What happens when my app no longer supports older iOS

Question

If a user was using my app on iOS 8 and my latest release supports iOS 9 only. The user update OS to iOS9.

1. Does he get al alert to update app when he opens my app?
2. Can the user still use my app without updating it?
3. What happens for the deprecated code? Will the app crash while trying to access such APIs?

Answer 1

1. No alerts. If user will not update iOS your application will not be updated. Your project should be updated that now it requires iOS 9, see deployment target in your project settings. If you don't change this setting than it will get updated and you can have a crash if new API is used on old iOS.
2. If you didn't change server side in such way that it breaks compatibility than old version should still work.
3. This should be investigated by you. It depends on many factors. If you have setup project "deployment target" properly than update will not be performed on older iOS versions. This may be tricky if your application should support older versions of iOS. Not updated application will continue to work on new iOS even if deprecated API is used. Apple will simply reject updates witch are using deprecated API but still maintains that API. Once I had a problem since UIAlertView was deprecated and old application had new bug where UIAlertView was not respecting application orientation. I had to do a tricky code which detected if UIAlertController (I prefer this approach instead detecting iOS version) is available than use it and if not than fallback to UIAlertView was used.

During application update the biggest problem you can create is setting compatibility or database comp compatibility. So you should be careful with that.