来自箭头函数的Angular 2 Observable单值

Question

我的组件中有以下代码：

  private katalog : Katalog;
  private allKatalogs : Katalog[] = [];

  constructor(private adminService : AdminService) { }

  ngOnInit() {
    this.adminService.get_kataloge().subscribe((items : Katalog[]) => this.allKatalogs = items);
    this.adminService.get_kataloge().subscribe((items : Katalog[]) => this.katalog = items[0]);
  }
  showObject() {
    console.log(this.katalog); // this returns undefined
    console.log(this.allKatalogs); // this returns [Object, Object, Object, Object, Object]
  }

为什么this.katalog未定义？

这里的服务：

get_kataloge() : Observable<Katalog[]> { return this.kataloge.asObservable(); }

katalog模型：

export class Katalog {
    _id: string;
    bezeichnung: string;
}

模板html

<p (click)="showObject()">show object</p>