React-Hooks之useRef

1.什么是useRef Hook?

useRef就是createRef的Hook版本用来获取元素, 只不过比createRef更强大一点
createRef只能获取普通元素和类元素，并且只能获取到类组件的实例对象，不能拿到函数式组件

import React, {createRef, useRef} from 'react';
class Home extends React.PureComponent{
    render() {
        return (
            <div>Home</div>
        )
    }
}
function About() {
    return (
        <div>About</div>
    )
}
function App() {
    const pRef = createRef();
    const homeRef = createRef();
    function btnClick() {
        console.log(pRef.current);
        console.log(homeRef.current);
    }
    return (
        <div>
            <p ref={pRef}>我是段落</p>
            <Home ref={homeRef}/>
            <About/>
            <button onClick={()=>{btnClick()}}>获取</button>
        </div>
    )
}
export default App;

import React, {createRef, useRef} from 'react';
class Home extends React.PureComponent{
    render() {
        return (
            <div>Home</div>
        )
    }
}
function About() {
    return (
        <div>About</div>
    )
}
function App() {
    // const pRef = createRef();
    // const homeRef = createRef();
    const pRef = useRef();
    const homeRef = useRef();
    function btnClick() {
        console.log(pRef.current);
        console.log(homeRef.current);
    }
    return (
        <div>
            <p ref={pRef}>我是段落</p>
            <Home ref={homeRef}/>
            <About/>
            <button onClick={()=>{btnClick()}}>获取</button>
        </div>
    )
}
export default App;

两种方法都能拿到ref,结果如下：

createRef.png

createRef和useRef

1. useState和useRef区别:
   useRef中保存的数据, 除非手动修改, 否则永远都不会发生变化

const age = useRef(18);

如果说useRef也可以保存数据的话那么为什么还要有useState呢？

function App() {
    const pRef = createRef();
    const homeRef = useRef();
    function btnClick() {
        console.log(pRef); // {current: p}
        console.log(homeRef); // {current: Home}
    }
    const [numState, setNumState] = useState(0);
    // const age = useRef(18); // {current: 18}
    const age = useRef(numState); // {current: 0}
    useEffect(()=>{
        age.current = numState;
    }, [numState]);
    return (
        <div>
            <p>上一次的值: {age.current}</p>
            <p>当前的值  :{numState}</p>
            <button onClick={()=>{setNumState(numState + 1)}}>增加</button>
        </div>
    )
}
export default App;

总结一下：
useRef可以获取元素，可以保存数据
和createRef的区别是可以保存数据
和useState的区别是：useRef中保存的数据, 除非手动修改, 否则永远都不会发生变化