gistfile1.swift

protocol TrackProvider {
    var track: Track? { get }
    func observeTrack(_ block: (TrackProvider, NSKeyValueObservedChange<Track>)) -> NSKeyValueObservation
}

extension TrackProvider where Self: NSObject {
    func observeTrack(_ block: (TrackProvider, NSKeyValueObservedChange<Track?>)) -> NSKeyValueObservation {
        return self.observe(\TrackProvider.track) { obj, change in
            
        }
    }
}