observeValue(forKeyPath:of:change:context:) in Swift 3

Cocoa Programming for OS X (5th Edition) 11. NSUndoManager
1

What I really learn most in this section is debugging and patience…

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
  if context != &KVOContext {
    // If the context does not match, this message must be intended for our superclass.
    super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    return
  }
  
  guard let keyPath = keyPath, let change = change, object is Employee else {
    return
  }
  
  var oldValue = change[NSKeyValueChangeKey.oldKey] as Any?
  if oldValue is NSNull {
    oldValue = nil
  }
  
  let undo: UndoManager = undoManager!
  Swift.print("oldValue=\(oldValue)")
  undo.registerUndo(withTarget: self) { (myself) in
    (object as! Employee).setValue(oldValue, forKeyPath: keyPath)
  }
  undo.setActionName("Typing")
}