What to Do with Interruptions to Your iOS Application
On an iOS device running iOS 4.2 or newer versions, various events besides termination can interrupt your app to allow the user to respond — for example, calendar alerts or the user pressing the Sleep/Wake button — and your app moves into the inactive state.
If the user chooses to ignore an interruption, your app moves back into the active state and continues running as before. If the user decides to tap the alert to deal with it (or if the interruption was from the user touching the Home button to switch out of your application), your app then moves into its background state, where it's suspended but remains in memory.
iOS sends you a number of messages to let you know exactly what’s happening as well as to give you the opportunity to take actions such as save user data and state information, which means saving at the point where the user was in the application. (If an app needs to continue running, it can request execution time from the system.)
Because the app is in the background (running or suspended) and still in memory, relaunching is nearly instantaneous. An app’s objects (including its windows and views) remain in memory, so they don’t need to be re-created when the app relaunches. If memory becomes constrained, iOS may purge background apps to make more room for the foreground app.
Because these interruptions cause a temporary loss of control by your app, touch events are no longer sent to your app. When developing your app, you need to take this fact into account. For example, if your app is a game, you should pause the game when your game is interrupted.
In general, your app should store information about its current state when it moves to the inactive state and be able to restore itself to the current state upon a subsequent relaunch.
In all cases, the sequence of events starts the same way — with the applicationWillResignActive: message sent to your application delegate when the application is about to move from active to inactive state. In this method, you should pause ongoing tasks, disable timers, throttle down OpenGL ES frame rates (that is, you should use this method to pause the game), and generally put things on hold.
What happens after this depends on a) the nature of the interruption, and b) how the user responds to the interruption. Your application may be either moved to the background or reactivated.