如何根据设备类型启动特定的视图

在启动时候有三个视图，想要实现的是可以根据设备类型来选择三个中的一个。目前我写的代码在AppDelegate.m中

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // 应用启动后重写制定
    if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone && [UIScreen mainScreen].bounds.size.height == 568.0) {
        self.viewController = [[ViewController alloc] initWithNibName:@"ViewController_Portrait5" bundle:nil];
    }

    else if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone && [UIScreen mainScreen].bounds.size.height == 480.0) {
        self.viewController = [[ViewController alloc] initWithNibName:@"ViewController_Portrait4" bundle:nil];
    }

    else {
        self.viewController = [[ViewController alloc] initWithNibName:@"ViewController_PortraitPad" bundle:nil];
    }
}

但是运行之后，应用启动的时候屏幕一片漆黑。
怎么解决，谢谢。

在方法的最后添加：

self.window.rootViewController = self.viewController;
[self.window makeKeyAndVisible];

用以下代码，更简洁一些。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

    NSString *nibName;
    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {
        nibName = @"ViewController_PortraitPad";
    } else {
        if ([UIScreen mainScreen].bounds.size.height == 480.0) {
            nibName = @"ViewController_Portrait4";
        } else {
            nibName = @"ViewController_Portrait5";
        }
    }

    self.viewController = [[ViewController alloc] initWithNibName:nibName bundle:nil];
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];