title: why my iOS11 navigationController doesn't to work
body: <p>I only want to show a <strong>large title</strong>.<br>
  My <code>ViewController.swift</code> look like:</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          self.view.backgroundColor = UIColor.white
  
          title = "Hello world"
          self.navigationController?.navigationBar.prefersLargeTitles = true
      }
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
      }
  
  }
  </code></pre>
  
  <p>But it doesn't work.</p>