学步园

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(Notice
  that applicationWillEnterForeground
  will not be called in the first launch.)
• 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 dont return within ~5 seconds the app is terminated.
  • you can ask for more time with beginBackgroundTaskWithExpirationHandler: