title: How do I check if an UIViewController is currently being displayed?
body: <p>How do I check if an <code>UIViewController</code> is currently being displayed?</p>
  
  <p>My <code>UIViewControllers</code> are listening for <code>NSNotifications</code> - even if when they are not displayed (ie not shown). So I could have 10 <code>UIViewController</code> in the background observing <code>NSNotifications</code> from <code>NSNotificationCenter</code>. When an <code>NSNotification</code> is posted and received by the <code>UIViewController</code>, I'd like to find out if it is currently being shown. If it is not, I will just set a boolean so that it will processed when the View is presented. If it currently being display, I will do more things like update tables immediately, and so forth...</p>