Swift3 关闭崩溃

Question

Crashing while creating an instance of URLSessionTask with the completion handlers

func sessionTaskPostRequest (_ urlRequest : URLRequest , responseHandler: @escaping  ResponseHandler) -> URLSessionTask {
    // 5
     let sesstionTask : URLSessionTask = networkSession.dataTask(with: urlRequest, completionHandler: { (data : Data? , urlResponse : URLResponse? , error : NSError? )     in

        var json: NSDictionary!
        do {
            json = try JSONSerialization.jsonObject(with: data!, options: JSONSerialization.ReadingOptions()) as? NSDictionary
        } catch {
            print(error)
        }

        // Did the JSONObjectWithData constructor return an error? If so, log the error to the console
        if(error != nil) {
            responseHandler (false , nil , error , nil)
        }
        else {
            // The JSONObjectWithData constructor didn't return an error. But, we should still
            // check and make sure that json has a value using optional binding.
            if let parseJSON = json {
                let errorJSON = parseJSON ["Err"] as! String
                if !errorJSON.isEmpty {
                    responseHandler (false , nil , nil , errorJSON)
                }else {
                    responseHandler (true , parseJSON , nil , nil)
                }
                print("Succes: \(parseJSON)")
            }
            else {
                // Woa, okay the json object was nil, something went worng. Maybe the server isn't running?
                let jsonStr = NSString(data: data!, encoding: String.Encoding.utf8.rawValue)
                responseHandler (false , nil , error , "Error could not parse JSON")
                print("Error could not parse JSON: \(jsonStr)")
            }
        }
    } as! (Data?, URLResponse?, Error?) -> Void)

    return sesstionTask

}

And created a type alias for response handler that returns the response JSON Object . Type alias as follows

typealias ResponseHandler = (_ successFlag :Bool , _ data : NSDictionary? , _ errorObject : NSError? , _ errorString : String?) -> Void

Answer 1

Seems like Response handler is getting the Error Object as NSError which will get crashed if Error object is nil and failed to cast it to NSError

Error = nil and when you receive it as NSError Typecasting fails because of nil object , but the same thing is handled wisely if you don't do any typecasting