self.navigationController为null

Question

在我的viewcontroller中，我有一个按钮，当按下按钮时，输入navigationController，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];   
}

但我可以看到self.navigationController为null，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

仅当视图控制器位于导航控制器的导航堆栈中时，视图控制器的 navigationController 属性才会返回有效的导航控制器对象。可以通过以下方式将视图控制器添加到导航堆栈。

1. 使用 UINavigationController

的 initWithRootViewController：方法，使视图控制器成为导航控制器的rootViewController

2. 使用 UINavigationController 的 pushViewController：方法推送视图控制器。

确保以上述任何方式将视图控制器添加到导航堆栈中。

---

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
}

Answer 2

此代码将产生您正在寻找的解决方案：

-(IBAction)ShangHaiButtonPressed:(id)sender {
     UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:self];
     [self.view removeFromSuperview];
     [appDelegate.window addSubview:nav.view];   // appDelegate is the delegate of your Application
     marketViewController = [[MarketViewController alloc] initWithNibName:@"MarketViewController" bundle:nil];
     [nav pushViewController:marketViewController animated:YES];
     [marketViewController release];    
}

Answer 3

在AppDelegate.m文件中创建第一个用于导航的RootView视图：

-(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
    myView *Obj=[[myView alloc]initWithNibName:@"myView" bundle:nil];
    UINavigationController *nav = [[UINavigationController alloc]initWithRootViewController:Obj];
    nav.navigationBar.barStyle = UIBarStyleBlackOpaque;
    [window addSubview:nav.view];
    [self.window makeKeyAndVisible];
    return YES;
}

在myView.m文件中添加以下代码以从myView导航到myNewView：

-(void) registerMethod {
    myNewView *obj = [[myView alloc] initWithNibName:@"myNewView" bundle:nil];
    [self.navigationController pushViewController:obj animated:YES];

    [obj release];
}