Programming iOS 6, 3rd Edition

Application Lifetime Events

As I explained in Chapter 6, 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. As I’ve discussed earlier in this chapter, the application immediately starts reporting lifetime events through method calls to its delegate; subsequently, your other instances can also register to receive these same events as notifications. I can’t think of a better place in this book to describe these events, so I’m going to do so here and now.

What application lifetime events you can receive depends on whether or not your app participates in multitasking. 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 under iOS 4 and later, 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.

The suite of basic application lifetime events received by the app delegate in a nonmultitasking app is pretty simple:

application:didFinishLaunchingWithOptions: ...