export class AuthApplicationService {

  // inject all needed services
  constructor(
    private sessionService: SessionService,
    private authService: AuthService
  ) {}

  // orchestrate multi service execution
  // note that subscription and cancelation are responsibility of the caller
  login(username: string, password: string): Observable<boolean> {
    return this.authService.login(username, password)   // returns Observable<User>
      .do(user => this.sessionService.setUser(user))    // .do() to perform side-effects, mutate model (sync)
      .mapTo(true);                                     // return boolean flag
      .catch(err => this.sessionService.setUser(null)); // handle failure, mutate model (sync)
  }

  /* ... */
}