AngularFire -如何将firestore文档映射到valueChanges上的数组？

npm install firebase @angular/fire

import { AngularFireModule } from '@angular/fire';
import { AngularFirestoreModule } from '@angular/fire/firestore';

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

import { Component } from '@angular/core';
import { AngularFirestore } from '@angular/fire/firestore';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-example',
  template: `
    <ul>
      <li *ngFor="let item of items | async">{{ item.name }}</li>
    </ul>
  `
})
export class ExampleComponent {
  items: Observable<any[]>;

  constructor(private firestore: AngularFirestore) {
    this.items = this.firestore.collection('collectionName').valueChanges();
  }
}