激活路线。paramMap。switchMap错误

在我的组件的ngOnInit方法中,下面的行产生了一个错误。

this.products$ = this.route.paramMap.switchMap((params: ParamMap) =>
  this.getProductsForType(params.get('type'))); 

这是产生的错误:

BrandComponent_Host.ngfactory.js? [sm]:1 ERROR TypeError: this.route.paramMap.switchMap is not a function
at BrandComponent.ngOnInit (brand.component.ts:22)
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] (BrandComponent_Host.ngfactory.js? [sm]:1)
at Object.debugUpdateDirectives [as updateDirectives] (core.js:14655)
at checkAndUpdateView (core.js:13802)
at callViewAction (core.js:14153)

这是组件:

export class BrandComponent implements OnInit {
  private products:Product[];
  private products$: Observable<Product[]>;

  constructor(private route:ActivatedRoute,
    private brandService: BrandService) { }

  ngOnInit() {    
    this.products$ = this.route.paramMap.switchMap((params: ParamMap) =>
      this.getProductsForType(params.get('type'))); 
  }

  private getProductsForType(type) {
    console.log('BrandComponent.getProductsForType() called')
    return this.brandService.getProductsForType(type);
  }

}

目前,BrandService。getProductsForType()方法返回空数组:

@Injectable()
export class BrandService {
  private productsUrl:string = '/api/products/all';
  private products:Product[];

  constructor(private http:HttpClient,
    private dataService:DataService) { }

  public getProductsForType(type:string)  : Observable<Product[]> {
    console.log('BrandService.getProductsForType() called')
    // return this.dataService.getProductsForType(type);
    return of([])
  }
}