title: need advise about NSClassFromString
body: <p>according to this code
  i dont understand about NSClassFromString</p>
  
  <p>how did i know "viewControllerName"
  can i use it from another source ?</p>
  
  <p>could you please tell me more information how to use NSClassFromString?</p>
  
  <pre><code>UIViewController *viewController = [[NSClassFromString(viewControllerName) alloc] init];
  </code></pre>