What is need to upgrade an app from IOS3 to IOS4?

I have an app I have written for IOS3, but I want to polish it up for IOS4. What do I have to do differently (or in addition). I have noticed that every o开发者_如何学Gone in a great while, it will crash on the hardware at startup, but I haven't yet gotten a good debug session while it does it. I am hoping that the info received here will help me solve that problem also. (Note that I didn't have the crashes pre-IOS4.)

Thanks

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Really, there isn't much that is required. If your build is clean of warnings and you have implemented applicationWillResignActive to save all your data you should be fine with iOS 4 and the multi-tasking features added. Supporting anything else isn't required to run on iOS 4...