title: Use Core data as browser cache in AFNetworking
body: <p>I read something about multi thread access on DB but still not sure best way to do that for reading/writing in conjunction with async network download.
  For instance I will have a page with images from the web, so I'm retrieving them by their URLs, using AFNetworking but I want to check first on my DB and write on it (or disk) the retrieved image for future use.
  What can be the best way to do that without blocking the UI (like when scrolling)?
  If I do that with a singleton that reads/writes it block the main thread.
  Thanks for any tips on that.</p>