ViewController.swift

// `.` indicates code left out. 
// Please refer to the sample app for reference
// https://github.com/josh-marasigan/ObserverDesignExample
class ViewController: UIViewController {
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  // MARK: - Button Configuration
  private func configChangeBackgroundButtonUI() {
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        self.changeBackgroundButton.addTarget(
          self, 
          action: #selector(changeBackgroundButtonAction), 
          for: .touchUpInside)
        .
        .
  }
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  // MARK: - Button Action
  @objc func changeBackgroundButtonAction(sender: UIButton!) {
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    /*
    Then we set the notification name.
    This will allow the observers who care about 
    this event that the event is occuring.
    (Observer keys are unique identifiers used 
    to distinguish events from one another)
    We also check if the current background 
    is colored teal, and if so,
    send an event to change it to pink. 
    If not, then send an event to change it to teal
    */
    let notificationIdentifier = (isBackgroundTeal) ?
            Notification.Name(rawValue: ObserverKeys.pink) :
            Notification.Name(rawValue: ObserverKeys.teal)

    /*
    Post the notification informing any observers 
    listening to this event (indicated by its name) 
    that an event had occured.
    If an observer registered to listen for 
    an event with name 'notificationIdentifier',
    then they will receive this notification
    */
    NotificationCenter.default.post
    (
        name: notificationIdentifier, 
        object: nil
    )
    .
    .
  }
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