iOS code for downloading a file using HTTP only if it was changed

I a开发者_JS百科m looking for a complete piece of code for iOS for downloading a file in background using HTTP but only if the file was updated on the server.

Mainly in order to do that you have to send a request and set if-modified-since for it.

Shortly, I want to obtain the same functionality as wget --timestamp <url>

You can set the if-modified-since header field (or any other header field) in a NSMutableURLRequest object.