Getting error when receiving JSON after post method in swift

Question

I am sending a POST request to my backend however i am getting this error:

The given data was not valid JSON.",
underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840
"JSON text did not start with array or object and option to allow
 fragments not set."
UserInfo={NSDebugDescription=JSON text did not start with array
or object and option to allow fragments not set.})))

And this is the code that i am using to send/fetch data:

func fetchDataWithParameters(){

    struct Response: Codable {
        let status: String?
        let error: String?
    }

    let decoder = JSONDecoder()
    HTTP.POST("somelinkhere", parameters: ["date": self.weekDays[self.itemSelectedIndex]]) { response in
        if let error = response.error {
            print("got an error: \(error)")
            return
        }
        do {
            let resp = try decoder.decode(Response.self, from: response.data)
            if let err = resp.error {
                print("got an error: \(err)")
            }
            if let status = resp.status {
                print("completed: \(status)")

            }
        } catch let error {
            print("decode json error: \(error)")
        }
    }
}

Using my terminal i am trying to do a manual POST request and i get this:

Admins-MacBook-Pro:hello-world admin$ curl -i -H "Accept: application/json" -H "Content-Type: application/json" somelinkhere
HTTP/1.1 200 OK
Server: openresty/1.9.15.1
Date: Thu, 03 May 2018 23:42:04 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 39
Connection: keep-alive
X-Clacks-Overhead: GNU Terry Pratchett

"{\"name\": \"Timo\", \"age\": \"39\"}"

This leaves me wondering that the only possible fault can be how i am decoding the JSON. Why else would it work with a terminal? Any ideas?

As @patru suggested i included the print here:

catch let error {
    print(String(data:response.data, encoding: .utf8)!)
    print("decode json error: \(error)")
}

This was the result:

"{\"name\": \"Mergim\", \"age\": \"39\"}"
decode json error: dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.})))

It seems as if i am getting the JSON as i did with the curl but for some reason swift does not see it as valid JSON? This is how my post method looks like in the backend:

@app.route('/', methods=['GET', 'POST'])
def index():

    jsonData = {"name": "Timo", "age": "39"}
    jsonData1 = {"name": "Mergim", "age": "39"}

    if request.method=='GET':
        return json.dumps(jsonData)

    elif request.method=='POST':
        return json.dumps(jsonData1)

EDIT

    jsonData = '{"name": "Timo", "age": "39"}'
    jsonData1 = '{"name": "Mergim", "age": "39"}'

changed to:

    jsonData = {"name": "Timo", "age": "39"}
    jsonData1 = {"name": "Mergim", "age": "39"}

Answer 1

This is subtle, but, believe it or not, the problem is on the Server!

Actually your code looked ... a little off, but still quite OK, so I ran this Playground:

import Cocoa

let jsonData = "{\"name\": \"Mergim\", \"age\": \"39\"}".data(using: .utf8)!

struct Response: Codable {
    let status: String?
    let error: String?
}

struct NameAge: Codable {
    let name: String
    let age: String
}

do {
    let resp = try JSONDecoder().decode(Response.self, from: jsonData)
    print(resp)
    let na = try JSONDecoder().decode(NameAge.self, from: jsonData)
    print(na)
} catch {
    print(error)
}

print(String(data:jsonData, encoding: .utf8)!)

Since your Response struct does not quite match the structure of your data I implemented NameAge in order to have something more fitting. Still both do parse, although the value of Response remains questionable. I was puzzled too, until I checked out your error message: it complains about the first character in your data! I finally added the last line in the Playground and the problem became clear.

You provided your server (I guess it is in ruby or something close to it) with a valid String object and it turned it into the corresponding JSON which still represents just a String in JSON. This explains the double quotes which you have and my Playground does not produce. You probably should provide your jsonData as a Hash and the server will convert it properly (without the double quotes). For a random reason JSONDecoder does not want to decode a simple JSON-string, but that is something I excuse it for.

Btw: You probably want to change the type of age to Int once you can pass it correctly.