为什么我无法获得服务中的数据路由

Question

我希望能够在服务中动态获取路由信息，但发现数据为空（例如，作为数据属性..），但它可以从组件中获取完整数据

live example

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// AppRoutingModule
const routes: Routes = [
  { path: '', redirectTo: '/child', pathMatch: 'full' },
  {
    path: 'child',
    component: ChildComponent,
    data: {
      pageName: 'child page!!'
    }
  },
];
========================
// ChildComponent
export class ChildComponent implements OnInit {

  constructor(
    private activateRoute: ActivatedRoute,
    private childService: ChildService
  ) { 

  }

  getPageName() {    
    this.activateRoute.data
      .subscribe(val => console.log('child_component_pageName:', val['pageName']));
  }
  
  ngOnInit() {
    // has Data
    this.getPageName();
   // empty 
   this.childService.getPageName();
   // empty
   this.childService.getPageNameBySnapshot();

   /*
   output:
   ---
   child_component_pageName: child page!!
   child_service_pageName: undefined
   child_service_snapshot_pageName: undefined
   */
  }

}

==================================
// ChildService
@Injectable()
export class ChildService {
    constructor(private activateRoute: ActivatedRoute) {

    }

    getPageName() {
        this.activateRoute.data
        .subscribe(val => console.log('child_service_pageName:', val['pageName']));
    }

    getPageNameBySnapshot() {
         console.log('child_service_snapshot_pageName:', this.activateRoute.snapshot.data['pageName']);
    }
}

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我的英语不好，如果语法不对，请告诉我。