊按鈕復(fù)制鏈接，但是顯示TypeError: Cannot read property 'select' of undefined代碼：import React, { Component } from 'react'import MuiThemeProvider from 'material-ui/styles/MuiThemeProvider'import {Card, CardActions, CardHeader, CardMedia, CardTitle, CardText} from 'material-ui/Card'import RaisedButton from 'material-ui/RaisedButton'import RemoveRedEye from 'material-ui/svg-icons/image/remove-red-eye'import ActionGrade from 'material-ui/svg-icons/action/grade'class card extends Component {  state = {    width: window.screen.availWidth > 1280 ? '20%' : '100%'  }  //復(fù)制事件函數(shù)  copysvn = () => {    this.refs.ref.select()    document.execCommand("copy")    console.log(1111)  }  render() {    const ref = 'svn' + this.props.index    const string = '喜歡' + this.props.like    return (      <MuiThemeProvider>      <Card style={{width: this.state.width, margin: '10px 2.5%', float: 'left'}}>        <CardHeader        title={this.props.headtitle}        subtitle={this.props.headsubtitle}        avatar={this.props.avatar}        />        <CardMedia        >          <img src={this.props.imgsrc} alt="" />        </CardMedia>        <CardTitle title={this.props.cardtitle} subtitle={this.props.subtitle} />        <CardText style={{height: '100px', overflow: 'hidden'}}>          {this.props.cardtext}        </CardText>        <CardActions>          <input type="text" ref={ref} value={this.props.svn} style={{display: 'none'}}/>          <RaisedButton label="svn地址" primary={true} onClick={this.copysvn} icon={<RemoveRedEye />}/>          <RaisedButton label={string} primary={true} icon={<ActionGrade />}/>        </CardActions>        </Card>      </MuiThemeProvider>    )  }}export default card網(wǎng)上也搜不到合理的解釋，問題要怎么解決呢？