Need iOS reference for what happens in sleep, standby and/or idle modes

I'm a new ObjC dev and after searching the Web and Apple's developer docs, I can't find a good succinct explanation of the various iOS device modes, and what goes on in these modes. I see a lot of little discussions, but nothing overarching about the big picture.

For example, I see that some Alarm applications for iPhone/iPad claim to be able to execute alarms (presumably not just system alerts) in the morning. I'm struggling to understand how that happens, even if the user lets the app just keep running overnight, assuming the device goes into sleep mode after no use.

So I don't know what happens when the device goes into sleep or standby mode. Is it just the screen dimming? Does the application in the foreground continue to execute code, go into some interim state like background mode in a saved state, or does it completely (even if temporarily) shut down?

I'd just like to be pointed to a website, a book or the portion of the Apple documentation that explains the various device states, wh开发者_StackOverflowat's going on under iOS 3 & 4 with the application in each state, and how to manage that if you'd like to build an app similar to a simulated bedside alarm clock that's "always on," etc.

Thanks in advance.

Those 'alarms' are Local Notifications, and are very handy. They're fired even when an application is closed, and even when the device is asleep.

The concept of asleep is not too tricky; when the user 'quits' your application (pressing the home button), your application will either be halted and placed into a frozen state (4.x), or quit entirely (3.x). Nonetheless, you are given the opportunity to do a few cleanup operations before the latter happens with the - (void)applicationWillTerminate:(UIApplication *)application method, and before the former happens with the - (void)applicationDidEnterBackground:(UIApplication *)application method (both are called upon your delegate class).

The concept of 'sleep' means that in both iPhone OS 3 and iOS 4 you'll receive the following method when your app loses focus (and also when an SMS comes in, or an alert, etc):

- (void)applicationWillResignActive:(UIApplication *)application

Really, most applications (with the exceptions of applications that are designed to work in the background, like voip, audio streaming, etc) simply quit on close. But if you need to, you can also suspend the freezing of your app and ask for more time to complete a task.

This was a lot easier to grasp in the 3.x days, but now with 4.x's 'multitasking' it becomes a bit harder. It's not too daunting though, and a read of this will help a bit.

This may be of help:

http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIApplication_Class/Reference/Reference.html

see the applicationState for UIApplicationState.