KVObserver.swift

import Foundation

class KVObserver: NSObject {
    let subject: NSObject
    let keyPath: String
    let block: (Any?) -> Void
    
    init(subject: NSObject, keyPath: String, block: @escaping (Any?) -> Void) {
        self.subject = subject
        self.keyPath = keyPath
        self.block = block
        
        super.init()
        
        subject.addObserver(self, forKeyPath: keyPath, options: [], context: nil)
    }
    
    deinit {
        subject.removeObserver(self, forKeyPath: self.keyPath)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard keyPath == self.keyPath else { return }
        
        self.block(object)
    }
}

TrackProvider.swift

import Foundation

protocol TrackProvider {
    var track: Track? { get }
    
    func observeTrack(_ block: @escaping (TrackProvider) -> Void) -> KVObserver
}

extension TrackProvider where Self: NSObject {
    func observeTrack(_ block: @escaping (TrackProvider) -> Void) -> KVObserver {
        return KVObserver(subject: self, keyPath: "track", block: { obj in
            guard let provider = obj as? TrackProvider else { return }
            block(provider)
        })
    }
}