When the app goes to inactive state,app delegate's applicationWillResignActive: is called and when the app goes to background state,app delegate's applicationDidEnterBackground:method is called.
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After the app moves to background state ,it may move to the suspended state afterward or still continue to run for a while longer.
Apps that is waiting for some background tasks (such as playing music) or that has requested for some extra execution time from system may continue some background task for a while .