Basic error in iPad application launch - ios

Question

I have a file named MMAppDelegate.m:
#import "MMAppDelegate.h"
#implementation MMAppDelegate
#synthesize window;
#synthesize viewController;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
viewController = [[MainViewController alloc] init];
[window addSubview:viewController.view];
// Override point for customization after application launch.
[window makeKeyAndVisible];
return YES;
}
//and more standard methods
MMAppDelegate.h:
#import <UIKit/UIKit.h>
#import "MainViewController.h"
#interface MMAppDelegate : UIResponder <UIApplicationDelegate> {
UIWindow *window;
MainViewController *viewController;
}
#property (nonatomic, retain) UIWindow *window;
#property (nonatomic, retain) MainViewController *viewController;
#end
MainViewController.m:
#import "MainViewController.h"
#implementation MainViewController
- (void)viewDidLoad {
NSLog(#"view did load main view controller");
[super viewDidLoad];
self.view.backgroundColor = [UIColor redColor];
}
#end
And yet when I run the program, I get a black screen, and the output:
2012-02-12 15:44:48.160 MoreMost[44076:f803] view did load main view controller
2012-02-12 15:44:48.163 MoreMost[44076:f803] Applications are expected to have a root view controller at the end of application launch
What is the problem?

Answer 1

Change this line:
[window addSubview:viewController.view];
to this:
window.rootViewController = viewController;
And you'll need to create the window, like this:
window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];

Answer 2

You don't need/want to add the view controller's view as a subview. You want to make your controller the root view controller and iOS will take care of the rest:
window.rootViewController = viewController;
That is, as of iOS 4. (Not sure the answer if you're going back earlier than that.)
You also need to create the window before you use it. Something like
window = [[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];
Not sure if you aren't or if you just didn't show it ...