applicationWillEnterForeground vs. applicationDidBecomeActive, applicationWillResignActive vs. applicationDidEnterBackground

Question

When waking up i.e. relaunching an app (either through springboard, app switching or URL) applicationWillEnterForeground: is called. It is only executed once when the app becomes ready for use, after being put into the background, while applicationDidBecomeActive: may be called multiple times after launch. This makes applicationWillEnterForeground: ideal for setup that needs to occur just once after relaunch.

applicationWillEnterForeground: is called:

• when app is relaunched
• before applicationDidBecomeActive:

applicationDidBecomeActive: is called:

• when app is first launched after application:didFinishLaunchingWithOptions:
• after applicationWillEnterForeground: if there's no URL to handle.
• after application:handleOpenURL: is called.
• after applicationWillResignActive: if user ignores interruption like a phone call or SMS.

applicationWillResignActive: is called:

• when there is an interruption like a phone call.
  • if user takes call applicationDidEnterBackground: is called.
  • if user ignores call applicationDidBecomeActive: is called.
• when the home button is pressed or user switches apps.
• docs say you should
  • pause ongoing tasks
  • disable timers
  • pause a game
  • reduce OpenGL frame rates

applicationDidEnterBackground: is called:

• after applicationWillResignActive:
• docs say you should:
  • release shared resources
  • save user data
  • invalidate timers
  • save app state so you can restore it if app is terminated.
  • disable UI updates
• you have 5 seconds to do what you need to and return the method
  • if you don't return within ~5 seconds the app is terminated.
  • you can ask for more time with beginBackgroundTaskWithExpirationHandler:

The official documentation.

Managing Your App's Life Cycle is helpful to your questions. For quick concept, you can see Figures in that document. You can also read the comment from the code generated by the XCode Wizard. Listed as follows:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    return YES;
}

- (void)applicationWillResignActive:(UIApplication *)application
{
    /*
     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.
     */
}

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    /*
     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.
     */
}

- (void)applicationWillEnterForeground:(UIApplication *)application
{
    /*
     Called as part of the transition from the background to the active state; 
     here you can undo many of the changes made on entering the background.
     */
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    /*
     Restart any tasks that were paused (or not yet started) while the 
     application was inactive. If the application was previously in the 
     background, optionally refresh the user interface.
     */
}

- (void)applicationWillTerminate:(UIApplication *)application
{
    /*
     Called when the application is about to terminate.
     Save data if appropriate.
     See also applicationDidEnterBackground:.
     */
}

For more detailed explanations, please refer to official document for UIApplicationDelegate

I was still a bit confused with Dano's answer so I did a little test to get the flow of events in certain scenarios for my reference, but it might be useful to you too. This is for apps that DO NOT use UIApplicationExitsOnSuspend in their info.plist. This was conducted on an iOS 8 simulator + confirmed with iOS 7 device. Please excuse Xamarin's event handler names. They are very similar.

• Initial and all subsequent launches from a not-running state:

FinishedLaunching

OnActivated

• Interruption (phone call, top slide-down, bottom slide-up):
• Home button double-press listing inactive apps, then reselecting our app:

OnResignActivation

---

OnActivated

• Home button double-press listing inactive apps, selecting another app, then relaunching our app:
• Home button single press, then relaunch:
• Lock (on/off button), then unlock:

OnResignActivation

DidEnterBackground

---

WillEnterForeground

OnActivated

• Home button double-press, and terminate our app: (subsequent relaunch is first case)

OnResignActivation

DidEnterBackground

DidEnterBackground (iOS 7 only?)

Yes, DidEnterBackground is called twice on iOS7 device. Both times UIApplication state is Background. However, iOS 8 simulator does not. This needs testing on iOS 8 device. I will update my answer when I get my hand on it, or someone else could confirm.

applicationWillEnterForeground is called:

when app is relaunched(comes from background to foreground) This method is not invoked when app starts for the first time i.e when applicationDidFinishLaunch is called but only when comes from background applicationDidBecomeActive

applicationDidBecomeActive is called

when app is first launched after didFinishLaunching after applicationWillEnterForeground if there’s no URL to handle. after application:handleOpenURL: is called. after applicationWillResignActive if user ignores interruption like a phone call or SMS. after disappearing of alertView anywhere from the application

applicationWillResignActive is called when system is asking for permissions. (in iOS 10). Just in case someone hit into the same trouble as me...

In iOS 8+ there is a subtle but important difference for taking phone call.

In iOS 7 if user takes phone call both applicationWillResignActive: and applicationDidEnterBackground: are called. But in iOS 8+ only applicationWillResignActive: is called.