public static void AddObserver(this NSObject self, NSObject observer, string keypath)
{
self.AddObserver(observer, (NSString)keypath, NSKeyValueObservingOptions.New, IntPtr.Zero);
}
public static IObservable <NSObservedChange> ObserveKey(this NSObject obj, string key) => Observable.Create <NSObservedChange>(obs => obj.AddObserver(key, NSKeyValueObservingOptions.New, obs.OnNext));
public static void AddObserver(this NSObject obj, string key, Action action)
{
obj.AddObserver(helper, (NSString)key, NSKeyValueObservingOptions.New, IntPtr.Zero);
helper.Add(obj, key, action);
}