create a JSON using swift

You can use the Encodable protocol to convert an object to JSON string or JSON object upto your choice. Please try to understand the example

//This the the customer structure
struct Customer: Codable {

    var name: String?
    var id: String?
    var account: Account?
}

//This is customer's account structure
struct Account: Codable {

    var acId: String?
    var openedDate: Date?
}

//Main View Controller.
class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        //Call this fnctions to initiate the process.
        self.makeJson()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    func makeJson() {

        //Make the account object
        let accountInfo = Account.init(acId: "100100234", openedDate: Date())

        //Make the customer object
        let customer = Customer.init(name: "Naresh", id: "dfg-2561", account: accountInfo)

        do {

            //make data object
            let data = try JSONEncoder.init().encode(customer)

            //print as JSON String
            let jsonString = String.init(data: data, encoding: String.Encoding.utf8)
            print(jsonString ?? "Not parsed")

            //print as JSON Object
            let jsonObject = try JSONSerialization.jsonObject(with: data, options: [])
            print(jsonObject)

        }
        catch {
            print(error.localizedDescription)
        }
    }
}

Look at the terminal. This will print the json string & json object both.

The easy way is to create a multiline String:

let myJSON: String = """
{ 
  "id" : 12,
  "foo": "bar"
}
"""

First of all take advantage of the new literal string syntax to avoid the confusion of the escaped double quotes.

Second of all please conform to the naming convention that variables are camelCased

Assumed variables :

let name = "Foo"
let id = 1
let subid = 2

The strings look much clearer.

let nlpJsonString = """
{"appId":"**********","cloud":true}
"""

let cmdJsonString = """
{"asr":"","id":\(id),"name":"\(name)","nlp":"\(nlpJsonString)","subid":\(subid)}
"""

As you can see (better now) nlpJsonString is inserted as String (note the wrapping double quotes) so the result is

{"asr":"","id":1,"name":"Foo","nlp":"{"appId":"**********","cloud":true}","subid":2}

If you want to insert nlpJsonString as a dictionary remove the double quotes

let cmdJsonString = """
{"asr":"","id":\(id),"name":"\(name)","nlp":\(nlpJsonString),"subid":\(subid)}
"""

then you get

{"asr":"","id":1,"name":"Foo","nlp":{"appId":"**********","cloud":true},"subid":2}

An alternative is to create dictionaries and serialize them

let name = "Foo"
let id = 1
let subid = 2

let nlpDict : [String:Any] = ["appId":"**********","cloud":true]
let cmdDict : [String:Any] = ["asr":"","id":id,"name":name,"nlp":nlpDict,"subid":subid]

do {
    let jsonData = try JSONSerialization.data(withJSONObject: cmdDict)
    let json = String(data: jsonData, encoding: .utf8)!
    print(json)
} catch { print("something went wrong", error) }