observer.swift

import Foundation

class Singleton: NSObject {
    static let sharedInstance = Singleton()

    @objc dynamic var aProperty = false

    func updateDoesntWork() {
        aProperty = !aProperty
    }

    func updateDoesWork() {
        Singleton.sharedInstance.aProperty = !aProperty
    }
}

class Observer: NSObject {
    override init() {
        super.init()
        Singleton.sharedInstance.addObserver(self, forKeyPath: #keyPath(Singleton.aProperty), options: [.new], context: nil)
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print("observeValue(forKeyPath: \(String(describing:keyPath)), of: \(String(describing:object)), change: \(String(describing:change)), context:\(String(describing:context)))")
    }
}

let observer = Observer()
Singleton.sharedInstance.updateDoesWork()
Singleton.sharedInstance.updateDoesntWork()