Sad View Controller example for https://medium.com/@PatrykPeszko/notification-pattern-a862ceff4226

SadPaymentViewController.swift

import UIKit

struct Payment { }

enum PaymentResponse {
    case Accepted(Payment)
    case Rejected(Payment)
}

// It's just an example for notifications. The rest can be safely ignored :)
class SadPaymentViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let notificationCenter = NSNotificationCenter.defaultCenter()

        // MARK: ლ( `Д’ ლ)
        // This tells Notfication Center that we are interested in updates.
        notificationCenter.addObserver(self,
            selector: #selector(SadPaymentViewController.paymentNotificationReceived(_:)),
            name: "PaymentViewControllerNotificationRejected",
            object: nil)
        notificationCenter.addObserver(self,
            selector: #selector(SadPaymentViewController.paymentNotificationReceived(_:)),
            name: "PaymentViewControllerNotificationAccepted",
            object: nil)
    }

    func paymentServiceDidReturn(paymentResponse: PaymentResponse) {
        let notificationCenter = NSNotificationCenter.defaultCenter()

        switch paymentResponse {
        case .Accepted(_):
            // MARK: •́︵•̀
            // And this is how we post notifications
            notificationCenter.postNotificationName("PaymentViewControllerNotificationRejected",
                object: nil,
                userInfo: nil)
        case .Rejected(_):
            notificationCenter.postNotificationName("PaymentViewControllerNotificationAccepted",
                object: nil,
                userInfo: nil)
        }
    }

    func paymentNotificationReceived(notification: NSNotification) {
        //FIXIT: Write this code before release, okay? Bye, I'm off for vacation. ᕕ( ᐛ )ᕗ
    }

    deinit {
        NSNotificationCenter.defaultCenter().removeObserver(self)
    }

}