import Foundation

public class ObserverContext: NSObject {
    public let keyPath: String
    public let options: NSKeyValueObservingOptions
    public var context = 0
    public init(keyPath: String, options: NSKeyValueObservingOptions = nil) {
        self.keyPath = keyPath
        self.options = options
    }
}

public extension NSObject {

    public func addObserver(observer: NSObject, context: ObserverContext) {
        addObserver(observer, forKeyPath: context.keyPath, options: context.options, context: &context.context)
    }

    public func removeObserver(observer: NSObject, context: ObserverContext) {
        removeObserver(observer, forKeyPath: context.keyPath, context: &context.context)
    }

}

// USAGE:
class ViewController: UIViewController {
  private let contentSizeContext = ObserverContext(keyPath: "contentSize")
  private lazy var scrollView: UIScrollView = { return UIScrollView() }()

  override func viewDidLoad() {
    super.viewDidLoad()
    scrollView.addObserver(self, context: contentSizeContext)
  }

  override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        switch context {
        case &contentSizeContext.context:
          print(scrollView.contentSize)
        default:
         super.observerValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
    }

  deinit {
    if isViewLoaded() {
      scrollView.removeObserver(self, context: contentSizeContext)
    }
  }
}