Storekit Appstore购买促销

当用户点击促销app store产品时,苹果开发者网站上给出的代码告诉用户检查是否可以完成交易?我该怎么检查呢?因为如果交易失败或延期,我必须考虑,而且似乎不知道怎么做。

//MARK: - SKPaymentTransactionObserver

func paymentQueue(_ queue: SKPaymentQueue, shouldAddStorePayment payment: SKPayment,
    forProduct product: SKProduct) -> Bool {
      // Check to see if you can complete the transaction.
      // Return true if you can.
 return true
}

我还必须满足以下场景,我发现这些场景与检查交易是否可以完成的场景相同

func paymentQueue(_ queue: SKPaymentQueue, shouldAddStorePayment payment: SKPayment,
    forProduct product: SKProduct) -> Bool {

       // ... Add code here to check if your app must defer the transaction.
     let shouldDeferPayment = ...

     // If you must defer until onboarding is completed, then save the payment and return false.

     if shouldDeferPayment {
        self.savedPayment = payment
        return false
     }

    // ... Add code here to check if your app must cancel the transaction.
    let shouldCancelPayment = ...
    // If you must cancel the transaction, then return false:
    if shouldCancelPayment {
       return false
    }
}

      // (If you canceled the transaction, provide feedback to the user.)

     // Continuing a previously deferred payment
     SKPaymentQueue.default().add(savedPayment)

  )

如何检查付款是否失败或需要延期,或者是否可以按照两个代码部分中的说明完成?