iPhone App transiting from active to inactive state - ios

Question

Assume that I am build an iPhone application which accepts user name and password in one of the screens. While I am entering my username, I get an high priority event like a phone call.
My app will transit from being active to an inactive state at this point.
My question here is: What are the steps that I need to do in my app so that I can save the current application state (and entered info) so that I can restore the same when my app becomes active later on?
This question was asked in one of iOS interviews.
My answer was that handle the active to inactive state transition in the applicationWillResignActive delegate method to save the partially entered user info details and restore it in applicationDidBecomeActive method. The App UI state, we don't have to handle as the OS will take care of it.
It looked like the interviewer was not convinced with my answer. He kept asking me as to how would you handle (steps that you would take in your app) the transition from an active state to inactive state and then to active state in your app so that app state is restored/intact?

Answer 1

Your answer is correct, the interviewer was probably looking for you to name each of these methods and you most likely only named the two you stated. There are a few other delegate methods for certain App UI states, such as applicationDidEnterBackground and applicationWillEnterForeground
If you read the default description of these methods in the comments you will get a better understanding of what each is used for. For example, applicationWillResignActive specifically refers to incoming phone calls or SMS messages:
// Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
// Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
And applicationDidEnterBackground is for saving user data when a user closes your app:
// 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.
// If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
Check out your AppDelegate.m file of a new project and the comments really are very, very useful for understanding exactly which ones will handle what states and when you should use each.
Also, be sure to read up on Apple's documentation on handling application states: The App Life Cycle

Answer 2

Apple has already provided decent documentation on your question:
link.
And for interruptions:
link for apple developer site on handling interruptions

Answer 3

I think you are talking much more about restore state.
First of all, implement App delegate protocol:
func application(application: UIApplication, shouldRestoreApplicationState coder: NSCoder) -> Bool {
return true
}
func application(application: UIApplication, shouldSaveApplicationState coder: NSCoder) -> Bool {
return true
}
Then in your ViewController, implement UIStateRestoring protocol. Here you should notice that if you want archive your own complicated object, don't forget to implement NSCoder.
Please go deep into "State Restoration".
Besides, when your app go to background, you need to do some work such as stop timer, pause ongoing task.
When go to foreground, you need to do something like start the paused task, and prepare to refresh page.