title: Determine if UIView is visible to the user?
body: <p>is it possible to determine whether my <code>UIView</code> is visible to the user or not?</p>
  
  <p>My View is added as <code>subview</code> several times into a <code>Tab Bar Controller</code>.</p>
  
  <p>Each instance of this view has a <code>NSTimer</code> that updates the view.</p>
  
  <p>However I don't want to update a view which is not visible to the user.</p>
  
  <p>Is this possible?</p>
  
  <p>Thanks</p>