SOURCE

class FormComponent extends React.Component {
    constructor(props){
        super(props)
        this.state = {value: ''}
    }

    componentDidUpdate(){
        console.log(this.state.value);
    }

    render() {
        return (
            <form>
                <input type='text' placeholder='请输入' ref={input=>this.input=input} onChange={()=>this.setState({value: this.input.value})} />
            </form>
        )
    }
}


ReactDOM.render(
    <FormComponent/> , 
    document.getElementById("root")
);

<div id="root"></div>