语义问题：指针不兼容警告

Question

我启用了ARC，在我的didFinishLaunchingWithOptions方法中，我编写了以下代码：

AppDelegate.h：

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) ViewController *viewController;

@end

AppDelegate.m：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    ViewController * vc = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    UINavigationController * nav = [[UINavigationController alloc] initWithRootViewController:vc];
    self.viewController = nav;
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];
    return YES;
}

但声明：self.viewController = nav;收到编译警告，警告信息为：

file://.../AppDelegate.m: warning: Semantic Issue: Incompatible pointer types passing 'UINavigationController *__strong' to parameter of type 'ViewController *'

如何删除警告？

感谢。

Answer 1

我假设ViewController是UIViewController的自定义子类，它与或是UINavigationController本身的子类完全不同。这就是为什么它是错误的：超类不能完全充当它的子类（例如，它可能没有某些属性/方法等），因此警告。

Answer 2

编译器告诉你：“nav，UINavigationController的一个实例，不是'ViewController'或'ViewController'的子类”。如果您真的想要同时使用导航控制器和视图控制器，则可以添加第二个属性：

@property (nonatomic, strong) UINavigationController *navController;

然后在application:didFinishLaunchingWithOptions:

中设置它

self.viewController = vc;
self.navController = nav;

这里的另一个解决方案就是抓住导航控制器并使用'topViewController'属性来访问你的VC。

编辑：或者更好的是，不关心导航控制器。只需：

self.viewController = vc;
self.window.rootViewController = nav;

Answer 3

请尝试以下代码： -

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    UINavigationController * nav = [[UINavigationController alloc] initWithRootViewController:self.viewController];

    self.window.rootViewController = nav;
    [self.window makeKeyAndVisible];
    return YES;
}

Answer 4

你可以这样做

 self.viewController =[nav.viewControllers objectAtIndex:0];

然后它不会显示类似“不兼容的指针类型将'UINavigationController * __ strong'传递给'ViewController *'类型参数的警告