react.js 给标识ref，获取内容

import React,{Component} from 'react'
import ReactDOM from 'react-dom'
class App extends Component{
//给一个初始化状态
constructor(){
super();
this.state={
result:0
}
}
handChange=()=>{
//获取input a的类容，这个a就是我们给的标识ref="a"
let a=this.refs.a.value||0;
let b=this.refs.b.value||0;
//改变状态，更新视图
this.setState({result:parseFloat(a)+parseFloat(b)})
}
render(){
return (
<div>
<input type="text" ref="a" onChange={this.handChange}/>
+
<input type="text" ref="b" onChange={this.handChange}/>
<p>{this.state.result}</p>
</div>
)
}
}

ReactDOM.render(<App/>,document.querySelector("#root"))