title: Auto Layout constraint change does not animate
body: <p>I was learning the auto layout with animations from the tutorial</p>
  
  <p><a href="http://weblog.invasivecode.com/post/42362079291/auto-layout-and-core-animation-auto-layout-was" rel="nofollow noreferrer">http://weblog.invasivecode.com/post/42362079291/auto-layout-and-core-animation-auto-layout-was</a> </p>
  
  <p>and things were working perfect.</p>
  
  <p>When I tried to use this concept in my application, trying to animate a settings screen(a UIView) from bottom to top,it works great when the settings screen is just an empty UIView,</p>
  
  <p>But in case I add a UILabel as a subview to this settings screen, the animation just vanishes. 
  On removing this UILabel form the settings screen, the animation is visible.</p>
  
  <p>Here are the outlets that I have connected</p>
  
  <pre><code>__weak IBOutlet UIView *settingsView;
  __weak IBOutlet NSLayoutConstraint *settingsBottomConstraint;
  __weak IBOutlet NSLayoutConstraint *settingsViewHeightConstraint;
  </code></pre>
  
  <p>View did load setup method(setupViews)</p>
  
  <pre><code>-(void)setupViews
  {
      settingsBottomConstraint.constant = - settingsViewHeightConstraint.constant;
      [settingsView setNeedsUpdateConstraints];
      [settingsView layoutIfNeeded];
      isSettingsHidden = YES;
  }
  </code></pre>
  
  <p>Hide/Unhide Method</p>
  
  <pre><code>- (IBAction)showSettingsScreen:(id)sender {
  
      if (isSettingsHidden) {
  
          settingsBottomConstraint.constant = 0;
          [settingsView setNeedsUpdateConstraints];
          [UIView animateWithDuration:.3 animations:^{
              [settingsView layoutIfNeeded];
          }];
      }
      else{
  
          settingsBottomConstraint.constant = - settingsViewHeightConstraint.constant;
          [settingsView setNeedsUpdateConstraints];
          [UIView animateWithDuration:0.3 animations:^{
              [settingsView layoutIfNeeded];
          }];
  
      }
      isSettingsHidden = !isSettingsHidden;
  }
  </code></pre>
  
  <p>My issue seems similar to the 
  <a href="https://stackoverflow.com/questions/21168873/issue-with-uiview-auto-layout-animation">Issue with UIView Auto Layout Animation</a></p>