Downloading JSON data fails at NSHTTPURLResponse

There are a few issues:

1. If there is any error in the request, response will be nil, and thus your attempt to force cast it will result in fatal error. Do not use forced unwrapping/casting when dealing with network responses.

2. There is a deeper problem here that you're trying to return data from a method that runs asynchronously. You should change your method to not return anything, per se, but rather supply a completion handler by which you can asynchronously pass back the relevant data:

    class func downloadJSONFile(completionHandler: @escaping (Any?) -> Void) {
        let requestURL = URL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
        let urlRequest = URLRequest(url: requestURL)
        let session = URLSession.shared
        let task = session.dataTask(with: urlRequest) { data, response, error in
            // check for fundamental networking errors
   
            guard
                error == nil,
                let data = data
            else {
                print(error ?? "Other error")
                completionHandler(nil)
                return
            }
   
            guard
                let httpResponse = response as? HTTPURLResponse,
                (200 ..< 300) ~= httpResponse.statusCode
            else {
                print("Invalid status code")
                completionHandler(nil)
                return
            }
   
            do {
                let json = try JSONSerialization.jsonObject(with: data)
                completionHandler(json)
            } catch let parseError {
                print("Error parsing: \(parseError)")
                completionHandler(nil)
            }
        }
   
        task.resume()
    }
   

   and then you call it, using the completion handler (or use trailing closure syntax, like shown below):

    APIClass.downloadJSONFile() { json in
        guard json != nil else {
            print("there was some problem")
            return
        }
   
        // now you can use `json` here
   
        dispatch_async(dispatch_get_main_queue()) {
            // and if you're doing any model or UI updates, dispatch that back to the main queue
        }
    }
   
    // but do not use `json` here, as the above runs asynchronously
   

3. Note, if you wanted to supply the error information back to the calling routine, you could change it to also return the error information, e.g.:

   enum DownloadError: Error {
       case networkError(Error)
       case notHTTPResponse
       case invalidHTTPResponse(Int)
       case parseError(Error)
   }
   
   class func downloadJSONFile(completionHandler: @escaping (Result<Any, Error>) -> Void) {
       let requestURL = URL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
       let urlRequest = URLRequest(url: requestURL)
       let session = URLSession.shared
       let task = session.dataTask(with: urlRequest) { data, response, error in
           if let error = error {
               completionHandler(.failure(DownloadError.networkError(error)))
               return
           }
   
           guard let httpResponse = response as? HTTPURLResponse, let data = data else {
               completionHandler(.failure(DownloadError.notHTTPResponse))
               return
           }
   
           guard 200 ..< 300 ~= httpResponse.statusCode else {
               completionHandler(.failure(DownloadError.invalidHTTPResponse(httpResponse.statusCode)))
               return
           }
   
           do {
               let json = try JSONSerialization.jsonObject(with: data)
               completionHandler(.success(json))
           } catch let parseError {
               completionHandler(.failure(DownloadError.parseError(parseError)))
           }
       }
   
       task.resume()
   }
   

   And, obviously, the call would change to take both parameters:

   APIClass.downloadJSONFile() { result in
       switch result {
       case .failure(let error):
           print(error)
   
       case .success(let value):
           // and then it would be like before ...
       }
   }
   

4. When using URLSession in iOS 9 and later, it will not permit cleartext requests (i.e. "http" is not permitted, only "https" is, by default). You can force the app to permit non-https requests by adding the following to your info.plist. See https://stackoverflow.com/a/31254874/1271826 for more information

    <key>NSAppTransportSecurity</key>
    <dict>
        <key>NSExceptionDomains</key>
        <dict>
            <key>learnswiftonline.com</key>
            <dict>
                <!--Include to allow subdomains-->
                <key>NSIncludesSubdomains</key>
                <true/>
                <!--Include to allow HTTP requests-->
                <key>NSTemporaryExceptionAllowsInsecureHTTPLoads</key>
                <true/>
                <!--Include to specify minimum TLS version-->
                <key>NSTemporaryExceptionMinimumTLSVersion</key>
                <string>TLSv1.1</string>
            </dict>
        </dict>
    </dict>