Helper to encapsulate loading JSON over the network.

JSONService.swift

//
// MARK: generic networking and JSON-decoding helper
//

// give it a NSURLRequest and a session, it gives you back a aresult with an AnyObject or an NSError  

public class JSONService
{
  /// returns JSON result on statusCode 200, and an error otherwise
   public class func taskWithJSONRequest(
    request:NSURLRequest,
    session:NSURLSession,
    completion:(JSONResult)->()) -> NSURLSessionDataTask
  {
    let task = session.dataTaskWithRequest(request, completionHandler: {
      (data:NSData!, response:NSURLResponse!, error:NSError!) -> Void in
      if let error = error {
        dispatch_async(dispatch_get_main_queue()) {
          completion(JSONResult.Fail(error))
        }
      }
      else {
        // assert: sub-HTTP networking succeeded
        if let httpResponse = response as? NSHTTPURLResponse {
          if httpResponse.statusCode < 200 || httpResponse.statusCode > 299 {

            let error = NSError(domain: NSBundle.mainBundle().bundleIdentifier!, code: 0, userInfo: [NSLocalizedFailureReasonErrorKey : "http returned non-200 status code","result":httpResponse])

            dispatch_async(dispatch_get_main_queue()) {
              completion(JSONResult.Fail(error))
            }
          }
          else {
            // assert: ... and HTTP succeeded (200)
            if let data = data {
              var JSONError:NSError?
              let JSONValue: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.allZeros, error: &JSONError)
              if let JSONValue: AnyObject = JSONValue {
                // assert: ... and JSON decoding succeeded
                dispatch_async(dispatch_get_main_queue()) {
                  completion(JSONResult.Success(JSONValue))
                }
              }
              else {
                if let JSONError = JSONError {
                  // JSON decoding failed
                  dispatch_async(dispatch_get_main_queue()) {
                    completion(JSONResult.Fail(JSONError))
                  }
                }
                else {
                  Log_asWarn("ERROR: Apple violated its API contract. NSJSONSerialization.JSONObjectWithData returned a nil value, but did not populate its error object")

                  let myJSONError = NSError(domain: NSBundle.mainBundle().bundleIdentifier!, code: 0, userInfo: [NSLocalizedFailureReasonErrorKey :"NSJSONSerialization.JSONObjectWidthData returned nil and no error"])

                  dispatch_async(dispatch_get_main_queue()) {
                    completion(JSONResult.Fail(myJSONError))
                  }
                }
              }
            }
          }
        }
        else {
          let errString = "ERROR: Apple violated its API contract. NSURLSession.dataTaskWithRequest returned a nil NSHTTPURLResponse object even though there was no NSError"
          Log_asWarn(errString)
          
          let myError = NSError(domain: NSBundle.mainBundle().bundleIdentifier!, code: 0, userInfo: [NSLocalizedFailureReasonErrorKey :errString])
          
          dispatch_async(dispatch_get_main_queue()) {
            completion(JSONResult.Fail(myError))
          }
        }
      }
    })
    task.resume()
    return task
  }
  
  public enum JSONResult {
    case Success(AnyObject)
    case Fail(NSError)
  }
}