react hook 使用注意点

1.useEffect

　　useEffect执行环境：

// componentDidMount
useEffect(() => {
    // do something    
}, [])

// componentUpdate  count变量更新时执行
useEffect(() => {
    // do something
}, [count])

// componentWillUnmount
useEffect(() => {
     console.log('进入');
     return () => {
       console.log('离开'); 
     }
},[])

2. 当在useEffect 使用 变量或者函数，并且没有添加依赖时, 会报如下错误　　

　　错误：React Hook useEffect has missing dependencies: 'fn1' and 'menuList'. Either include them or remove the dependency array
　　const fn1 = (list) => {
            let hashMap = new Map();
            fn2(list, hashMap);
            return hashMap.get(location.pathname);
    }

    const fn2 = (list, map) => {
        list.forEach(item => {
            map.set(item.path, item.id)
            if(item.children && item.children.length > 0){
                mapList(item.children, map);
            }
        });
    }

    useEffect(() => {
        fn1(menuList);
    },[])

变量的话直接添加到以来的数组里即可，函数的话添加到依赖数组后会报这个警告

The 'fn1' function makes the dependencies of useEffect Hook (at line 208) change on every render. Move it inside the useEffect callback. Alternatively, wrap the 'fn1' definition into its own useCallback() Hook;

解决办法： 使用useCallback

注意：
　　1. 使用useCallback时,函数中使用到的变量也需要添加到依赖的数组中
　　2. 使用useCallback时,函数中调用另外一个函数时,需在当前函数前声明
　　const fn2 = useCallback((list, map) => {
        list.forEach(item => {
            map.set(item.path, item.id)
            if(item.children && item.children.length > 0){
                mapList(item.children, map);
            }
        });
    },[])

    const fn1 = useCallback((list) => {
            let hashMap = new Map();
            mapList(list, hashMap);
            return hashMap.get(location.pathname);
    },[location, mapList])

    useEffect(() => {
        let path = findNowPath(menuList).toString();
        setSelectedKeys([path])
    },[menuList, findNowPath])