While waiting for swift4 observe method

Prop.swift

//
//  Prop.swift
//  GQLC
//
//  Created by Stefano Brilli on 01/08/2017.
//  Copyright © 2017 Stefano Brilli. All rights reserved.
//

import Cocoa

class Prop<T>: NSObject {
    
    var target: NSObject
    var keyPath: String
    var callback: (T?, T?) -> Void

    init(target: NSObject, keyPath: String, callback: @escaping (T?, T?) -> Void) {
        self.target = target
        self.keyPath = keyPath
        self.callback = callback

        super.init()
        
        target.addObserver(self, forKeyPath: keyPath, options: [.initial, .new, .old], context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        let new = change?[.newKey] as? T
        let old = change?[.oldKey] as? T
        self.callback(new, old)
    }
    
    deinit {
        target.removeObserver(self, forKeyPath: keyPath)
    }
}