错误TypeError：this.afAuth.authState.map不是函数

Question

我只想验证用户是否登录，所以我在auth.service中实现方法getAuth（），我在navbar.component中的方法ngOnInit（）上调用它，但是我有一个错误，我找不到任何解决方案

这是我的auth.service的代码

import { Observable } from 'rxjs';
import { AngularFireAuth } from 'angularfire2/auth';
import { Injectable } from '@angular/core';


@Injectable()
export class AuthService {

  constructor(
    public afAuth: AngularFireAuth,
  ) { }


  login(email:string, password:string){
    return new Promise((resolve,reject) => {
      this.afAuth.auth.signInWithEmailAndPassword(email,password)
      .then(userData => resolve(userData),
      err=>reject(err));
    });
  }


  getAuth(){
     return this.afAuth.authState.map(auth => auth); -->
  }

  logout(){
    this.afAuth.auth.signOut();
  }
}

**我在navbar.components **

中的ngOnInit（）上调用了该函数

import { Observable } from 'rxjs';
import { AuthService } from './../../services/auth.service';
import { Router } from '@angular/router';
import { FlashMessagesService } from 'angular2-flash-messages';
import { Component, OnInit } from '@angular/core';


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

  isLoggedIn: boolean = true;
  loggedInUser:string;
  showRegister:boolean;

  constructor(
    private authService: AuthService,
    private route: Router,
    private flashMessagesService : FlashMessagesService
  ) { }

  ngOnInit() {
    this.authService.getAuth().subscribe(auth => {
      if (auth){
        this.isLoggedIn = true;
        this.loggedInUser = auth.email;
      }else{
        this.isLoggedIn = false;
      }
    });
  }

  onLogoutClick(){
    this.authService.logout();
    this.flashMessagesService.show('You are logged out', {cssClass:'alert-success', timeout:4000});
    this.route.navigate(['/login']);
  }

}

错误

ERROR TypeError: this.afAuth.authState.map is not a function
    at AuthService.getAuth (auth.service.ts:24)
    at NavbarComponent.ngOnInit (navbar.component.ts:26)
    at checkAndUpdateDirectiveInline (core.js:12369)
    at checkAndUpdateNodeInline (core.js:13893)
    at checkAndUpdateNode (core.js:13836)
    at debugCheckAndUpdateNode (core.js:14729)
    at debugCheckDirectivesFn (core.js:14670)
    at Object.eval [as updateDirectives] (AppComponent.html:1)
    at Object.debugUpdateDirectives [as updateDirectives] (core.js:14655)
    at checkAndUpdateView (core.js:13802)