Improve the type of callback invoke params

[VanTanev]

type Event = { type: 'START' } | { type: 'STOP' }

const machine = createMachine<{}, Event>({
  invoke: {
    id: 'callback-service',
    src: () => (callback, onReceive) => {
      onReceive(event => {
        // event is { type: 'START' } | { type: 'STOP' }
      })
      callback({ type: 'I_DONT_EXIST' }) // type errors
    }
  }
})

[changeset-bot]

🦋 Changeset detected

Latest commit: a61d01c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
xstate	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[VanTanev]

@Andarist @mattpocock this should be a pretty straightforward type improvement. It didn't break any of my projects, but it would be nice if you could test with yours too.

[davidkpiano]

This makes sense to me. I think an initial motivation for having it as any was that in some situations, it's not easy for the child to know what event types it could send the parent. But there's ways around that.

[mattpocock]

@davidkpiano I 100% agree with @VanTanev that this should be typed as a Sender<TEvent>. I really like using the invoked callback pattern but having to manually type this is not particularly fun. I also haven't found a case where I want it typed as anything else.