Observing window key changes in Marzipan

ObserveUINSWindow.swift

import UIKit

extension NSNotification.Name {

    static let didResignKey = Self.init("UISBHSMainHostWindowDidResignKeyNotification")
    static let didBecomeKey = Self.init("UISBHSMainHostWindowDidBecomeKeyNotification")

}

extension Notification {

    func extractUIWindow() -> UIWindow? {
        let UINSWindow = object as AnyObject // Apple private `UINSWindow` type 
        let keyWindow = UINSWindow.value(forKey: "keyUIWindow") as? UIWindow
        return keyWindow
    }

}

func yourFunctionToObserveTheWindow(_ window: UIWindow) {
    let notificationCenter = NotificationCenter.default
    notificationCenter.addObserver(forName: .didResignKey, object: nil, queue: nil) { notification in
        guard let extractedWindow = notification.extractUIWindow(), extractedWindow == window else { return }
        // Update your UI now that your window is no longer key
    }

    notificationCenter.addObserver(forName: .didBecomeKey, object: nil, queue: nil) { notification in
        guard let extractedWindow = notification.extractUIWindow(), extractedWindow == window else { return }
        // Update your UI now that your window is key
    }

    // Don't forget to capture the token returned by the above calls to
    // Notification Center & remove observer on deinit, or use selectors
    // which handles removal for you.
}