Generic way of loading data from the network.

Resource.swift

struct Resource<A> {
  let pathComponent: String 
  let parse: AnyObject -> A?
}

extension Resource {
  func loadAsynchronous(callback: A? -> ()) {
    let session = NSURLSession.sharedSession()
    let resourceURL = webserviceURL.URLByAppendingPathComponent(pathComponent)
    session.dataTaskWithURL(resourceURL) {
      data, response, error in
        let json = data.flatMap {
          try? NSJSONSerialization.JSONObjectWithData($0, options: NSJSONReadingOptions())
        }
        callback(json.flatMap(self.parse))
    }.resume()
  }
}

func jsonArray<A>(f: AnyObject -> A?) -> AnyObject -> [A]? {
  return {
    arr in
      (arr as? [AnyObject]).map { $0.flatMap(f) }
  }
}

let usersResource: Resource<[User]> = Resource(pathComponent: "/users", parse: jsonArray(User.init))
let postsResource: Resource<[BlogPost]> = Resource(pathComponent: "/posts", parse: jsonArray(BlogPost.init))