Swift 的觀察者模式實作：

static let CHANGE_BACKGROUND_COLOR = "changeBackgroundColor"

func viewDidLoad(){
  super.viewDidLoad()
  
  //加入觀察者
  NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(changeBackgroundColor), name: CHANGE_BACKGROUND_COLOR, object: nil)
}

//移除觀察者
func viewWillDisappear(animated: Bool){
  NSNotificationCenter.defaultCenter().removeObserver(self, name: CHANGE_BACKGROUND_COLOR, object: nil)
}

//實作
func changeBackgroundColor(){
  self.view.backgroundColor = UIColor.redColor()
}

func button_Click(){
  NSNotificationCenter.defaultCenter().postNotificationName(viewcontroller.CHANGE_BACKGROUND_COLOR, object: nil)
}