presentModalViewController and UINavigationController

今天遇到一个问题，直到现在还是有点不明白，看不懂，记录下来

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];    [NSThread   sleepForTimeInterval:.1];                //设置启动时长    //    [[Global sharedGlobal] setupAtLaunching];    [self.window addSubview:[Global sharedGlobal].mainViewController.view];        //注意这里添加的视图        //  presentModalViewController    UILoginViewController* loginViewController=[[UILoginViewController alloc] initFromNib];          [[Global sharedGlobal].mainViewController presentModalViewController:loginViewController                                                                 animated:NO];        navigationController = [[UINavigationController alloc] initWithRootViewController:loginViewController];    navigationController.navigationBarHidden=YES;    [self.window addSubview:navigationController.view];        [loginViewController release];        self.window.backgroundColor = [UIColor whiteColor];    [self.window makeKeyAndVisible];    return YES;}