App Delegate life Cycle Methods

• application:willFinishLaunchingWithOptions:—This method is your app’s first chance to execute code at launch time.
  
• application:didFinishLaunchingWithOptions:—This method allows you to perform any final initialization before your app is displayed to the user.
  
• applicationDidBecomeActive:—Lets your app know that it is about to become the foreground app. Use this method for any last minute preparation.
  
• applicationWillResignActive:—Lets you know that your app is transitioning away from being the foreground app. Use this method to put your app into a quiescent state.
  
• applicationDidEnterBackground:—Lets you know that your app is now running in the background and may be suspended at any time.
  
• applicationWillEnterForeground:—Lets you know that your app is moving out of the background and back into the foreground, but that it is not yet active.
  
• applicationWillTerminate:—Lets you know that your app is being terminated. This method is not called if your app is suspended.

iOS Application States

Not Running State
The app has not been launched or was running but was terminated by the system. 

Inactive
The app is running in the foreground but is currently not receiving events. (It may be executing other code though.) An app usually stays in this state only briefly as it transitions to a different state.

Active
The app is running in the foreground and is receiving events. This is the normal mode for foreground apps.

Background
The app is in the background and executing code. Most apps enter this state briefly on their way to being suspended. However, an app that requests extra execution time may remain in this state for a period of time. In addition, an app being launched directly into the background enters this state instead of the inactive state. For information about how to execute code while in the background,

Suspended
The app is in the background but is not executing code. The system moves apps to this state automatically and does not notify them before doing so. While suspended, an app remains in memory but does not execute any code.
When a low-memory condition occurs, the system may purge suspended apps without notice to make more space for the foreground app.