Download in background in iPhone

Is it possible to download a开发者_开发技巧 file while the app is in background in iPhone 4?

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Yes, it is.

It's supported in ASIHTTPRequest as of v1.8:

http://allseeing-i.com/ASIHTTPRequest/How-to-use#background_downloads_ios

(There was also an experimental branch of v1.7 with it at http://github.com/pokeb/asi-http-request/commits/backgroundtask but this appears not to exist anymore.)

It's also supported in AFNetworking, see Does AFNetworking have backgrounding support?

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Under iOS 4.x, you can use request some background time using UIApplication beginBackgroundTaskWithExpirationHandler: and perhaps continue a file download for a limited period of time.

See Apple's document on "multitasking" background execution for details.