objective-c - 如何在Appdelegate.m的 active方法中modal出一个UIAlertController

//初始化UIAlertController

UIAlertController *alertCtl = [UIAlertController alertControllerWithTitle:@"提示" message:@"AppDelegate中" preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction *alertAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleDefault handler:nil];
[alertCtl addAction:alertAction];
//初始化UIWindows
UIWindow *aW = [[UIWindow alloc]initWithFrame:[UIScreen mainScreen].bounds];
aW.rootViewController = [[UIViewController alloc]init];
aW.windowLevel = UIWindowLevelAlert + 1;
[aW makeKeyAndVisible];
[aW.rootViewController presentViewController:alertCtl animated:YES completion:nil];