In Xcode 4.2 the MainWindow.xib is not included for some of the project templates. Its means now we have to generate GUI elements by code or we can reconstruct the MainWindow.xib in project. I am describing you the second option here.

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In Xcode 4.2 the MainWindow.xib is not included for some of the project templates. Its means now we have to generate GUI elements by code or we can reconstruct the MainWindow.xib in project. I am describing you the second option here.

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iOS: (Tips) Creating MainWindow.xib in Xcode 4.2
There is always a new thing to discover when it comes to updating one's Xcode. So today I decided to create a new project based on Tab Bar Navigation to fool around and to my amazement, where is the ever important MainWindow.xib?
So I gave a read on Google and found out that MainWindow is not generated on the templates and instead the view controllers and its related component are generated programmatically in the AppDelegate.
Luckily, I found this wonderful blogpost!
Recreating MainWindow in project =========================
http://www.makebetterthings.com/iphone/where-is-mainwindow-xib-in-xcode-4-2/
Tested and worked like a charm, but as the above walkthrough makes use of an empty application template, do not forget to add the newly added view controller in the MainWindow as root at the didFinishLaunchingWithOptions.
self.window.rootViewController = self.[your new view controller here];