title: Are NSLayoutConstraints animatable?
body: <p>I am trying to animate up some views so that they are blocked by the giant keyboard in landscape.  It works well if I simply animate the frames, but others have suggested that this is counter-productive and I should be updating the NSLayoutConstraints instead.  However, they don't seem to be animatable.  Has anyone gotten them to work with success?</p>
  
  <pre><code>//heightFromTop is an NSLayoutConstraint referenced from IB
  [UIView animateWithDuration:0.25 animations:^{
      self.heightFromTop.constant= 550.f;
  }];
  </code></pre>
  
  <p>The result is an instant jump to the height in question.</p>