将Angular http调用迁移到v6

Question

我正在尝试使用http get从api中检索布尔值，并遇到Angular 6的问题（代码在v5中运行）：

我的userservice.ts：

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { environment } from '../../../environments/environment';
import { Observable } from 'rxjs/Observable';

@Injectable({
    providedIn: 'root',
})


export class UserService {

    readonly baseUrl = `${environment.apiUrl}/user`;

    constructor(private http: HttpClient) { }

    public isAdmin(): Observable<boolean> {
        return this.http.get<boolean>(this.baseUrl + '/isAdmin');
    }

}

我的组件：

export class TestComponent{
  public admin: boolean;

  constructor(private userService: UserService) {
    this.userService.isAdmin()
      .subscribe(
        result => {
          this.admin = result;
        }
      );

  }
}

当我导航到我的组件时，我在控制台中遇到的错误是this.userService.isAdmin不是函数