title: Animating auto layout constraint change on UICollectionView
body: <p>Following on from a <a href="https://stackoverflow.com/questions/21014087/uiview-animated-changes-with-auto-layout-constraints">previous SO question</a> which I asked, I am attempting to animate the change in height of a <code>UICollectionView</code> (which is <code>yellowBox</code> in the example). This change in height is being triggered by modifying the auto layout constraints on the collection view with the following code:</p>
  
  <pre><code>- (IBAction)expandYellowBox:(id)sender {
  
  [UIView animateWithDuration:0.5
                   animations:^{
                       self.yellowBoxHeightConstraint.constant += 50;
                       [self.view layoutIfNeeded];
                   }];
  }
  </code></pre>
  
  <p>However, when I call <code>[self.view layoutIfNeeded]</code> this results in the collection view being reloaded, so it flashes visibly to the user. However, I don't want the collection view to reload but instead just to animate its height change. Is there any way to either avoid <code>-layoutIfNeeded</code> reloading the collection view, or an alternative way to animate the constraint height change which doesn't call a method which has the side-effect of reloading the collection view?</p>