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Originally Posted by oilers_fan
I don't know man. Wait for someone who knows for sure, but I don't think they update like that. For example, the twitter application I use does not update until I re-open it from the task switcher. I could be entirely wrong, but I don't know how it would use data.
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Apps can only do the following in the background:
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Background audio - Allows your app to play audio continuously. So customers can listen to your app while they surf the web, play games, and more.
Voice over IP - Your VoIP apps can now be even better. Users can now receive VoIP calls and have conversations while using another app. Your users can even receive calls when their phones are locked in their pocket.
Background location - Navigation apps can now continue to guide users who are listening to their iPods, or using other apps. iOS 4 also provides a new and battery-efficient way to monitor location when users move between cell towers. This is a great way for your social networking apps to keep track of users and their friends' locations.
Push notifications - Receive alerts from your remote servers even when your app isn't running.
Local notifications - Your app can now alert users of scheduled events and alarms in the background, no servers required.
Task finishing - If your app is in mid-task when your customer leaves it, the app can now keep running to finish the task.
Fast app switching - All developers should take advantage of fast app switching, which allows users to leave your app and come right back to where they were when they left - no more having to reload the app.
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Unless you are streaming internet radio or actively engaged on a VOIP call, or navigating, there is no way apps are chewing up data in the background. Even the task finishing is hard limited to like 10 minutes of multitasking, after which your app gets put to sleep. Similarly, having many apps in the task switcher is of no consequence to performance or battery life. Everything is suspended, unless it's using one of the above, and even then, its only the background audio, VOIP, and location services that you need to be concerned about.
So if your phone is making noise (eg. streaming audio or VOIP), or if the little directional arrow is active in the top right corner of the status are, those are the ONLY situations in which an app is chewing battery or data because its running in the background. Everything else is in suspended animation.