Programming iOS 6, 3rd Edition

State Restoration

In the multitasking world, when the user leaves your app and then later returns to it, one of two things might have happened in the meantime (Chapter 11):

Your app was suspended

Your app was suspended in the background, and remained suspended while the user did something else. When the user returns to your app, the system simply unfreezes your app, and there it is, looking just as it did when the user left it.

Your app was terminated

Your app was suspended in the background, and then, as the user worked with other apps, a moment came where the system decided it needed the resources (such as memory) being held by your suspended app. Therefore it terminated your app. When the user returns to your app, the app launches from scratch.

For most apps, a general goal should be to make those two situations more or less indistinguishable to the user. It should always feel to the user as if the app is being resumed from where it left off the last time it was in the foreground, even if in fact the app was terminated while suspended in the background. This goal is state restoration. Your app has a state at every moment: some view controller’s view is occupying the screen, and views within it are displaying certain values (for example, a certain switch is set to ON, or a certain table view is scrolled to a certain position). The idea of state restoration is to save that information when the app goes into the background, and use it to make all those things true again if the ...