ios - iPad (iOS 5.0/5.1) 上的 UIModalPresentationFullScreen 很烂，为什么？

Question

//在当前VC内部的一个按钮上单击此代码应该执行

 AViewController *Controller = [[AViewController alloc] initWithWithString:@"Hello"];  
    Controller.delegate = self;

  UINavigationController *navController = [[UINavigationController alloc]   initWithRootViewController:Controller];
    navController.navigationBarHidden = YES;

//在 iPAD 上，iOS 6.0 或更高版本一切正常，但在 iOS 5.0/5.1 上出现空白白屏，上面什么都没有。如果我使用 UIModalPresentationCurrentContext 而不是一切正常但 AViewController 上的手势事件被传递给我不想要的父控制器。

navController.modalPresentationStyle = UIModalPresentationFullScreen;//why this is not working

    navController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
    [self presentViewController:navController animated:YES completion:nil];
    [Controller release];
    [navController release];

// AViewController 自定义初始化

- (id) initWithString:(NSString*) string
{
  self = [self initWithNibName:@"AViewController" bundle:nil];
  if(self)
  {
    self.text = string;
  }
  return self;
}

-(id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
   self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
   if (self) {
      // Custom initialization

   }
   return self;
 }