Allow optional actor in useSelector

[davidkpiano]

This PR allows an actor to be undefined here, with proper typing:

const someState = useSelector(maybeActor, (snapshot) => ...);

someState; // derived state or undefined

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Latest commit: c4c49e4

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@xstate/react	Minor
@xstate/vue	Minor

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What particular use case(s) are you envisioning here?

[davidkpiano]

What particular use case(s) are you envisioning here?

const maybeChild = useSelector(parentActor, s => s.children.child);
// const maybeCount = useSelector(maybeChild ?? createEmptyActor(), s => s?.context.count);
const maybeCount = useSelector(maybeChild, s => s?.context.count);

[Necmttn]

Is there a similar method for a non-react environment?

const $count = actor.watch((state) => state.context.count)

something like this, maybe ?

cc: @davidkpiano

[davidkpiano]

Is there a similar method for a non-react environment?

const $count = actor.watch((state) => state.context.count)

something like this, maybe ?

cc: @davidkpiano

I'd like to introduce something like this:

// PSEUDOCODE
import { fromSelector } from 'xstate';

const totalActor = fromSelector(someActor, s => {
  return s.context.items.reduce((sum, item) => sum + item.price, 0)
});

totalActor.subscribe(s => s.context /* = the total */)

[Andarist]

If we are changing this in @xstate/react then we should probably do a similar change in @xstate/vue and @xstate/svelte.

[davidkpiano]

I added it for @xstate/vue but I'm not familiar enough with Svelte to get it working there.

[Andarist]

but I'm not familiar enough with Svelte to get it working there.

Yeah, me neither. I don't think we even support actorRef changes in its useSelector. Let's leave it out for now then