Knowing new iOs features?

Everytime when New iOs comes, Apple gives features in end user perspectively in their website.but how can developer know the programming features(which is not available in previous versions)? For Example App will start from beginning (with first tab in tabbar controller)in iphone 3.0.but in iphone 开发者_如何转开发4.0 , app starts from the last status user opened(third tab bar).where can developer see this development oriented features ?

The iOS Developer Site has changelogs and API differences:

http://developer.apple.com/devcenter/ios/index.action

For Example App will start from beginning (with first tab in tabbar controller)in iphone 3.0.but in iphone 4.0 , app starts from the last status user opened(third tab bar).

You probably won't find this anywhere in the docs, because this is not an actual feature.
This happens because apps are allowed to (and if you don't change the defaults they will) "run" in the background now. That the app starts at the last view is just a "side-effect".

It still needs some thinking on the developer-side to figure out what effect a new feature has in your application.