如何检测使用AngularFire加载的可观察异步项目

import { AngularFireModule } from '@angular/fire';
import { AngularFireDatabaseModule } from '@angular/fire/database';

@NgModule({
  imports: [
    AngularFireModule.initializeApp(environment.firebaseConfig),
    AngularFireDatabaseModule
  ],
  // ...
})
export class AppModule { }

import { AngularFireDatabase } from '@angular/fire/database';

@Component({
  // ...
})
export class MyComponent {
  items$: Observable<any[]>;

  constructor(private db: AngularFireDatabase) {
    this.items$ = this.db.list('items').valueChanges();
  }
}

this.items$.subscribe(items => {
  // 处理数据更新
});

this.items$.subscribe(items => {
  console.log('数据已更新：', items);
  // 执行其他操作
});