React 中 ref 的使用

实例：获取button元素离页面顶部的距离

ref写在html元素上

import react,{component,fragment} from 'react';
import child from './child';

class counter extends component{

    constructor(props){
        super(props);
        this.addcount=this.addcount.bind(this);

        this.state={
            counter:1
        }
    }

    addcount(){
        console.log(this.button);
        console.log(this.button.clienttop);

        this.setstate({
            counter:this.state.counter+1
        })
    }

    render(){
        return(
            <fragment>
                <button onclick={this.addcount} ref={button=>this.button=button}>点击</button>
                <child num={this.state.counter}/>
            </fragment>
        )
    }
}

export default counter;

 

 

ref写在组件上，获取的是组件的实例

import react,{component,fragment} from 'react';
import child from './child';

class counter extends component{

    constructor(props){
        super(props);
        this.addcount=this.addcount.bind(this);

        this.state={
            counter:1
        }
    }

    addcount(){
        console.log(this.child);

        this.setstate({
            counter:this.state.counter+1
        })
    }

    render(){
        return(
            <fragment>
                <button onclick={this.addcount}>点击</button>
                <child num={this.state.counter} ref={child=>this.child=child}/>
            </fragment>
        )
    }
}

export default counter;

 

 

总结：ref写在html标签上，获取的是dom节点

ref写在组件标签上，获取的是组件的js实例

 

setstate异步，实例：

import react,{component,fragment} from 'react';

class counter extends component{

    constructor(props){
        super(props);
        this.addcount=this.addcount.bind(this);

        this.state={
            counter:1
        }
    }

    addcount(){
        console.log(this.div.innerhtml);

        this.setstate({
            counter:this.state.counter+1
        })

        console.log(this.div.innerhtml);
    }

    render(){
        return(
            <fragment>
                <button onclick={this.addcount}>点击</button>
                <div ref={div=>this.div=div}>{this.state.counter}</div>
            </fragment>
        )
    }
}

export default counter;

 

 

可以看到两次输出的都是1，证明setstate操作是异步执行的

 

解决方法：

import react,{component,fragment} from 'react';

class counter extends component{

    constructor(props){
        super(props);
        this.addcount=this.addcount.bind(this);

        this.state={
            counter:1
        }
    }

    addcount(){
        console.log(this.div.innerhtml);

        this.setstate(()=>{
            return{
                counter:this.state.counter+1
            }
        },()=>{
            console.log(this.div.innerhtml);
        })
    }

    render(){
        return(
            <fragment>
                <button onclick={this.addcount}>点击</button>
                <div ref={div=>this.div=div}>{this.state.counter}</div>
            </fragment>
        )
    }
}

export default counter;

这种写法，第二次打印会在setstate执行完毕之后再执行

结果：