如標(biāo)題所述，在顯示價格和數(shù)量方面存在問題。這是我到目前為止所做的：購物車項目 export class ShoppingCartItem {  id?: string;  name: string;  imgCover: string;  price: number;  quantity: number;}購物車服務(wù)async getCart(): Promise<Observable<ShoppingCartItem[]>> {  let cartId =  await this.getOrCreateCartId();  let cartCollection = this.afs.collection(`shopping-carts/${cartId}/items`, ref => ref.where('id','==', cartId));  return cartCollection.snapshotChanges().pipe(map( actions => {    return actions.map(a => {      const data = a.payload.doc.data() as ShoppingCartItem;      const id = a.payload.doc.id;      return { id, ...data };    });  }));}navbar.component  totalCart$: Observable<ShoppingCartItem[]>;  items: ShoppingCartItem[]  total: number = 0;  constructor(   public dialog: MatDialog,   private shoppingCartService: ShoppingCartService,  ) {}  ngOnInit() {    this.getCart();  }  async getCart() {    this.totalCart$ = await this.shoppingCartService.getCart();    this.totalCart$.subscribe(data => {      data.forEach(element => {       this.total += element.quantity       console.log(this.total);   })  }) }使用這種方法，我可以在第一次加載時顯示正確的數(shù)據(jù)，然后將該數(shù)量乘以相同數(shù)量+ 1（因為再次調(diào)用現(xiàn)有數(shù)據(jù)）就可以顯示正確的數(shù)據(jù)。如何合并數(shù)量和價格字段？