[Javascript] Create an Actor model in Javascript

Refer to: https://stately.ai/docs/actor-model

What defines an “actor”?​

Actors are independent “live” objects that can communicate with each other via asynchronous message passing. In XState, we refer to these messages as events.

• An actor has its own internal, encapsulated state that can only be updated by the actor itself. An actor may choose to update its internal state in response to a message it receives, but it cannot be updated by any other entity.
• Actors communicate with other actors by sending and receiving events asynchronously.
• Actors process one message at a time. They have an internal “mailbox” that acts like an event queue, processing events sequentially.
• Internal actor state is not shared between actors. The only way for an actor to share any part of its internal state is by:
  • Sending events to other actors
  • Or emitting snapshots, which can be considered implicit events sent to subscribers.
• Actors can create (spawn/invoke) new actors.

function counterBehavior(state, event) {
  if (event.type === "INCREMENT") {
    return {
      ...state,
      count: state.count + 1,
    };
  }

  return state;
}

function createActor(behavior, initialState) {
  let currentState = initialState;
  const listeners = new Set();
  function notifyListeners() {
    for (const listener of listeners) {
      listener(currentState);
    }
  }
  return {
    send: (event) => {
      currentState = behavior(currentState, event);
      notifyListeners();
    },
    subscribe: (listener) => {
      listeners.add(listener);
      listener(currentState);
    },
    getSnapshot: () => {
      return currentState;
    },
  };
}

const actor = createActor(counterBehavior, { count: 0 });

actor.subscribe((state) => {
  console.log("State is now", state);
});

actor.send({ type: "INCREMENT" });