jotai/dist/vanilla/utils/selectAtom.js

👻 Primitive and flexible state management for React

import { atom } from '../../vanilla.js';
const getCached = (c, m, k) => (m.has(k) ? m : m.set(k, c())).get(k);
const cache1 = new WeakMap();
const memo3 = (create, dep1, dep2, dep3) => {
    const cache2 = getCached(() => new WeakMap(), cache1, dep1);
    const cache3 = getCached(() => new WeakMap(), cache2, dep2);
    return getCached(create, cache3, dep3);
};
export function selectAtom(anAtom, selector, equalityFn = Object.is) {
    return memo3(() => {
        const EMPTY = Symbol();
        const selectValue = ([value, prevSlice]) => {
            if (prevSlice === EMPTY) {
                return selector(value);
            }
            const slice = selector(value, prevSlice);
            return equalityFn(prevSlice, slice) ? prevSlice : slice;
        };
        const derivedAtom = atom((get) => {
            const prev = get(derivedAtom);
            const value = get(anAtom);
            return selectValue([value, prev]);
        });
        // HACK to read derived atom before initialization
        derivedAtom.init = EMPTY;
        return derivedAtom;
    }, anAtom, selector, equalityFn);
}