Xcode 4 has a “development” setting inside the file inspector in the right sidebar. This installs the earliest version of Xcode that can open the file. This setting in Xcode 5 will provide backward compatibility.
Since Xcode developer previews are under the NDA, I cannot answer this part of the question. I do not know if such an installation exists or whether it will work if it exists .; -)
Using both the stable and beta versions of Xcode in parallel usually works quite well.
If you are expecting a stable version of Xcode 5 and the SDK, there is a high probability that your application will not be ready for iOS7 when iOS7 hits the shelves.
From what I can imagine from viewing publicly available information, it seems like I have to work hard for the iOS6 application to look in iOS7. Not only on the code side, but if your application needs to be fully integrated into iOS7, you may need to rethink the concept of your application.
Keep in mind that if you want to publish before the SDK is publicly available, you must use the Xcode 4 and iOS 6 SDKs to build your application. If you use the iOS 7 code, your application will not compile.
I would start testing and adaptation as early as possible. But if your “fast release” means within the next one or two months, I would forget about adding the code to support iOS 7 so far and complete the release.
Btw: A place where you can freely discuss technology covered by apples. NDA is a beta area in the Apple developer forums .
Matthias bauch
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