public static void NotifyWhenDeallocated(NSObject obj, Action deallocated)
{
// Add an associated object which will be 'released'd when the obj
// to watch is deallocated. When 'release' is sent, then invoke
// the 'deallocated' callback.
var notifier = new ReleaseNotifier(deallocated);
obj.SetAssociatedObject(IntPtr.Zero, notifier, AssociatedObjects.AssociationPolicy.Retain);
notifier.Dispose(); // remove any managed references.
notifier.enabled = true; // notify on the next 'release' message.
}