仅在设置了数据后如何调用完成处理程序？

Question

The function seen below appears to deliver the orderId via completion handler before the data has been set in firebase, which causes a backend function that uses the orderId to retrieve associated data from Firestore to fail. It's possible that the backend function is failing for other reasons, but it's probably because the function has not finished setting the data to firestore when it delivers the orderId. How do I make it so my function delivers the orderId only once the data has been set?

public func uploadTransactionData(_ menuItems: [MenuItem], tip: Int, tax: Int, rewardAmountApplied: Int, totalPaidFromCredit: Int, discountAmount: Int, subTotal: Int, balanceId: String, locationId: String, completion: @escaping ((String?) -> ())) {
    guard let userId = Auth.auth().currentUser?.uid else { completion(nil); return }
    let utilitiesManager = UtilitiesManager()
    let timestamp = utilitiesManager.timestamp()
    var listOfItems = [Any]()
    for item in menuItems {
        do {
            let jsonData = try JSONEncoder().encode(item)
            let jsonObject = try JSONSerialization.jsonObject(with: jsonData, options: [])
            listOfItems.append(jsonObject)
        }
        catch let err {
            print("$-- error uploading transaction data \(err)")
            completion(nil)
            return
        }
    }
    let orderRef = Firestore.firestore().collection("order").document()
    let orderId = orderRef.documentID
    let totalPrice = subTotal + tax + tip + discountAmount
    let params: [String: Any] = ["date": timestamp,
                                "total_amount": totalPrice,
                                "tip_amount": tip,
                                "tax_amount":tax,
                                "discount_amount": discountAmount,
                                "reward_amount": rewardAmountApplied,
                                "balance_amount": totalPaidFromCredit,
                                "balance_id": balanceId,
                                "subtotal": subTotal,
                                "account_id": userId,
                                "location_id": locationId,
                                "status": "PENDING",
                                "notes": "",
                                "line_items": listOfItems
    ]
    orderRef.setData(params)
    { err in
        if let e = err {
            print("$-- error uploading transaction data \(e)")
            completion(nil)
        } else {
            completion(nil)
        }
    }
    completion(orderId)
}

Answer 1

setData is asynchronous and returns immediately, before the database generates any results. You're calling completion() at the end of your function unconditionally, before the results are complete. Perhaps you meant to say something like this instead:

orderRef.setData(params)
{ err in
    if let e = err {
        print("$-- error uploading transaction data \(e)")
        completion(nil)
    } else {
        // Change this line to yield the order id to the callback on success
        completion(orderId)
    }
}
// Remove this line that always calls the completion before the result
// completion(orderId)