Question

Is applicationDidEnterBackground ALWAYS called before applicationWillTerminate in an iOS app? I know that applicationWillTerminate is not always called (multitasking) – but when it is called, is applicationDidEnterBackground ALWAYS called first? I don’t want to duplicate code unnecessarily by including it in applicationWillTerminate if it is already included in applicationDidEnterBackground, for an app that supports multitasking.

Answer 1

in ios 4.0 and later applicationDidEnterBackground is called instead of applicationWillTerminate so you don’t have to call both of them. Here is the portion of the Apple docs:

Discussion

In iOS 4.0 and later, this method is called instead of the
applicationWillTerminate: method when the user quits an application
that supports background execution. You should use this method to
release shared resources, save user data, invalidate timers, and store
enough application state information to restore your application to
its current state in case it is terminated later. You should also
disable updates to your application’s user interface and avoid using
some types of shared system resources (such as the user’s contacts
database). It is also imperative that you avoid using OpenGL ES in the
background.

Your implementation of this method has approximately five seconds to
perform any tasks and return. If you need additional time to perform
any final tasks, you can request additional execution time from the
system by calling beginBackgroundTaskWithExpirationHandler:. In
practice, you should return from applicationDidEnterBackground: as
quickly as possible. If the method does not return before time runs
out your application is terminated and purged from memory.

You should perform any tasks relating to adjusting your user interface
before this method exits but other tasks (such as saving state) should
be moved to a concurrent dispatch queue or secondary thread as needed.
Because it’s likely any background tasks you start in
applicationDidEnterBackground: will not run until after that method
exits, you should request additional background execution time before
starting those tasks. In other words, first call
beginBackgroundTaskWithExpirationHandler: and then run the task on a
dispatch queue or secondary thread.

The application also posts a
UIApplicationDidEnterBackgroundNotification notification around the
same time it calls this method to give interested objects a chance to
respond to the transition.

For more information about how to transition gracefully to the
background, and for information about how to start background tasks at
quit time, see iOS App Programming Guide.

Hope this helps clear the issue for you man.
Adrian

Here is the link to the technical note that is available on developer section. It is dealing with networking and multitasking. The actual method used in this doc deals with only applicationDidEnterBackground and since iOS 5 they have a system called watchdog which terminates the app if the network is unresponsive automatically. Hence there is no need to actually call applicationWillTerminate and try to execute codes to allow your app to finish its task before the app is terminated. The app will enter the background and will continue its task until the last task is completed. I hope that makes sense, but here is the link. Please read the watchdog section.
https://developer.apple.com/library/ios/#technotes/tn2277/_index.html#//apple_ref/doc/uid/DTS40010841

Hope this helps. 🙂