observer.swift

func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
        for transaction in transactions {
            switch(transaction.transactionState) {
            case .purchasing:
                break;
            case .purchased:
            case .restored:
                handlePurchase(transaction);
            case .failed:
                handleFailure(transaction);
            case .deferred:
                break;
            }
        }
    }
}