Differences between iPhone OS v2 and v4?

Can any one say difference between the iPhone OS 2+ till 4 and what's the main difference in the OS as well as the d开发者_如何转开发evelopment in xcode..

Thanks

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For the difference between the versions go to http://developer.apple.com/iphone/library/navigation/index.html#section=Resource%20Types&topic=Release%20Notes and see the Documents called iOS x.x API Diffs

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In Release notes section of iOS Reference Library you can find "API diffs" docs for every iOS version starting from 2.1.

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The answer of this question should be very much in detail but may be following my answer in other post will help you to understand how apple world is working:

Are there any Hardware Requirements to test iPhone Apps on your iPhone