title: How to find out if a UITableViewCell has scrolled away and is invisible
body: <p>I have a <code>UITableViewCell</code> that observes (via <code>KVO</code>) some properties. This only makes sense if the cell is visible. I want to remove this observer when the cell scrolls away, and before <code>prepareForReuse</code> is called. </p>
  
  <p>How can I do this?</p>