How does selector memoization work?

[RunDevelopment]

The RFC proposes that both the context and the selector are memoized.

However, it seems like this library works differently. E.g.:

  const count1 = useContextSelector(context, v => v[0].count1);

From my understanding of the RFC, this should defeat the memorization and make useContextSelector ineffective as a performance optimization.

Obviously, the above snippet works for this library, so could you please explain how? Is the selector memoized, and if so, how?

To cite a more complex example, let's look at this snippet:

function Foo({ id }) {
  const value: number = useContextSelector(Context, c => c.values[id])
  return <>{id} = {value}</>
}

How will this behave if both Context and id may change? (If props are too constant, then id may also be state.)

[dai-shi]

As far as I understand, the RFC says if the selector is memoized like

  const count1 = useContextSelector(context, useCallback(v => v[0].count1, []));

It skips re-evaluation. But, if not

  const count1 = useContextSelector(context, v => v[0].count1);

It will re-evaluate.

So, it's just about optimization of re-evaluation of the selector function.

This library doesn't include such optimization, assuming the selector function is fairly lightweight.

How will this behave if both Context and id may change?

It will use the latest values, so it behaves correctly.

Is it clear? I'm afraid I don't get the point of your question.

[RunDevelopment]

Thank you for your explanation.

In that case, I think there is a bug in the useContextSelector implementation of this library. This dependency array should also include selector.

Right now, the selector captured by the reducer function can become stale. This happens with the following steps:

1. Let the context be the number 1, and the selector be () => 1.
2. Change the selector to be c => c (yield the whole context).
   This will cause the selector in useReducer to be become stale because the deps array is still [value, selected] = [1, 1]
3. Change the context to 2.
   The stale selector will still return 1.

I think those steps will produce incorrect results.

[dai-shi]

Sorry for code readability, but that's not a dependency array. It's an initial value for useReducer. selector will never be stale.

[RunDevelopment]

I see. Thank you for the clarification.