[Angular] Design API for show / hide components based on Auth

Simple Auth service:

import { Injectable } from '@angular/core';
import {HttpClient} from '@angular/common/http';
import {Observable} from 'rxjs/Observable';
import {User} from '../model/user';
import {BehaviorSubject} from 'rxjs/BehaviorSubject';
import 'rxjs/add/operator/map';

export const ANONYMOUS_USER: User = {
  id: undefined,
  email: ''
};

@Injectable()
export class AuthService {

  subject = new BehaviorSubject<User>(ANONYMOUS_USER);
  user$: Observable<User> = this.subject.asObservable();
  isLoggedIn$: Observable<boolean> = this.user$.map(user => !!user.id);
  isLoggedOut$: Observable<boolean> = this.isLoggedIn$.map(isLoggedIn => !isLoggedIn);


  constructor(private http: HttpClient) { }

  signUp(email: string, password: string) {

  }

}

Using Observable is a easy way to implement reactive application. 

Create a BehaviorSubject and then convert subject to Observable. BehaviorSubject also takes a init param. 

The isLoggedIn$ & isLoggedOut$ based on user$.

The in the component, we inject the auth service:

import {Component, OnInit} from '@angular/core';
import {AuthService} from './services/auth.service';
import {Observable} from 'rxjs/Observable';
import {User} from './model/user';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {

  user$: Observable<User>;
  isLoggedIn$: Observable<boolean>;
  isLoggedOut$: Observable<boolean>;

  constructor(private auth: AuthService) {

  }

  ngOnInit() {
    this.user$ = this.auth.user$;
    this.isLoggedIn$ = this.auth.isLoggedIn$;
    this.isLoggedOut$ = this.auth.isLoggedOut$;
  }
}

HTML:

        <li *ngIf="isLoggedOut$ | async">
            <a routerLink="/signup">Sign Up</a>
        </li>
        <li *ngIf="isLoggedOut$ | async">
            <a routerLink="/login">Login</a>
        </li>
        <li *ngIf="isLoggedIn$ | async">
            <a >Logout</a>
        </li>