add builtin weakmap version of atomFamily?

[himself65]

In my source code, there are so many weak maps from an object to an atom. I'm not sure if the jotai could add it as the builtin utility.

[dai-shi]

I have been interested in the idea of weakAtomFamily. It can be the Jotai way.
Would it be pretty simple?

export function weakAtomFamily(initializeAtom) {
  const atoms = new WeakMap();
  return (obj) => {
    if (!atoms.has(obj)) {
      atoms.set(obj, initializeAtom(obj));
    }
    return atoms.get(obj);
  };
}

Maybe, it's better in a recipe.
(tbh, I'd like to move atomFamily to recipes in docs, if possible.)

[grzesiek-ds]

@dai-shi are You planning to work on weakAtomFamily? If not then is this topic still open and if yes, then re You accepting any contributions in this field, or You would prefer to keep it outside of jotai?

[dai-shi]

I am interested in weakAtomFamily (more than atomFamily), but I'm not planning to work on it (because I don't really understand the usage requirement.) We don't plan to add it in jotai/utils (I actually would like to move atomFamily out too, eventually). For the moment, it should be in the recipe in docs and/or 3rd-party libs.

[jaens]

Here's a use case description:

atomFamily requires "stable" keys (eg. database IDs from the backend) so it would not leak memory.

Sometimes though, it's convenient to calculate and cache atoms based on a parameter that is an actual object (which can eg. contain extra information), but the object could be "temporary"/"dynamic" (eg. created purely based on client-side data, or re-created multiple times from backend data on periodic refresh).

In this case, instead of trying to invent a stable ID (which is possible in 90% of cases, but can be inconvenient), temporary objects could be directly used as a family keys, but the Map would have to Weak to prevent memory leaks.

[dai-shi]

Does the implementation in #2239 (comment) work for your case?

[jaens]

Yes, I use the slightly optimized:

export function atomFamilyWeak<K extends WeakKey, V>(create: (key: K) => V): (key: K) => V {
    const cache = new WeakMap<K, V>();
    return (key) => {
        let value;
        if (!cache.has(key)) {
            value = create(key);
            cache.set(key, value);
        } else {
            value = cache.get(key)!;
        }
        return value;
    };
}