Simple KVO example in Swift based on: http://stackoverflow.com/questions/24092285/is-key-value-observation-kvo-available-in-swift

KVO.swift

import Foundation

class User: NSObject {
  dynamic var name: String?
}

class Observer: NSObject {
  var user: User
  
  init(user: User) {
    self.user = user
    
    super.init()
    
    self.user.addObserver(self, forKeyPath: "name", options: NSKeyValueObservingOptions.New, context: nil)
  }
  
  override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
      if let newValue = change?[NSKeyValueChangeNewKey] {
        print("Name changed: \(newValue)")
      } else {
        super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
    }
  }
  
  deinit {
    self.user.removeObserver(self, forKeyPath: "name")
  }

  
}

var user = User()
user.name = "Test"

var observer = Observer(user: user)
user.name = "Test 2"