Create an Empty Application step in XCode 4 (create and associate the xib view file)

1. Create new project in Xcode 4.2 beta and choose “Empty Application” template 2. Add “New File” to the project by choosing “iOS -> User Interface -> Empty” 3. Choose “iPhone” and name “MainWindow” ( .xib will be added automatically)
4. Open “MainWindow.xib” in project navigator 5. Click “File’s Owner” in “Placeholders” Change Class as “UIApplication” in “Identity Inspector” 6. Drag “Object” in the library into “Objects” panel on the left 7. Change the class of the Object as your delegate class, such as “DemoAppDelegate” (The name of object will be automatically change like “Demo App Delegate”) 8. Drag “Window” in the library into “Objects” panel on the left
9. Open “DemoAppDelegate.h” in project navigator 10. Find below code

@interface DemoAppDelegate :
UIResponder
@property (strong, nonatomic)UIWindow *window;
@end

11. Change as below:

@interface DemoAppDelegate :
UIResponder
@property (strong, nonatomic) IBOutlet UIWindow *window;
@end

11. Open “MainWindow.xib” in project navigator 12. Click “File’s Owner” in “Placeholders” 13. Control-Drag “delegate” of “Outlets” in “Connections Inspector” to “Demo App Delegate” in “Objects” 14 . Click “Demo App Delgate” in “Objects” 15. Control-Drage “window” of “Outlets” in “Connections Inspector” to “Window” in “Objects”
16. Open “Summary” of your project 17. Find “iPhone/iPod Deployment Info” 18. Change “Main Interface” as “MainWindow”
19. Open “DemoAppDelegate.m” in project navigator 20. Find below code

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
// Override point for customization after application launch.
self.window.backgroundColor = [UIColor whiteColor];

[self.window makeKeyAndVisible];
return YES;
}

21. Change as below

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
//self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
// Override point for customization after application launch.
//self.window.backgroundColor = [UIColor whiteColor];

[self.window makeKeyAndVisible];
return YES;
}

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Repost: http://chrislee.kr/wp/2011/07/24/mainwindow-xib-absence-in-xcode-4-2-beta-4-with-ios-5-sdk/