升级到xcode4.2后，无法建立windowbased application了，只能建empty，建了这个之后里面没有mainWindow.xib，只能自己手动添加，霍比麻烦。请参考：http://www.trappers.tk/site/2011/06/16/mainwindow-xib/

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;
}
—- Finish —–