Purchase.swift

func purchase(_ product: SKProduct) {
    guard SKPaymentQueue.canMakePayments() else {
        return
    }
    
    let payment = SKPayment(product: product)
    let queue = SKPaymentQueue.default()
    queue.add(payment)
    queue.add(self) //Delegate
}

//SKPaymentTransactionObserver
func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
    transactions.forEach{
        switch $0.transactionState {
        case .purchasing, .deferred:
            print("IAP: Waiting response")
        case .purchased, .restored:
            let originalTransactionId = $0.original?.transactionIdentifier ?? $0.transactionIdentifier ?? ""
            print("IAP: Bought (\(originalTransactionId))")
        case .failed:
            print("IAP: Fail")
        @unknown default:
            fatalError()
        }
    }
}