Allow existing actors in spawn() to be referenced by name

[davidkpiano]

Existing actors can now be identified in spawn(...) calls by providing an id. This allows them to be referenced by string:

const machine = createMachine({
  context: () => ({
    someRef: spawn(someExistingRef, 'something')
  }),
  on: {
    SOME_EVENT: {
      actions: send('AN_EVENT', { to: 'something' })
    }
  }
});

[changeset-bot]

🦋 Changeset detected

Latest commit: 1def6cf

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

This PR includes changesets to release 1 package

Name	Type
xstate	Patch

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[mattpocock]

Does this also make them available in service.children.get('something')

[davidkpiano]

Does this also make them available in service.children.get('something')

Yes it does. But maybe not on state.children['something'], will investigate.