非受控组件（不推荐使用）

import React, { PureComponent,createRef } from 'react'

export default class App extends PureComponent {

    constructor(props){

        super(props)

        this.usernameRef = createRef()

    }

    render() {

        return (

            <div>

                <form onSubmit={e=> this.handleSubmit(e)}>

                    <label htmlFor="username">

                      用户名: <input type="text" ref={this.usernameRef}   id="username" />

                    </label>

                    <input type="submit" value="提交" />

                </form>

            </div>

        )

    }

    handleSubmit(e){

        e.preventDefault();

        console.log(this.usernameRef.current.value);

    }

 

}