新建了一個(gè)工程使用下面獲取rootVC用來彈窗時(shí)，彈出不來，發(fā)現(xiàn)RootViewControlle是nil

UIViewController *RootViewController()
{
    id<UIApplicationDelegate> delegate= [UIApplication sharedApplication].delegate;
    if([delegate respondsToSelector:@selector(window)]){
        UIWindow *window = [(NSObject *)delegate valueForKey:@"window"];
        UIViewController *controller = window.rootViewController;
        while (controller.presentedViewController) {
            controller = controller.presentedViewController;
        }
        return controller;
    }
    return nil;
}

需要?jiǎng)h除 SceneDelegate 使用 AppDelegate

Xcode11之后新創(chuàng)建的工程會(huì)多出兩個(gè)文件SceneDelegate。那么我們?nèi)绾巫屗兓刂暗哪菢拥墓こ棠亍?/p>

一、將這兩個(gè)文件刪除。
會(huì)報(bào)錯(cuò)There is no scene delegate set. A scene delegate class must be specified to use a main storyboard file.
二、將Info.plis 的 UIApplicationSceneManifest 刪除
三、將AppDelegate.m中的UISceneSession lifecycle注釋掉或者刪掉。
四、在AppDelegate.h加入window。在didFinishLaunchingWithOptions中加入U(xiǎn)IWindow。

@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (nonatomic, strong) UIWindow *window;

@end



- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];

    self.window.rootViewController = [[ViewController alloc] init];;
    
    [self.window makeKeyAndVisible];
    return YES;
}

刪除后繼續(xù)報(bào)錯(cuò) whose view is not in the window hierarchy .我是demo里面遇到的，在viewDidLoad彈一個(gè)alertViewController，所以移到外面就好了

1. 在 一個(gè) ViewController 里面調(diào)用另外一個(gè) ViewController 是出現(xiàn)這個(gè)錯(cuò)誤：

該錯(cuò)誤一般是由于在 viewDidLoad 里面調(diào)用引起的，解決辦法是轉(zhuǎn)移到 viewDidAppear 方法里面

2. 在 AppDelegate.m 中調(diào)用遇到這個(gè)錯(cuò)誤

解決辦法1：

UIViewController *topRootViewController = [UIApplication sharedApplication].keyWindow.rootViewController;
while (topRootViewController.presentedViewController)
{
   topRootViewController = topRootViewController.presentedViewController;
}

//[topRootViewController presentViewController:yourController animated:YES completion:nil];
//or
[topRootViewController myMethod];

解決辦法2：

UIStoryboard *mainstoryboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
   LoginViewController* loginViewController = [mainstoryboard instantiateViewControllerWithIdentifier:@"LoginViewController"];
   [self.window makeKeyAndVisible];
//[LoginViewController presentViewController:yourController animated:YES completion:nil];
//or
[LoginViewController myMethod];