public func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
    for transaction in transactions {
        switch transaction.transactionState {
        case .purchased:
            complete(transaction: transaction)
        case .failed:
            fail(transaction: transaction)
        case .restored:
            restore(transaction: transaction)
        case .deferred:
            break
        case .purchasing:
            break
        default:
            break
        }
    }
}