How to handle shouldAddStorePayment for In-App Purchases in iOS 11?

Here's a class I did that can help you achieve what you want, simply copy the code below and paste it inside a new file and then you can simply access the class StoreManager.shared to whatever method/variable you want to access.

1- To init this class, just call from your didFinishLaunchingWithOptions StoreManager.shared.Begin() and then payment observer is added.

import Foundation
import StoreKit


class StoreManager: NSObject{

    /**
     Initialize StoreManager and load subscriptions SKProducts from Store
     */
    static let shared = StoreManager()

    func Begin() {
        print("StoreManager initialized"))
    }

    override init() {
        super.init()

        // Add pyament observer to payment qu
        SKPaymentQueue.default().add(self)
    }

    func requestProductWithID(identifers:Set<String>){

        if SKPaymentQueue.canMakePayments() {
            let request = SKProductsRequest(productIdentifiers:
                identifers)
            request.delegate = self
            request.start()
        } else {
            print("ERROR: Store Not Available")
        }
    }

    func buyProduct(product: SKProduct) {

        print("Buying \(product.productIdentifier)...")
        let payment = SKPayment(product: product)
        SKPaymentQueue.default().add(payment)
    }

    func restorePurchases() {

        SKPaymentQueue.default().restoreCompletedTransactions()
    }
}

// MARK:
// MARK: SKProductsRequestDelegate

//The delegate receives the product information that the request was interested in.
extension StoreManager:SKProductsRequestDelegate{

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

        var products = response.products as [SKProduct]

        var buys = [SKProduct]()

        if (products.count > 0) {
            for i in 0 ..< products.count {
                let product = products[i]
                print("Product Found: ",product.localizedTitle)
            }
        } else {
            print("No products found")
        }

        let productsInvalidIds = response.invalidProductIdentifiers

        for product in productsInvalidIds {
            print("Product not found: \(product)")
        }
    }

    func request(_ request: SKRequest, didFailWithError error: Error) {
        print("Something went wrong: \(error.localizedDescription)")
    }
}

// MARK:
// MARK: SKTransactions

extension StoreManager: SKPaymentTransactionObserver {

    public func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        for transaction in transactions {
            switch (transaction.transactionState) {
            case .purchased:
                completeTransaction(transaction: transaction)
                break
            case .failed:
                failedTransaction(transaction: transaction)
                break
            case .restored:
                restoreTransaction(transaction: transaction)
                break
            case .deferred:
                // TODO show user that is waiting for approval

                break
            case .purchasing:
                break
            }
        }
    }

    private func completeTransaction(transaction: SKPaymentTransaction) {

        print("completeTransaction...")

        deliverPurchaseForIdentifier(identifier: transaction.payment.productIdentifier)
        SKPaymentQueue.default().finishTransaction(transaction)
    }

    private func restoreTransaction(transaction: SKPaymentTransaction) {


        guard let productIdentifier = transaction.original?.payment.productIdentifier else { return }

        print("restoreTransaction... \(productIdentifier)")


        deliverPurchaseForIdentifier(identifier: productIdentifier)
        SKPaymentQueue.default().finishTransaction(transaction)
    }

    private func failedTransaction(transaction: SKPaymentTransaction) {

        if let error = transaction.error as NSError? {
            if error.domain == SKErrorDomain {
                // handle all possible errors
                switch (error.code) {
                case SKError.unknown.rawValue:
                    print("Unknown error")

                case SKError.clientInvalid.rawValue:
                    print("client is not allowed to issue the request")

                case SKError.paymentCancelled.rawValue:
                    print("user cancelled the request")

                case SKError.paymentInvalid.rawValue:
                    print("purchase identifier was invalid")

                case SKError.paymentNotAllowed.rawValue:
                    print("this device is not allowed to make the payment")

                default:
                    break;
                }
            }

        }

        SKPaymentQueue.default().finishTransaction(transaction)
    }

    private func deliverPurchaseForIdentifier(identifier: String?) {

        guard let identifier = identifier else { return }

    }
}

//In-App Purchases App Store
extension StoreManager{

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

        //To hold
        //return false

        //And then to continue
        //SKPaymentQueue.default().add(savedPayment)
    }
}