title: Autolayout invalid related to hidden navigationbar in xib
body: <p>I am working on an problem related to navigation bar in xib file. The problem is as follows:</p>
  
  <p>I add a xib file, and manually put a <code>UINavigationBar</code> on top of it, and use constraints to make it stays on top of the screen. Then I add a <code>UILabel</code> below it, and also use constraints to make it just below the navigation bar. That is, <code>let label.top</code> equals <code>navigationbar.bottom</code>. Then I need to add a fullscreen function. I add a button and after pressing it, I hide the navigation bar by code like this:</p>
  
  <pre><code>self.navbar.hidden = YES;
  </code></pre>
  
  <p>The navbar is hidden, but the label is somehow still keeping its position, and doesn't move to the top of the screen, which is what I expected to see. </p>
  
  <p>So the question is:</p>
  
  <ol>
  <li><p>Is this effect normal? Because now the navbar is hidden, the constraints should make the label move to top.</p></li>
  <li><p>What should I do to achieve the effect I want if it's normal/abnormal.</p></li>
  </ol>
  
  <p>Thank you guys! Any idea can help!</p>
  
  <p>And pardon me for my English, for it's not my native language.</p>