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iPhone App Life Cycle

One will have notice after NSObject there is pair of triangle brackets in which we declare delegates protocols,

which we will implement. Thus AppDelegate class needs to implement UIApplicationDelegate

protocol, as it is this UIApplicationDelegate defines the iPhone application life cycle methods

which iPhone OS sends the notification when ever application changes its state. Basically delegates are the

components, which captures any events such as state change. Or in other words delegates are objects, which

implements the protocol methods. Also it helps to implements a class various methods of the class, which is not

the class hierarchy.

Thus by declaring UIApplicationDelegate; AppDelegate class can implement the following methods (someare compulsory while some are optional)

-(BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions;

-(void)applicationWillResignActive:(UIApplication *)application;

-(void)applicationDidEnterBackground:( UIApplication *)application;

-(void)applicationWillEnterForeground:( UIApplication *)application;

-(void)applicationDidBecomeActive:(UIApplication *)application;

-(void)applicationWillTerminate:(UIApplication *)application;

These all above functions defines the life cycle of the iPhone Apps. One need to write its own implementation

depending upon what app needs to do in the respective states.

When OS prepares the application to run it sends control to didFinishLaunchingWithOptions.This method will

call when user presses the app icon on the iPhone screen. In this function we define the window of our respective

application on which content of the application will be displayed to user.

In order to start write code in this function we first see how life cycle function is called.

To understand the life cycle properly we will do simple exercise. We will put NSLog statement in each of these

functions. NSLog function allows you get output on the console at particular point in the code. So you come toknow what part of code was executing when any associated NSLog message is printed.

So after putting the NSLog function inside all the life cycle method we get following output on the console

didFinishLaunchingWithOptions

applicationDidBecomeActive

Thus as we discussed before, first DidFinishLaunchingWithOptions is called which prepares the application for

running in the system. Once OS prepares the application it puts the application in active state thus

applicationDidBecomeActive is called. The application remains in this state unless it is interrupted like

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incomeing call/sms or user pressing the home button.

If user press the home button on system running 4.0 or above than we get following console output for the projectwe have started.

applicationWillResignActive

applicationDidEnterBackground

This output tells that OS has deactivated the application and send it to the background. But when user press home

button on system running below 4.0 than following is output

applicationWillTerminate

This indicates that application is died. With OS 4.0 we can switch between the active applications, which is

called as Multitasking. Thus on system with OS 4.0 and above application is not terminated rather it is push into

background.

When user double tap on the home button multitasking panel is visible & user clicks on the application icon say

FirstProject we get following output

applicationWillEnterForeground

applicationDidBecomeActive

This means OS brings application from background to foreground and after application to foreground it is taken

to active state.

Note: When you press icon of the application in multitasking panel till it shows up cross image so that one can

close the application. Ideally applicationWillTerminate delegate function needs to be called. But it is not called.

So it is always advisable to store the application required states when OS calls

applicationDidEnteredBackground.