Wait for state with always transition

[alexamy]

Hi!

For state that have only always transition, how to assert that the machine is (or was) in that state?

Since always transitions are synchronous, waitFor helper cannot be used for that. Even subscribe does not show this state.

Sandbox (open devtools console in browser to see logs): https://stackblitz.com/edit/vitejs-vite-fb8gt9ws?file=src%2Fmain.js

[davidkpiano]

You can use the inspection API and listen for @xstate.microstep inspection events: https://stately.ai/docs/inspection

const actor = createActor(machine, {
  inspect: (inspectionEv) => {
    if (inspectionEv.type === '@xstate.microstep') {
      // ...

[alexamy]

@davidkpiano Hi! I just want to clarify - if there is no way to use the inspection api, it is not possible to track machine state with always transition (it's the intended design decision)?

I should mention that only waitFor is available in my current app setup, because it is separated from the actor initialization. Below are other solutions which I came up with, but they looks like hacks for me.

Using zero delay:

const states = { 
  //...
  "current target": {
    after: {
      0: "next target"
    }
  }
}

await waitFor(actor, (snapshot) => {
  return snapshot.matches("current target");
});

Using extra event just for delaying transition:

const states = { 
  //...
  "current target": {
    on: {
      "#intermediate proceed": "next target"
    }
  }
}

await waitFor(actor, (snapshot) => {
  return snapshot.matches("current target");
});

actor.send({ type: "#intermediate proceed" });

[FloNeu]

I agree with you... 'after' should really not skip emitting values/tags... not really transparent when you look at the diagram of the machine, why you shouldn't be able to match those.

[davidkpiano]

That's right - examining this is only currently possible through the inspection API. If you have access to the actor, you can inspect via actor.system.inspect(…).

[FloNeu]

Do i understand that correctly that i can't match for a state that has an always transition to another state, as there is no snapshot fired? I was just debugging for a few hours because i was trying to match a tag i put on this 'Refreshed' state that returns to a 'Pending'.

Isn't that pretty intransparent and confusing? I know it's for me...

I mean i can put actions on it and they work - put i can't use tags or match? But it would work if i put a delay on it?

[FloNeu]

yes it works even with delay of zero... but that's really a hard thing to explain to people... it seems not many people have fallen into this 'trap' but i thing that's very annoying behavior.

Seems to me using inspection api or the other solutions mentioned here are really just workarounds that shouldn't be needed.

[FloNeu]

But changing it would probably be a breaking change.... hmm. But maybe one could make the state 'transparent' , add a warning label and/or not allow tags in the editor - although that would not make this behavior visible in the json-format - it would be a start.

Also i just again checked the docs and this doesn't mention it either.

[alexamy]

@FloNeu If you are interested, these are xstate source code parts for eventless transition logic:

1. actor notification with current snapshot
2. macrostep, which creates new snapshot
3. this part of macrostep is responsible for iteration of all eventless transition before creating a new snapshot

As far as I understand, because all eventless transitions are happening inside single macrostep, there is no way to waitFor to be notified.