title: A background view remains visible when you try to hide SearchBar through the alpha
body: <p>I'm having trouble hiding SearchBar on NavigationItem.(Xcode 9 | Swift 4)
  ex: <a href="https://www.dropbox.com/s/6is5mukpk1yew8e/bgSearch.gif?dl=0" rel="nofollow noreferrer">https://www.dropbox.com/s/6is5mukpk1yew8e/bgSearch.gif?dl=0</a>
  Using this code:</p>
  
  <pre><code>func scrollViewDidScroll (_ scrollView: UIScrollView) {  
      let transp: Float = Float (1 - (mTableView.contentOffset.y / (mTableView.contentSize.height - mTableView.frame.size.height) * 60))  
      self.navigationController! .navigationBar.alpha = CGFloat (transp)  
      self.mSearchController.searchBar.alpha = CGFloat (transp)  
      titleMsgView.alpha = CGFloat (transp)  
  }
  </code></pre>
  
  <p>Everything works fine, except for a background that I could not access to hide with the alpha.
  I'm using:</p>
  
  <pre><code>viewDidLoad() {  
      mSearchController = UISearchController (searchResultsController: nil)  
      mSearchController.delegate = self as? UISearchControllerDelegate  
      mSearchController.searchResultsUpdater = self        
      navigationItem.searchController = mSearchController  
  } 
  </code></pre>
  
  <p>As the SerchBar search field disappears correctly, you will understand that it is something with the NavigationItem to which it has been added. Could someone help me solve the problem?</p>