iOS 6 App Development: Multitasking, Background Processing, and Notifications
Although iOS doesn’t have true multitasking, it has instant-on task switching that reduces application startup and makes it easier to continue right where you left off. For certain kinds of applications, you can also process events in the background. Such applications include the following:
Audio: Many apps play audio in the background.
Location: Many apps process location events (information the iOS sends to your app about changes in location) in the background.
VoIP: Many apps provide the ability for the user to make Voice over Internet Protocol calls — turning a standard Internet connection into a way to place phone calls.
Newsstand-Content: The Newsstand apps download and process content in the background.
External-Accessory: Some apps work with a hardware accessory that delivers updates.
iOS also offers push notifications for receiving alerts from your remote servers even when your app isn’t running, and local notifications that you can use in your app to alert users of scheduled events and alarms in the background (no servers required).
You can use local notifications to get a user’s attention; for example, a driver navigation application running in the background can use local notifications to alert the user when it’s time to make a turn. Applications can also schedule the delivery of local notifications for a future date and time and have those notifications delivered even if the application isn’t running.