[Feature Request] support receiving respond events when using an interpreter to run a machine

[AlCalzone]

When executing a machine using the built-in interpreter, I cannot find a way to receive events that were created by the machine using the respond action, e.g.:

App (using interpreter) ----EVENT---→   Machine
                                           ↓
                                     react to event
                                           ↓
App (   interpreter   ) ←--RESPONSE-- respond(...)

IMO it would make sense to treat the interpreter as the origin of the original event and this would give some flexibility to run a machine either standalone or spawned from another machine.

[davidkpiano]

Can you prepare a code example with a comment of what you're asking? Code examples are really helpful when thinking through problems like this.

CodeSandbox (XState template)

[AlCalzone]

Ok, here you go:
https://codesandbox.io/s/example-for-receiving-respond-events-qt775

[Andarist]

Those events are given to the originating machine in this situation and you can act upon them:
https://codesandbox.io/s/example-for-receiving-respond-events-forked-guxzy?file=/src/index.js

Not sure though if this is what you are looking for. If not - could you describe your issue in more detail?

[AlCalzone]

The use case is basically that I want to pass a message to a machine. If that machine doesn't handle the message, it must be handled somewhere else and nothing should happen inside that machine.

I just tried your example without the transition to error and I also get the event back. I could have sworn that didn't work the last time I tried it...
However, the listener is now called for every transition and event inside the machine. I think it would be more idiomatic (and maybe more performant) if there was a listener that was only called for responses (respond and/or sendParent actions).

[Andarist]

Well, respond is actually just a very thin utility, meant to be used mainly for convenience purposes. It's not any separate concept - just look at its implementation:
https://github.com/davidkpiano/xstate/blob/4c3a274a8c9777e8076c4538fedbfc90966b5d87/packages/core/src/actions.ts#L298-L312
It's just a send in disguise 😉 So it's very unlikely that we'd like to special case this with an extra listener set or anything like that.

[AlCalzone]

Got it, thanks for your responses!