Support generic sent event types (sendBack events) to callback-based actors

[steveadams]

As mentioned in Discord, I don't think this is supported presently. With no typing, it's possible to place arbitrary events into the sendBack method. With this change (currently intended to be a suggestion; some things are broken), the types do work as I want them to in my use-cases so this seems to be on the right track.

One existing solution to this problem is to type the events like this:

export type SegmenterEvent =
  | {
      type: 'SEGMENT_CREATED';
      eventID: string;
      eventData: EventData;
    }
  | {
      type: 'SEGMENTER_FINISHED';
      segmenterID: string;
    }
  | {
      type: 'SEGMENTER_FAILED';
      segmenterID: string;
      error: unknown;
    };

export const segmenter = fromCallback(
  ({ input, sendBack }: { input: SegmenterInput; sendBack: (e: SegmenterEvent) => void }) => {
    const segmentEventData = async () => {

With this proposed change, the goal is to be able to type them like this:

export const segmenter = fromCallback<AnyEventObject, SegmenterInput, AnyEventObject, SegmenterEvent>(
  ({ input, sendBack }) => {
    const segmentEventData = async () => {

Both versions are quite verbose, but for callbacks with received, emitted, and sent events, it could be quite a bit cleaner.

Remaining:

• Determine if anyone else thinks this is a good idea
• Include changeset
• Figure out why typecheck is failing. It appears to be failing on main as well, but not sure if this is a caching/local issue of some sort. I'll look closer
• Review if the changes are made correctly, or only match my use case coincidentally while it might break others

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[Andarist]

It requires a little bit of manual plumbing but the preferred approach to get what you want is this (TS playground):

import { setup, fromCallback, ActorRef, Snapshot } from "xstate"; // types: 5.19.0

type ParentEvent = { type: "TO_PARENT" } | { type: "ANOTHER_TO_PARENT" };

setup({
  types: {} as {
    events: ParentEvent;
  },
  actors: {
    foo: fromCallback(
      ({
        input: { parent },
      }: {
        input: { parent: ActorRef<Snapshot<unknown>, ParentEvent> };
      }) => {
        parent.send({
          type: "TO_PARENT",
        });
        parent.send({
          // @ts-expect-error
          type: "unknown",
        });
        return () => {};
      },
    ),
  },
}).createMachine({
  invoke: {
    src: "foo",
    input: ({ self }) => ({ parent: self }),
  },
});

Introducing a new generic introduces coupling between actor definitions in a way that is meaningless for non-callback actors. Other actor types can't send events to their parents without using a pattern like the above after all.

[steveadams]

Ah, I see why this wasn't done already then. Thank you for explaining it to me. I'll stick with the manual plumbing!