Detect when, iframe(AVPlayer) of video player inside wkwebkit

When I am open any website with movie inside WKWebKit and press on players this movie will be open inside some player, where I can pause, remove etc. My question How I can detect when this iframe(window or player, I don't know how it is named) is open or closed and do something in background if window open or closed.

For clarity I am attached screenshot of simulator where this player was opened if I am press on player with movie on website.

I am solved this problem using NotificationCenter using UIWindowDidBecomeVisibleNotification & UIWindowDidBecomeHiddenNotification.

My code:

override func viewDidLoad() {
        super.viewDidLoad()

// listen for videos playing in fullscreen
        NotificationCenter.default.addObserver(self, selector: #selector(onDidEnterFullscreen(_:)), name: UIWindow.didBecomeVisibleNotification, object: view.window)

        // listen for videos stopping to play in fullscreen
    NotificationCenter.default.addObserver(self, selector: #selector(onDidLeaveFullscreen(_:)), name: UIWindow.didBecomeHiddenNotification, object: view.window)
}

@objc func onDidEnterFullscreen(_ notification: Notification) {
    print("Enter Fullscreen")
}

@objc func onDidLeaveFullscreen(_ notification: Notification) {
    print("Leave Fullscreen")
}