title: Why the Label in the ViewController is gone after I set the translucent to the background?
body: <p>I am new to iOS swift. I try to change page from A to B ,and set the background of page-B to alpha 0.5 like the following code:</p>
  
  <p>In <strong><code>A-ViewController</code></strong>:</p>
  
  <pre><code>    @IBAction func btnClick_aitrix_robot(_ sender: Any) {
              print("btnClick_aitrix_robot")
  
  
              let filterVC = QuestionViewController()
              filterVC.modalPresentationStyle = UIModalPresentationStyle.custom
              self.present(filterVC, animated: true, completion: nil)
      }
  </code></pre>
  
  <p>In <strong><code>B-ViewController</code>:</strong> </p>
  
  <pre><code>override func viewDidLoad() {
          super.viewDidLoad()
  
          // Do any additional setup after loading the view.
  
          self.view.backgroundColor = UIColor.black.withAlphaComponent(0.5)
  }
  </code></pre>
  
  <p>There has a <strong><code>Label</code></strong> in the <strong><code>page-B</code></strong> , but it disappear after I set the translucent to the background of page-B. </p>
  
  <p>Did i missing something ? Thanks in advance.</p>