title: When do I need to call setNeedsDisplay in iOS?
body: <p>When creating an iOS app, I'm confused as to when exactly I need to call <code>setNeedsDisplay</code>?  I know that it has something to do with updating/redrawing the UI; however, do I need to call this every time I change any of my views?</p>
  
  <p>For example, do I need to call it:</p>
  
  <ul>
  <li>After programatically changing the text in a text field</li>
  <li>When changing the background of a view?</li>
  <li>When I make changes in viewDidLoad? </li>
  <li>How about in viewDidAppear?</li>
  </ul>
  
  <p>Could someone give me some general guidelines regarding when to use this method?</p>