redux.ts

interface Action {
  type: string;
  payload?: any;
}

interface Reducer<T> {
  (state: T, action: Action): T
}

interface ListenerCallback {
  (): void;
}

interface UnsubscribeCallback {
  (): void;
}

class Store<T> {
  private _state: T;
  private _listeners: ListenerCallback[] = [];

  constructor( private reducer: Reducer<T>, initialState: T ) {
    this._state = initialState;
  }

  getState(): T {
    return this._state;
  }

  dispatch(action: Action): void {
    this._state = this.reducer(this._state, action);
    this._listeners.forEach((listener: ListenerCallback) => listener());
  }

  subscribe(listener: ListenerCallback): UnsubscribeCallback {
    this._listeners.push(listener);

    return () => { // returns an "unsubscribe" function
      this._listeners = this._listeners.filter(l => l !== listener);
    };
  }
}

let reducer: Reducer<number> = (state: number, action: Action) => {
  switch( action.type ) {
    case 'INCREMENT':
      return state + 1;
    case 'DECREMENT':
      return state - 1;
    case 'PLUS':
      return state + action.payload;
    default:
      return state;
  }
}

let incrementAction: Action = { type: 'INCREMENT' };
let decrementAction: Action = { type: 'DECREMENT' };

// By default, reducers return the original state.
// console.log(reducer(0, null));

console.log(reducer(0, incrementAction));
console.log(reducer(1, incrementAction));

console.log(reducer(2, decrementAction));

let unknownAction: Action = { type: 'UNKNOWN' };
console.log(reducer(100, unknownAction)); // -> 100

console.log( reducer(3, { type: 'PLUS', payload: 7 }) );
console.log( reducer(3, { type: 'PLUS', payload: 9000 }) );
console.log( reducer(3, { type: 'PLUS', payload: -2 }) );


// create a new store
let store = new Store<number>(reducer, 0);
// console.log(store.getState()); // -> 0

// subscribe
let unsubscribe = store.subscribe(() => {
  console.log('subscribed: ', store.getState());
});

store.dispatch(incrementAction);
console.log(store.getState()); // -> 1

store.dispatch(incrementAction);
console.log(store.getState()); // -> 2

unsubscribe();

store.dispatch(decrementAction);
console.log(store.getState()); // -> 1