iphone - self.navigationController 为空

Question

在我的 View Controller 中，我有一个按钮，当按下按钮时，进入导航 Controller ，我的代码如下:

-(IBAction)ShangHaiButtonPressed:(id)sender{
    marketviewcontroller = [[MarketViewController alloc]initWithNibName:@"MarketViewController" bundle:nil];
    NSLog(@"%@",self.navigationController);
    [self.navigationController pushViewController:marketviewcontroller animated:YES];
    [marketviewcontroller release];   
}

但是我可以看到 self.navigationController 为空，如何解决这个问题？谢谢。

更新:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    _switchviewcontroller = [[SwitchViewController alloc]initWithNibName:@"SwitchViewController" bundle:nil];
    [self.window addSubview:_switchviewcontroller.view];
    [self.window makeKeyAndVisible];
    return YES;
}

Answer 1

仅当 View Controller 位于导航 Controller 的导航堆栈中时， View Controller 的 navigationController 属性才会返回有效的导航 Controller 对象。可以通过以下方式将 View Controller 添加到导航堆栈中。

1. 通过使用 UINavigationController 的 initWithRootViewController: 方法使 View Controller 成为导航 Controller 的 rootViewController

2. 通过使用 UINavigationController 的 pushViewController: 方法推送 View Controller 。

确保您的 View Controller 已通过上述任何一种方式添加到导航堆栈中。

<小时/>

编辑:(在 didFinishLaunchingWithOptions: 代码添加到问题之后):

将 didFinishLaunchingWithOptions: 方法更改为此，

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    _switchviewcontroller = [[SwitchViewController alloc]initWithNibName:@"SwitchViewController" bundle:nil];
    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:_switchviewcontroller];
    [self.window addSubview:navController.view];
    [navController release];
    [self.window makeKeyAndVisible];
    return YES;
}

Swift 4(版本):

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey : Any]? = nil) -> Bool {
    switchviewcontroller = SwitchViewController(nibName: "SwitchViewController", bundle: nil)
    let navController = UINavigationController(rootViewController: switchviewcontroller)
    window.addSubview(navController.view)
    window.makeKeyAndVisible()
    return true
}