KVObserverEnumPlayground

import Foundation

// ============================================
// MARK: -  KVObserver Helper
// Modified from Source : https://stackoverflow.com/questions/27292918

class KVObserver: NSObject {
  private let keyPath: String
  private let block: () -> Void
  private weak var object: AnyObject?
  
  init(_ object: AnyObject, keyPath: String, block: () -> Void) {
    self.object = object
    self.keyPath = keyPath
    self.block = block
    super.init()
    object.addObserver(self, forKeyPath: keyPath, options: .New, context: nil);
  }
  
  override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
    if keyPath == self.keyPath {
      block()
    }
  }
  
  deinit {
    object?.removeObserver(self, forKeyPath: keyPath)
  }
}

// ============================================
// MARK: -  NSObject Subclass Defining enum to observe

// Must subclass NSObject for KVObserver to work
class MainViewModel: NSObject {
  enum ApplicationState: String  {
    case Default, Ready, Set, Go
  }
  
  dynamic private(set) var applicationStateRaw: String?
  var applicationState: ApplicationState? {
    didSet { applicationStateRaw = applicationState?.rawValue }
  }
}


// ============================================
// MARK: -  This could just as easily be your MainViewController

class Main: NSObject {

let mainViewModel = MainViewModel()
var applicationStateObserver: KVObserver!

  override init() {
    super.init()
    applicationStateObserver = KVObserver(mainViewModel, keyPath: "applicationStateRaw") { [unowned self] in
      
      guard let state = self.mainViewModel.applicationState else { return }
      switch state {
        case .Default: print("DEFAULT")
        case .Ready: print("READY")
        case .Set: print("SET")
        case .Go: print("GO")
      }
    }
  }
  
  deinit {
    applicationStateObserver = nil
  }

}


let main = Main()
main.mainViewModel.applicationState = .Ready
main.mainViewModel.applicationState = .Go

ToDo: Need to explore if this could be optimized/refactored to use @NoEscape in order to remove the need for [unowned self].
http://krakendev.io/blog/hipster-swift#noescape