Applications are expected to have a root view controller at the end of application launch

环境：XCode4.2

场景：这种问题多发生在XCode4.2 移植低版本项目时出现。

原因：在iOS5下，应用加载时，需要一个root view controller，在iOS5以下的版本会有MainWindow作为启动文件，iOS5以上没有了。

解决方案：手动创建一个root view controller，在application:didFinishLaunchingWithOptions中添加如下方法，同时为了避免新增加的view对已有的程序产生影响，把ViewController.xib文件的Alpha值设置为0，即完全透明。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // ===========add code===========
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    //self.viewController = [[ViewController alloc] init];
    self.window.rootViewController = self.viewController;
    // ===========add code============

    [self showTabBarController:1];
    [self.window makeKeyAndVisible];
    
	return YES;
}

参考资料：http://stackoverflow.com/questions/7520971/applications-are-expected-to-have-a-root-view-controller-at-the-end-of-applicati

http://alpascual.com/post/2011/10/09/Applications-are-expected-to-have-a-root-view-controller-at-the-end-of-application-launch.aspx