mini 状态管理

不通过context的状态管理(mini型）

const createEmitter = () => {
  const subscriptions = new Map();
  return {
    emit: (v) => subscriptions.forEach(fn => fn(v)),
    subscribe: (fn) => {
      const key = Symbol();
      subscriptions.set(key, fn);
      return () => subscriptions.delete(key);
    },
  }
};

const createStore = (init) => {
  // create an emitter
  const emitter = createEmitter();

  let store = null;
  const get = () => store;
  const set = (op) => (
    store = op(store),
    // notify all subscriptions when the store updates
    emitter.emit(store)
  );
  store = init(get, set);

  const useStore = () => {
    // intitialize component with latest store
    const [localStore, setLocalStore] = useState(get());

    // update our local store when the global
    // store updates. 
    //
    // emitter.subscribe returns a cleanup 
    // function, so react will clean this
    // up on unmount.
    useEffect(() => emitter.subscribe(setLocalStore), []);
    return localStore;
  };
  return useStore;
};