重點

  1. 只支援 NSObject 的子物件
  2. 要加入關鍵字 dynamic
class album: NSObject {
    dynamic var id:String?
    dynamic var name:String?
    dynamic var url:String?
}

class ViewController: UIViewController {

    let ab:album = album()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        ab.id = "0"
        ab.name = "album1"
        ab.url = "http://www.apple.com.tw"
        
        ab.addObserver(self, forKeyPath: "url", options: [.old,.new], context: nil)
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        
        ab.removeObserver(self, forKeyPath: "url")
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func Button_Click(_ sender: Any) {
        ab.url = "http://www.google.com.tw"
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "url"{
            var newStr:String = "", oldStr:String = ""
            
            if let dict = change?[.newKey] {
                newStr = dict as! String
            }
            if let dict = change?[.oldKey]{
                oldStr = dict as! String
            }
            print("old value = \(oldStr) -> new value = \(newStr)")
        }
    }
}