DeallocNotifier.swift

final class DeallocNotifier {
    private var _observers: [() -> Void] = []

    func observe(_ observer: @escaping () -> Void) {
        _observers.append(observer)
    }

    deinit {
        _observers.forEach { observer in
            observer()
        }
    }
}

private var deallocNotifierContext: UInt8 = 0

extension NSObject {
    var deallocNotifier: DeallocNotifier {
        /// This is a technique to use getter as if this object has additional property
        if let notifier = objc_getAssociatedObject(self, &deallocNotifierContext) as? DeallocNotifier {
            return notifier
        }
        let notifier = DeallocNotifier()
        objc_setAssociatedObject(self, &deallocNotifierContext, notifier, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        return notifier
    }
}