title: Can't display UIActivityIndicator on top of mapview
body: <p>In my code, I initialize a UIActivityIndicator, and add it as a subview, but it never appears. What's going wrong?</p>
  
  <pre><code>-(void) update {
  
      UIActivityIndicatorView *activityView=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
      activityView.center=self.view.center;
      [self.view addSubview:activityView];
      [activityView startAnimating];
  
      /** Stuff to  access to server, this stuff works correctly.*/
      [activityView stopAnimating];
  }
  </code></pre>