import { BehaviorSubject } from 'rxjs';
import { IStore } from './IStore';
import { proxyToBehaviorSubject } from './proxyToBehaviorSubject';
import { onFirstSubscription } from './onFirstSubscription';

export type IStateUpdaterFunc<State extends object> = (
  state: State
) => Partial<State>;
export type IStateUpdater<State extends object> =
  | Partial<State>
  | IStateUpdaterFunc<State>;

/**
 * Prostore which uses BehaviorSubject to hold its state.
 * It mimics the React class component state management.
 * You can use setState to change part of the state of the store.
 *
 * It performs synchronous state updates which means that two consequent calls to setState
 * will result in dispatching two store updates.
 */
export class BehaviorStore<State extends object> implements IStore<State> {
  state$: BehaviorSubject<State>;

  get state() {
    return this.state$.value;
  }

  constructor(initialState: State) {
    const subject$ = new BehaviorSubject<State>(initialState);
    this.state$ = proxyToBehaviorSubject(
      subject$.pipe(
        onFirstSubscription(
          () => this.onSubscribe(),
          () => this.onUnsubscribe()
        )
      ),
      subject$
    );
  }

  /**
   * This callback is called when the first subscriber subscribes to this store.
   * It may be called multiple times during the lifetime of the store.
   * You can use it to perform side-effects like subscribing to other
   * observables.
   */
  onSubscribe() {}

  /**
   * This callback is called when the last subscriber unsubscribes from this store.
   * It may be called multiple times during the lifetime of the store.
   * You can use it to cleanup side-effects created during onSubscribe.
   */
  onUnsubscribe() {}

  /**
   * Update part of the store state.
   *
   * @param stateUpdate - object containing partial state update or updater function.
   *  If the function is provided, it is called with the current state and should return the partial state update
   */
  setState(stateUpdate: IStateUpdater<State>) {
    this.state$.next(this._applyStateUpdater(this.state, stateUpdate));
  }

  _applyStateUpdater(state: State, stateUpdate: IStateUpdater<State>): State {
    if (typeof stateUpdate === 'function') {
      return {
        ...state,
        ...stateUpdate(state)
      };
    } else {
      return {
        ...state,
        ...stateUpdate
      };
    }
  }
}