observer_deinit.swift

class Baz: NSObject {
    dynamic var value: String = ""
    
    override class func automaticallyNotifiesObservers(forKey key: String) -> Bool {
        if key == "value" {
            return true
        }
        return super.automaticallyNotifiesObservers(forKey: key)
    }

}

class Foo: NSObject {
    var baz: Baz? {
        willSet {
            baz?.removeObserver(self, forKeyPath: "value")
        }
        didSet {
            baz?.addObserver(self, forKeyPath: "value", options: [.new, .old], context: nil)
        }
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "value" {
            print(keyPath as Any, object as Any)
        }
    }
    
    deinit {
//        baz?.removeObserver(self, forKeyPath: "value") ☆
        baz = nil
    }
}


do {
    let f = Foo()
    let baz = Baz()
    f.baz = baz
    baz.value = "!!!"
}