即使正确配置间谍，Angular 单元测试也失败

问：

我的组件如下所示：

  constructor(
    private readonly dialogRef: MatDialogRef<ChangeRolesDialogComponent>,
    private readonly rolesService: RolesService,
    private readonly usersService: UsersService
  ) {}

  ngOnInit(): void {
    this.rolesService.getAllRoles().subscribe((roles: Role[]) => {
      this.roles = roles;
    });
 
  }

在 .spec 文件的配置中，我这样做了：

fdescribe('ChangeRolesDialogComponent', () => {
  let component: ChangeRolesDialogComponent;
  let fixture: ComponentFixture<ChangeRolesDialogComponent>;

  let rolesServiceSpy: jasmine.SpyObj<RolesService>;
  let usersServiceSpy: jasmine.SpyObj<UsersService>;
  let matDialogRefSpy: jasmine.SpyObj<MatDialogRef<ChangeRolesDialogComponent>>;

  beforeEach(async () => {
    rolesServiceSpy = jasmine.createSpyObj('RolesService', ['getAllRoles']);
    usersServiceSpy = jasmine.createSpyObj('UsersService', [
      'getCurrentUserAuthentication',
    ]);

    matDialogRefSpy = jasmine.createSpyObj('MatDialogRef', ['close']);
    await TestBed.configureTestingModule({
      declarations: [ChangeRolesDialogComponent],
      providers: [
        { provide: RolesService, useValue: rolesServiceSpy },
        { provide: UsersService, useValue: usersServiceSpy },
        { provide: MatDialogRef, useValue: matDialogRefSpy },
        { provide: MatDialogRef, useValue: {} },
      ],

这是单元测试：

 it('should get all roles on init', () => {
    const roles: Role[] = [Role.ADMINISTRATORS, Role.PRICING_ADMIN];
    rolesServiceSpy.getAllRoles.and.returnValue(of(roles));

    component.ngOnInit();

    expect(component.roles).toEqual(roles);
  });

但它失败了，并出现错误：

“应该在 init FAILED 上获取所有角色 TypeError：无法读取 undefined 的属性（读取 'subscribe'）”

你可以帮我吗？