Avoid calling `selector` when the snapshot doesn't change

[Andarist]

alternative to https://github.com/statelyai/xstate/pull/5080/files

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[davidkpiano]

I think there may be a hidden problem with this logic:

const Component = (props) => {
  const items = useSelector(store, s => {
    return s.context.items.map(item => ({
      ...item,
      isSelected: props.selected === item.id
    }));
  });
}

Not the best example but the point is that if we're selecting something that mixes in data outside the snapshot (whether that's best practice or not), we may end up with stale data.

[Andarist]

That's a fair concern - this is how the React team has implemented' useSyncExternalStoreWithSelector', so I just used their logic here.

[TkDodo]

Not the best example but the point is that if we're selecting something that mixes in data outside the snapshot (whether that's best practice or not), we may end up with stale data.

Ah, I see. closing over values should still work - we could compare the selectorFn for equality but that wouldn't work with inline selectors :/

[Andarist]

For context, this is where the original useSyncExternalStoreWithSelector is returning early: https://github.com/facebook/react/blob/8dfbd16fce9077ab4e5fe85a7b86fa7c97a5ae04/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js#L83-L86

[davidkpiano]

Closing as we went with a different approach