Iphone 尝试将视图添加到navigationcontroller时是否重复动画？

我在一个应用程序中添加了这样一个视图，因为我需要它位于导航栏上方`

 AppDelegate *appDelegate = (AppDelegate  *)[[UIApplication sharedApplication] delegate];

    if(appDelegate.viewcontroller==nil)
    {
        appDelegate.viewcontroller = [[ViewController alloc] init];
        CATransition *animation = [CATransition animation];
        [animation setType:kCATransitionPush];
        [animation setSubtype:kCATransitionFromTop];
        [animation setDuration:0.5f];
        [animation setTimingFunction:[CAMediaTimingFunction functionWithName:
                                      kCAMediaTimingFunctionEaseInEaseOut]];
        [appDelegate.viewcontroller.view.layer addAnimation:animation forKey:@"someTransition"];
        [self.navigationController.view addSubview:appDelegate.viewcontroller.view];
        [appDelegate.viewcontroller viewWillAppear:YES];  
   }
    else
    {       
          CATransition *animation = [CATransition animation];
        [animation setType:kCATransitionPush];
        [animation setSubtype:kCATransitionFromTop];
        [animation setDuration:0.5f];
        [animation setTimingFunction:[CAMediaTimingFunction functionWithName:
                                      kCAMediaTimingFunctionEaseInEaseOut]];
        [appDelegate.viewcontroller.view.layer addAnimation:animation forKey:@"someTransition"];        
        [self.navigationController.view addSubview:appDelegate.viewcontroller.view];
        [appDelegate.viewcontroller viewWillAppear:YES];        

    }

它被移除了

self.view removefromsuperview

当appDelegate.viewcontroller为nil时，一切正常。但当它不为null时，当按下按钮时，它首先显示视图控制器，然后再次显示视图控制器，然后再次显示视图控制器。它首先没有动画，然后是动画。为什么会这样，有人能帮我吗