title: Cocoa - Notification on NSUserDefaults value change?
body: <p>Let's say I have a key <code>@"MyPreference"</code>, with a corresponding value stored through <code>NSUserDefaults</code>.</p>
  
  <p>Is there a way to be notified when the value is modified?</p>
  
  <p>Or could it be done through bindings? (But this case, instead of binding the value to a UI element, I wish my object to be notified of the change, so that I can perform other tasks.)</p>
  
  <p>I am aware that <code>NSUserDefaultsDidChangeNotification</code> can be observed, but this appears to be a all-or-nothing approach, and there does not appear to be a mechanism there to get at the specific key-value-pair that was modified. (Feel free to correct.)</p>