如果我的navigationController返回“（null）”，我该怎么办？

Question

可能重复：
Why can't I push a new view controller onto the current view?
Why is my new view controller not appearing?

我需要从我的navigationController推我的视图控制器，但毕竟已经做了的NSLog语句来找出为什么什么也没有用出下面的代码，我意识到它返回null：

-(IBAction)doChangePasscode{ 

NSLog(@"Change Passcode Screen Loaded!"); 

ChangePasscode *cpscreen = [[ChangePasscode alloc] initWithNibName:@"ChangePasscode" bundle:[NSBundle mainBundle]]; 
[self.navigationController pushViewController:cpscreen animated:YES]; 

NSLog(@"%@",self.navigationController); 

} 

这是怎么发生的？我能做些什么来返回除（null）之外的适当值？

UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:myRootViewController]; 

然后：

Answer 1

像大家已经解释过的和Why is my new view controller not appearing?，你需要先拥有一个导航控制器。使用该导航控制器推动您的视图控制器，然后您的视图控制器将不再为navigationController属性为null。

在这个例子中someSecondViewController将在上述代码中self：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { 
    RootViewController *rootViewController = [[RootViewController alloc] initWithNibName:@"RootViewController" bundle:nil]; 
    self.nav = [[[UINavigationController alloc] initWithRootViewController:rootViewController] autorelease]; 
    [rootViewController release]; 
    [self.window addSubview:nav.view]; 
    [self.window makeKeyAndVisible]; 
} 

- (void)someOtherMethod { 
    SecondViewController *someSecondViewController = [[SecondViewController alloc] initWithNibName:@"SecondViewController" bundle:nil]; 
    [self.nav pushViewController:someSecondViewController animated:YES]; 
    [someSecondViewController release]; 
} 

请仔细阅读这些：

View Controller Programming Guide for iOS
UINavigationController Docs
UIViewController Docs

Answer 2

我假设你要回到零，这意味着的viewController没有一个导航控制器，你可以有你的viewController是一个新创建的UINavigationController像这样从根本上解决这对于myRootViewController，self.navigationController将返回导航控制器。

我假设的viewController是在你的应用程序中的第一个，所以你想拥有这对你的appDelegate：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{ 
    MyViewController *viewController = [[[MyViewController alloc] initWithNibName:@"MyViewController" bundle:nil] autorelease]; 
    UINavigationController *myNavController = [[[UINavigationController alloc] initWithRootViewController:viewController] autorelease]; 

    self.window.rootViewController = myNavController; 
    [self.window makeKeyAndVisible]; 

    return YES; 
} 

然后若该IBAction为是MyViewController self.navigationController将返回导航控制器，而不是零。

希望有所帮助。