Programming iOS 8

Appendix A. Application Lifetime Events

Your app’s one and only application object (a UIApplication instance, or on rare occasions a UIApplication subclass instance) is created for you as the shared application object by UIApplicationMain, along with its delegate; in the Xcode project templates, this is an instance of the AppDelegate class. The application reports lifetime events through method calls to its delegate; other instances can also register to receive most of these events as notifications.

What application lifetime events you can receive depends on whether or not your app participates in multitasking. It almost certainly will. In the old days, before iOS 4, there was no multitasking. If the user pressed the Home button while running your app, your app was terminated. The next time the user launched your app by tapping its icon, your app launched from scratch. Even today, your app can opt out of multitasking and behave like a pre–iOS 4 app, if you set the “Application does not run in background” key (UIApplicationExitsOnSuspend) in your Info.plist. For some apps, such as certain games, this might be a reasonable thing to do.

In the multitasking world, however, the Home button doesn’t terminate your app; it backgrounds and suspends it. This means that your app is essentially freeze-dried in the background; its process still exists, but it isn’t actively running, and it isn’t getting any events — though notifications can be stored by the system for later delivery if your app ...