Programming iOS 9

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 delegate 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.

These events, notifying you of stages in the lifetime of your app as a whole and giving your code an opportunity to run in response, are extraordinarily important and fundamental. This appendix is devoted to a survey of them, along with some typical scenarios in which they will arrive.

Application States

In the early days of iOS — before iOS 4 — the lifetime of an app was extremely simple: either it was running or it wasn’t. The user tapped your app’s icon in the springboard, and your app was launched and began to run. The user used your app for a while. Eventually, the user pressed the Home button (the physical button next to the screen) and your app was terminated — it was no longer running. The user had quit your app. Launch, run, quit: that was the entire life cycle of an app. If the user decided to use your app again, the whole cycle started again.

The reason for this simplicity was that, before iOS 4, an iOS device, with its slow processor and its almost brutal paucity of ...