Allow for stopping spawned actors

[mattpocock]

There appears not to be a lucy-native way for stopping spawned actors. Pointed out by @VanTanev on a recent stream of mine.

[matthewp]

Normally you would call context.actorRef.stop() in an action, right? You an currently do that, I think. But it might be nice to have a lucy native way. Maybe:

machine todoItem {
  state idle {}
}

machine app {
  state idle {
    new => assign(todo, spawn(todoItem))

    delete => stop(todo)
  }
}

[mattpocock]

This looks cool. One thing which might affect spawn in general is how we represent arrays of spawned actors? Though perhaps this should be the topic for another issue.

[matthewp]

Yeah lists are going to be needed eventually, I intentionally avoided that so far.

[VanTanev]

Normally you would call context.actorRef.stop() in an action, right?

Unfortunately, no. The proper way to stop a spawned child is this:

{
  on: {
    STOP: actions.pure((ctx) => {
      const assignAction = assign({ actorRef: null })
      const stopAction = actions.stop(ctx.actorRef)
      
      return [assignAction, stopAction]
    })
  }
}

XState v5 should improve this, but for now, the above code is the only way to stop a child actor without leaking memory. Simply calling ctx.actorRef.stop() still leaves a dangling reference in machine.children, while actions.stop() will properly clean up all references.

Because assign() executes before all other actions, you can't simply have:

actions: [
  // doesn't work, assign() will execute before stop(), and `ctx.actorRef` will be null, so nothing will be stopped.
  actions.stop(ctx => ctx.actorRef),
  assign({ actorRef: null })
]

It must be wrapped in actions.pure().

[matthewp]

@VanTanev Thank you! Seems like something Lucy can do the gross part for you.