title: How to subClass a view which created from an Xib file
body: <p>I have a viewA is loaded from Xib.
  My question is that how to subClass this viewA.</p>
  
  <p>Every time when the my SubClass inited, and call <code>[super init]</code>, it will return <code>[viewA class]</code>. </p>
  
  <p>Similar situation happened in Storyboard, when I want to subClass a viewController which loaded from Storyboard.</p>