setState的回调

import React, { Component } from 'react';
class Test extends Component {
    constructor(props) {
        super(props);
        this.state = { 
            list:[1,2,3],
            inputValue:''
         }
    }
    handleChange=(e)=>{
         
        this.setState({
            inputValue:e.target.value
        })
    }

    handleAdd =()=>{
        let newList = [...this.state.list]
        this.setState({
            list:[...newList,this.state.inputValue],
            inputValue:''
        },()=>{
            // 获取li的个数
            let ul = document.querySelectorAll('ul')
            let childNodes = [...ul][0].childNodes
            console.log(childNodes.length);
        })
      

        // 获取li的个数 (错误方法，不应该写在这里，而是写在setState的回调里,因为setState是异步的)
        let ul = document.querySelectorAll('ul')
        let childNodes = [...ul][0].childNodes
        console.log(childNodes.length);
        
        
        
        
    }
    render() { 
        return (  
            <div>
                <input  value={this.state.inputValue} onChange={this.handleChange} />
                <button onClick={this.handleAdd}>添加</button>
                <ul ref={ul=>this.ul=ul}>
                  {
                        this.state.list.map((v,i)=>{
                          return   <li key={i}>{v}</li>
                      })
                  }
                </ul>
            </div>
        );
    }
}
 
export default Test;