为什么没有调用paymentQueue（：shouldAddStorePayment：：）？
我确信我已经做了我该做的一切。
我声明了我自己的类，它支持SKPaymentTransactionObserver协议：

import UIKit
import StoreKit
import AudioToolbox.AudioServices

class UTIPaymentTransactionObserver: NSObject, SKPaymentTransactionObserver {

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

        print("!!! shouldAddStorePayment")

        AudioServicesPlayAlertSound(kSystemSoundID_Vibrate)

        return false

    }

    func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {

        print("!!! updatedTransactions")

        for transaction in transactions {
            print("!!! transaction=", transaction)
            switch transaction.transactionState {
            // Call the appropriate custom method for the transaction state.
            case SKPaymentTransactionState.purchasing:
                showTransactionAsInProgress(transaction, deferred: false)
            case SKPaymentTransactionState.deferred:
                showTransactionAsInProgress(transaction, deferred: true)
            case SKPaymentTransactionState.failed:
                failedTransaction(transaction)
            case SKPaymentTransactionState.purchased:
                completeTransaction(transaction)
            case SKPaymentTransactionState.restored:
                restoreTransaction(transaction)
            }
        }

    }

    func showTransactionAsInProgress(_ transaction: SKPaymentTransaction, deferred: Bool) {

        print("!!! showTransactionAsInProgress")

    }

    func failedTransaction(_ transaction: SKPaymentTransaction) {

        print("!!! failedTransaction")

        SKPaymentQueue.default().finishTransaction(transaction)

    }

    func completeTransaction(_ transaction: SKPaymentTransaction) {

        print("!!! completeTransaction")

        SKPaymentQueue.default().finishTransaction(transaction)

    }

    func restoreTransaction(_ transaction: SKPaymentTransaction) {

        print("!!! restoreTransaction")

    }

}

我添加了代码来在调用paymentQueue（：shouldAddStorePayment：：）时振动设备，以指示该方法实际被调用。
我全局声明了一个观察者类的示例：

internal let paymentTransactionObserver = UTIPaymentTransactionObserver()

我确保我在AppDelegate中添加了观察者：

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.

    SKPaymentQueue.default().add(paymentTransactionObserver)

    return true
}

paymentQueue（：shouldAddStorePayment：：）方法中的print语句永远不会打印，设备也不会振动。看起来该方法没有被调用。
调用paymentQueue（_：updatedTransactions：）方法。执行该方法中的print语句。
在这段代码中，我为paymentQueue（：shouldAddStorePayment：：）方法返回了false，但这并没有什么区别。这个过程就像我返回了一个true。该产品之前已经购买过，因此它会通过并让用户/测试人员再次购买。
任何帮助将不胜感激。
下面是视图控制器的代码扩展，它从App Store中检索产品，并呈现允许用户购买产品的用户界面：
我调用validateProductIdentifiers（）来启动向用户销售产品的过程。

// MARK: - SKProductsRequestDelegate

extension CloudViewController: SKProductsRequestDelegate {

    func validateProductIdentifiers() {

        let url = Bundle.main.url(forResource: "Purchase", withExtension: "plist")!

        let nsArrayProductIdentifiers: NSArray = NSArray(contentsOf: url)!

        let productIdentifiers = nsArrayProductIdentifiers as! [String]

        print(productIdentifiers)

        let setProductIdentifers: Set = Set(productIdentifiers)

        let productsRequest = SKProductsRequest(productIdentifiers: setProductIdentifers)

        self.productsRequest = productsRequest

        productsRequest.delegate = self

        productsRequest.start()

    }

    func productsRequest(_ request: SKProductsRequest, didReceive response: SKProductsResponse) {

        print("!!! didReceive")

        self.products = response.products

        let alertMessage = "Would you like to purchase?"

        let alert = UIAlertController(title: nil, message: alertMessage, preferredStyle: .actionSheet)

        let actionYes = UIAlertAction(title: "Yes", style: .default) {

            action in

            // Purchase

            let product: SKProduct = response.products.first!

            let payment: SKMutablePayment = SKMutablePayment(product: product)
            SKPaymentQueue.default().add(payment)

        }

        let actionNo = UIAlertAction(title: "No", style: .cancel, handler: nil)

        alert.addAction(actionYes)
        alert.addAction(actionNo)

        alert.popoverPresentationController?.barButtonItem = barButtonItemEnableDropbox

        present(alert, animated: true, completion: nil)

    }

}