title: Problem with observers while engaging them with Uiapplication()
body: <pre><code>class ViewController: UIViewController {
  var check=UIApplication.didEnterBackgroundNotification{
  
  
      didSet{
  
          print("print some thing")
      }
  }
  
  override func viewDidLoad() {
      super.viewDidLoad()
  
  
   }}
  </code></pre>
  
  <p>I run this code in my iPhone, when I enter background it is not executing the printing </p>