class Foo: NSObject {
  dynamic var bar: Int = 0
}

class Observer: NSObject {

  private let observerKVOContext = 0

  // MARK: - Objects to observe

  var foo: Foo? {
    didSet {
      let observerKeyPath = #keyPath(Foo.bar)
      oldValue?.removeObserver(self, forKeyPath: observerKeyPath, context: &observerKVOContext)
      foo?.addObserver(self, forKeyPath: observerKeyPath, options: [.new, .initial, .old], context: &observerKVOContext)
    }
  }

  // MARK: - Deinit

  deinit {
    self.foo = nil
  }

  // MARK: - KVO
  override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if context == &observerKVOContext {
            guard let existingKeyPath = keyPath else {
                return
            }
            switch existingKeyPath {
            case #keyPath(Foo.bar):
                // Do KVO based update here
            default:
                return
            }
        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }

}