observe scroll view's 'dragging' property but received no notification

I have a custom UIView that is added to as a subView of a UIScrollView, and I want that UIView automatically repositioned whenever the UIScrollView scrolls. In addition to observe the UIScrollView's contentOffset property(which works fine), I also need observes its dragging property:

[scrollView addObserver:self 
             forKeyPath:@"dragging" 
                options:NSKeyValueObservingOptionNew 
                context:NULL];

but in the observeValueForKeyPath:ofObject:change:context method, I didn't receive any NSNotification for the change of the dragging property, what's the problem here? Thanks.

updated

Since i'm gonna reuse this custom UIView in multiple UIViewControllers, I want the custom view itself handles the repositioning logic, rather than duplicate the repositioning logic in each UIViewController's UIScrollViewDelegate methods(like scrollViewDidScroll thing).