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推送我的视图控制器，但在完成一个NSLog语句以找出为什么没有显示以下代码时，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); 

}

为什么会这样？除了（null）之外，我该怎么做才能返回正确的值？

Answer 1

就像每个人都在此和 Why is my new view controller not appearing? 中解释的那样，您需要先拥有导航控制器。使用该导航控制器推送视图控制器，然后视图控制器将不再为navigationController属性设置为空。

在此示例中，someSecondViewController将是您上面代码中的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];
}

请查看以下内容：

View Controller Programming Guide for iOS
UINavigationController Docs
UIViewController Docs

Answer 2

我假设你的意思是返回nil，这意味着viewController没有导航控制器，你可以通过让你的viewController成为新创建的UINavigationController的根来解决它：

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

然后对于myRootViewController，self.navigationController将返回导航控制器。

我假设viewController是你应用中的第一个，所以你想在你的appDelegate上有这个：

- (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，如果IBAction在MyViewController上，则self.navigationController将返回导航控制器而不是nil。

希望有所帮助。

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

2 个答案: