Swift4.2 foreground、background オブザーバー

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        NotificationCenter.default.addObserver( self,
 selector:#selector(background),
name: UIApplication.didEnterBackgroundNotification,object: nil)
        NotificationCenter.default.addObserver( self, selector: #selector(foreground),name: UIApplication.willEnterForegroundNotification,object: nil)
    }
    @objc func background() { print("background") }
    @objc func foreground() { print("foreground") }
}

参考箇所　スーパークラス