当按下viewcontroller时，navigationController为nil

Question

There are three viewController , MainViewController ViewControllerB and ViewControllerC . MainViewController will be loaded when the app launch.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    // Override point for customization after application launch.

    self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen] bounds]];

    MainViewController * main = [[MainViewController alloc]init];
    UINavigationController * navigationController = [[UINavigationController alloc]initWithRootViewController:main];
    self.window.rootViewController = navigationController;
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent];
    return YES;       
}

and there is a button on the MainViewController , present ViewControllerB ,

UIViewController *rootViewController = [[UIApplication sharedApplication].keyWindow rootViewController];
ViewControllerB * vcb=[[ViewControllerB alloc] init];
[rootViewController presentViewController:vcb animated:YES completion:nil];

After the ViewControllerB appear, click the button push ViewController C. but the navigationController is nil . It can't push ViewControllerC

[self.navigationController pushViewController:vcC animated:YES];

Answer 1

1. You have the MainViewController ( mvc ), which is embedded in a NavigationController ;

2. Then, on mvc you have the following code:

   [rootViewController presentViewController:vcb animated:YES completion:nil];

   You are calling presentViewController on the current ViewController, which will modally present the next ViewController, in this case ViewControllerB ( vcb );

3. Finally, you try to access the NavigationController inside ViewControllerB ( vcb ) in order to push ViewControllerC ( vcc ), with the following code:

   [self.navigationController pushViewController:vcC animated:YES];

The problem is that vcb is not aware of the NavigationController , since presentViewController presents the view controller modally, outside the existing navigation stack. Thus, resulting in a nil NavigationController in vcb .

You can refer to https://stackoverflow.com/a/14233252/9323816 for more information.