state.changed in v5

[adithep]

In v4 we can detect if the transition is successful like so:

https://xstate.js.org/docs/guides/states.html#state-changed

How do I know if the event I sent is successful in v5? Is it possible currently? If not I think it will be a very useful feature.

[davidkpiano]

In XState v5, it is a simple referential equality check:

let prevSnapshot;

actor.subscribe(snapshot => {
- snapshot.changed;
+ snapshot === prevSnapshot; // false if changed

  prevSnapshot = snapshot;
});

cc. @Andarist

[thirtified]

It really feels as if this should be a framework feature. Feels a bit weird having to manually keep track of the snapshot state of a state machine 😉.
That is, I'd very much appreciate having such a functionality. And maybe even an option to only trigger subscription events when the snapshot has changed?

[davidkpiano]

Now that we have the inspect API, I agree that this would be a good default behavior. However, it would be a breaking change in XState v5.

[yllan]

Maybe not a xstate problem but a typescript one: I'm trying to implement an utility function like this:

const onlyChangeWrapper = (actor) => {
  let prevSnapshot = undefined
  const subscribers = {}

  actor.subscribe((snapshot) => {
    if (snapshot !== prevSnapshot) {
      for (const fn of Object.values(subscribers)) 
        fn(snapshot)
    }
  })

  const subscribe = (fn) => {
    const key = uuid()
    subscribers[key] = fn
    return {
      unsubscribe: () => delete subscribers[key]
    }
  }

  return {
    actor,
    subscribe,
  }
}

However I don't know how to type this. Can you shed some light on this?

[zaccrites]

@yllan I think you can use ReturnType<typeof actor.getSnapshot>

[yllan]

@zaccrites Thanks for pointing out that for me. What about the type of actor?

[thirtified]

@zaccrites Thanks for pointing out that for me. What about the type of actor?

Easiest is probably Actor<any> as you likely won't care for the underlying state machine. If for some reason you do need to type the snapshots you may want to employ generics and ActorRef.