state-service.ts

import {BehaviorSubject, Observable} from "rxjs";
import {Inject, Injectable, InjectionToken} from "@angular/core";

export interface Action {
  type: string;
}

export interface ActionHandler {
  handleAction(currentState: any, action: Action): any;
}

export const ActionHandlers = new InjectionToken(
    'lookup for action handlers',
    {providedIn: 'root', factory: () => []}
)

export const InitialState = new InjectionToken(
    'lookup for initial state',
    {providedIn: 'root', factory: () => null}
)

@Injectable()
export class StateService {
 private _state: BehaviorSubject<any>;

 actions: BehaviorSubject<Action>;
 state$: Observable<any>;

 constructor(@Inject(InitialState) initialState: any,
             @Inject(ActionHandlers) actionHandlers: ActionHandler[]) {
   this._state = new BehaviorSubject<any>(initialState)
   this.actions = new BehaviorSubject<Action>({type: 'init'})

   this.actions.subscribe(action => {
     const nextState = this._state.getValue();
     actionHandlers.forEach( handler => {
       Object.assign(nextState, handler.handleAction(nextState, action))
     })
     this._state.next(nextState);
   })

   this.state$ = this._state.asObservable();
 }
}