공부기록, 2021-08-13(recoil callback)

- recoil callback
```
import {atom, useRecoilCallback} from 'recoil';

const itemsInCart = atom({
key: 'itemsInCart',
default: 0,
});

function CartInfoDebug() {
const logCartItems = useRecoilCallback(({snapshot}) => async () => {
const numItemsInCart = await snapshot.getPromise(itemsInCart);
console.log('Items in cart: ', numItemsInCart);
});

return (

Log Cart Items

);
}
```