ViewModel.swift

typealias GetProductsCompletionHandler = ([Product]) -> Void

func getProductsFromCart(_ completion: @escaping GetProductsCompletionHandler) {
  self.db.collection("cart").getDocuments { (snapshot, error) in

    // Handle error
    if error != nil {
      completion([])
      return
    }

    // Convert firebase dictionary data to Product object
    let products: [Product] = snapshot?.documents.compactMap({ Product(from: $0.data()) }) ?? []
    completion(products)
  }
}