Retain the State of iPhone Application after exit

Question

How can I retain the state of an iPhone after it has exited. What is the easy way to do it?

Answer 1

In your application delegate, you can define the -applicationWillTerminate: method to include code to save application state data.

- (void) applicationWillTerminate:(UIApplication *)application {
    // save state to data model here...
}

Your data model is up to you. For example, this could be a set of user defaults or a Core Data store.

The next time the app is started, you could check for saved state data in -applicationDidFinishLaunching: and initialize the app appropriately.

If you are using iOS 4 and your application supports multitasking features, you will get some of the state saving functionality "for free" because the app resigns focus, instead of terminating.

Answer 2

The first question is when do you save? The answer is in two places (assuming you want to support 3.x and 4.x devices).

First, for OS 3.x devices (and OS 4 devices that don't multi-task):

- (void)applicationWillTerminate:(UIApplication *)application

And second, for OS 4.x devices:

- (void)applicationDidEnterBackground:(UIApplication *)application

You need to do this on iOS4 devices because if the app is shutdown while it's in the background it is just killed; you never see the applicationWillTerminate message.

As for the how, well it depends on how complex your app is. I created a simple protocol that I implement for each view controller that might want to save its state:

@protocol SaveState

- (NSData*) saveState;
- (id) initWithSaveState:(NSData*)data;

@end

It saves the state by looping through view controllers in the main navigation controller and calling the save state method. It then does the reverse in the applicationDidFinishLaunching: method. More information on my blog.