hooks常见问题

1.用了hooks后生命周期函数放哪里

function App () {
    useEffect( ()=>{
        //等同于componetDidMount
    } )
    return () => {
        //等同于componentDidWillUnmount
    }
}

2.类实例成员变量映射到hooks

class App ...
    ... it = 0

function App ...
    ... const it = useRef(0)

3.hooks中获取历史props和state

用ref，因为ref不受重渲染影响

function App ...
    const [a,setA] = useState(0);
    const preARef = useRef();
    useEffect( ()=>{
        preARef.current = a;
    } )
    const preA = preARef.current;
    return {a},{preA}

4.强制更新组件

function App ...
    const [updater,setUpdater] = useState(0);
    
    function forceUpdate(){
        setUpdater(updater => updater+1)
    }