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// Singleton
static let sharedInstance = SingletonA()

// The KVO mechanism allows an object to observe changes to a property. 
// In your case, you can use KVO to observe changes to the image property of the UIImageView that holds the image.
init {
    object.addObserver(self, forKeyPath: "objectProperty", options: nil, context: nil)
}

deinit {
  object.removeObserver(self, forKeyPath: "objectProperty")
}

override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
  if keyPath == "objectProperty" {
    //do something
  }
}