Iphone 如何将按钮添加到UINavigationController

我有以下代码，其中向模式视图添加了导航控制器。导航栏和视图都显示正常，但右侧按钮不正常。我做错了什么

    UpgradesViewController* upgradesViewController = [[UpgradesViewController alloc] initWithNibName:@"UpgradesView" bundle:nil];
    upgradesViewController.title = @"Upgrades";

    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:upgradesViewController];
    navController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
    navController.navigationBar.barStyle = UIBarStyleBlack;
    UIBarButtonItem* doneButton = [[UIBarButtonItem alloc] initWithTitle:@"Show" style:UIBarButtonItemStylePlain target: self  action:nil];
    navController.navigationItem.rightBarButtonItem = doneButton;
    [self presentModalViewController:navController animated:YES];

    [navController release];
    [upgradesViewController release];

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将doneButton添加到upgradesViewController的navigationItem，而不是navController。导航控制器显示的是顶部控制器的导航项，而不是它本身。

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