Downloading JSON data fails at NSHTTPURLResponse

I am trying to get a JSON file from a URL and return the contents using Swift. However, the code fails at the line let httpResponse = response as! NSHTTPURLResponse in the following code. I get an exception at this line and Xcode goes into debug mode.

class func downloadJSONFile()->AnyObject
    {
        let requestURL: NSURL = NSURL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
        let urlRequest: NSMutableURLRequest = NSMutableURLRequest(URL: requestURL)
        let session = NSURLSession.sharedSession()
        var json:AnyObject = ""
        let task = session.dataTaskWithRequest(urlRequest) {
            (data, response, error) -> Void in

            let httpResponse = response as! NSHTTPURLResponse
            let statusCode = httpResponse.statusCode

            if (statusCode == 200) {

                do{
                    json = try NSJSONSerialization.JSONObjectWithData(data!, options:.AllowFragments)

                }catch {
                    print("Error with Json: \(error)")

                }

            }

        }

        task.resume()

        return json
    }

How can I fix this?

Solution

There are a few issues:

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.

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

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 ...
    }
}

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>