google-cloud-firestore - 用于收集的 Firestore 方法 snapshotChanges()

Question

以下是AngularFirestore 中的集合中提供的代码。

export class AppComponent {
  private shirtCollection: AngularFirestoreCollection<Shirt>;
  shirts: Observable<ShirtId[]>;
  constructor(private readonly afs: AngularFirestore) {
    this.shirtCollection = afs.collection<Shirt>('shirts');
    // .snapshotChanges() returns a DocumentChangeAction[], which contains
    // a lot of information about "what happened" with each change. If you want to
    // get the data and the id use the map operator.
    this.shirts = this.shirtCollection.snapshotChanges().map(actions => {
      return actions.map(a => {
        const data = a.payload.doc.data() as Shirt;
        const id = a.payload.doc.id;
        return { id, ...data };
      });
    });
  }
}

这里方法 snapshotChanges() 返回 DocumentChangeAction[] 的 observable。那么为什么在只有一个数组并且只会循环一次的情况下使用地图来读取它呢？