title: UIView animation stops when view disappears, doesn't resume when it re-appears
body: <p>Here's my config:</p>
  
  <p>I have a storyboard with a ListViewController.  ListViewController has a subview UIView called EmptyListView.  EmptyListView is shown only when there are no items in the UITableView, otherwise it is hidden.</p>
  
  <p>EmptyListView has a subview UIImageView called emptyListViewArrow which visually points towards the button to create a new entry in my UITableView.  This arrow is animated infinitely in an up &amp; down fashion.</p>
  
  <p>I listen for EmptyListView to send a notification when it has finished laying out it's subviews.  I do this because if not, animations that mutate constraints behave incorrectly. </p>
  
  <pre><code>- (void) layoutSubviews {
      [super layoutSubviews];
  
      [[NSNotificationCenter defaultCenter] postNotificationName:@"EmptyViewDidLayoutSubviews" object:nil];
  }
  </code></pre>
  
  <p>When ListViewController observes this notification, it begins the animation:</p>
  
  <pre><code>[UIView animateWithDuration:0.8f delay:0.0f options:(UIViewAnimationOptionRepeat | UIViewAnimationOptionAutoreverse) animations:^{
      self.emptyListViewArrowVerticalSpace.constant = 15.0f;
      [emptyListView layoutIfNeeded];
  } completion:nil];
  </code></pre>
  
  <p>If I put the app in the background or push another ViewController on the stack, once I come back to the app or the ListViewController, the animation is stopped/paused.  I can't get it to start again.</p>
  
  <p>I've tried:</p>
  
  <ol>
  <li>Listening for application will resign/become active.  When resigning, I did [self.view.layer removeAllAnimations] and then tried to start the animation again using the code above.</li>
  <li>Removing the UIImageView being animated and adding it back to the parent view, and then tried to start the animation.</li>
  </ol>
  
  <p>I'm regretting using constraints here, but would love any insight into what could be the problem.</p>
  
  <p>Thank you!</p>