Zustand useStore currently resubscribe to the store on each re-render when used with useShallow

[AlaricBaraou]

While this is not a bug, this could and maybe should be avoided ?

useStore uses React's useSyncExternalStore internally and React doc list in this hook caveats

If a different subscribe function is passed during a re-render, React will re-subscribe to the store using the newly passed subscribe function. You can prevent this by declaring subscribe outside the component.

As much as it's probably a very fast operation, why not avoid it all together by ensuring that useShallow returns a stable function ?

The change would be to change src/react/shallow.ts from

export function useShallow<S, U>(selector: (state: S) => U): (state: S) => U {
  const prev = React.useRef<U>(undefined)
  return (state) => {
    const next = selector(state)
    return shallow(prev.current, next)
      ? (prev.current as U)
      : (prev.current = next)
  }
}

to

export function useShallow<S, U>(selector: (state: S) => U): (state: S) => U {
  const prev = React.useRef<U>(undefined)
  return React.useCallback(
    (state) => {
      const next = selector(state)
      return shallow(prev.current, next)
        ? (prev.current as U)
        : (prev.current = next)
    },
    [selector],
  );
}

I'm happy to open a PR if this is something you agree should be handled at the library level.
Otherwise users are free to implement their own useShallow anyway so it's not blocking.
I also didn't try to benchmark the impact of either, just feels right to follow React docs when it comes to that.

[dai-shi]

Our subscribe function is stable regardless of useShallow.

zustand/src/react.ts

Line 31 in 39a391b

api.subscribe,

A bit technical note

Yes, the use of useCallback in useShallow technically optimizes it, but I don't think people use useCallback when they use useShallow, and we don't recommend it anyway as it's just micro optimization. If you need such micro optimization, I think avoiding useShallow would be better.

[AlaricBaraou]

You're right, seems I've mixed getSnapshot and subscribe in my reasoning.
Thank you for clarifying and sorry for the non issue!