Short answer is that Apple doesn't allow apps to run in the background, probably for battery saving reasons, so it would have to be done remotely on a computer/server for it to work reliably.
Long answer:
The issue isn't that it's difficult to code. It's more time-consuming than difficult if anything. But the main issue is that Apple does not really allow apps to run in the background unless it's a music app or voip or GPS app. I figured out a workaround that could potentially trigger the app to run in the background briefly. But it required strict conditions for it to happen, and there's no guarantee that it will even happen. Apple even said specifically to not expect it to occur at the time you want it to occur. The functionally was mainly for apps to be able to download newspaper/magazine content in the background at random times.
Anyway, I do want to tackle this issue, because it's frequently requested. I was working on a project with Parse.com to get it to work prior to my surgery. I also already coded the UI interface for it on Remote S prior to surgery and there's a picture of it sometime in a June post on this thread. But unfortunately, Facebook bought out Parse.com and is shutting down Parse later this year with no replacement service. So I'm back to square one, and will have to build up a web server and web service on my own to get this to work.