class CassiniViewController: UIViewController, UISplitViewControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    //22

    override func awakeFromNib() {
        super.awakeFromNib()
        self.splitViewController?.delegate = self
    }
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destinationViewController.
        // Pass the selected object to the new view controller.
        //17
        if let url = DemoURL.NASA[segue.identifier ?? "" ] {
            //18.
            if let imageVC = (segue.destination.contents as? ImageViewController) {
                imageVC.imageURL = url
                imageVC.title = (sender as? UIButton)?.currentTitle
            }
        }
    }

    //23
    func splitViewController(_ splitViewController: UISplitViewController, collapseSecondary secondaryViewController: UIViewController, onto primaryViewController: UIViewController) -> Bool {
        if primaryViewController.contents == self {
            if let ivc = secondaryViewController.contents as? ImageViewController, ivc.imageURL == nil {
                return true
            }
        }
        return false
    }

}

//19.
extension UIViewController {
    var contents: UIViewController {
        if let navcon = self as? UINavigationController {
            return navcon.visibleViewController ?? self
        } else {
            return self
        }
    }
}