title: How to add UIActivityIndicatorView at center of UIAlertView?
body: <p>As the title, I want to add an UIActivityIndicatorView instance at center of an UIAlertView instance. Here is my code:</p>
  
  <pre><code>    alertView  = [[UIAlertView alloc]initWithTitle:@"Processing" message:@"" delegate:nil cancelButtonTitle:nil otherButtonTitles:nil, nil];
  
      CGRect screenRect = [[UIScreen mainScreen] bounds];
  indicator = [[UIActivityIndicatorView alloc]initWithFrame:CGRectMake(screenRect.size.width/2, screenRect.size.height/2,50,50)];
  [alertView addSubview: indicator];
  [indicator startAnimating];
  [alertView show];
  </code></pre>
  
  <p>What I see is just the alertView. Did I make any mistake?</p>