Create JSON in swift

For Swift 3.0, as of December 2016, this is how it worked for me:

let jsonObject: NSMutableDictionary = NSMutableDictionary()

jsonObject.setValue(value1, forKey: "b")
jsonObject.setValue(value2, forKey: "p")
jsonObject.setValue(value3, forKey: "o")
jsonObject.setValue(value4, forKey: "s")
jsonObject.setValue(value5, forKey: "r")

let jsonData: NSData

do {
    jsonData = try JSONSerialization.data(withJSONObject: jsonObject, options: JSONSerialization.WritingOptions()) as NSData
    let jsonString = NSString(data: jsonData as Data, encoding: String.Encoding.utf8.rawValue) as! String
    print("json string = \(jsonString)")                                    

} catch _ {
    print ("JSON Failure")
}

EDIT 2018: I now use SwiftyJSON library to save time and make my development life easier and better. Dealing with JSON natively in Swift is an unnecessary headache and pain, plus wastes too much time, and creates code which is hard to read and write, and hence prone to lots of errors.

Creating a JSON String:

let para:NSMutableDictionary = NSMutableDictionary()
para.setValue("bidder", forKey: "username")
para.setValue("day303", forKey: "password")
para.setValue("authetication", forKey: "action")
let jsonData = try! NSJSONSerialization.dataWithJSONObject(para, options: NSJSONWritingOptions.allZeros)
let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding) as! String
print(jsonString)

• Swift 4.1, April 2018

Here is a more general approach that can be used to create a JSON string by using values from a dictionary:

struct JSONStringEncoder {
    /**
     Encodes a dictionary into a JSON string.
     - parameter dictionary: Dictionary to use to encode JSON string.
     - returns: A JSON string. `nil`, when encoding failed.
     */
    func encode(_ dictionary: [String: Any]) -> String? {
        guard JSONSerialization.isValidJSONObject(dictionary) else {
            assertionFailure("Invalid json object received.")
            return nil
        }

        let jsonObject: NSMutableDictionary = NSMutableDictionary()
        let jsonData: Data

        dictionary.forEach { (arg) in
            jsonObject.setValue(arg.value, forKey: arg.key)
        }

        do {
            jsonData = try JSONSerialization.data(withJSONObject: jsonObject, options: .prettyPrinted)
        } catch {
            assertionFailure("JSON data creation failed with error: \(error).")
            return nil
        }

        guard let jsonString = String.init(data: jsonData, encoding: String.Encoding.utf8) else {
            assertionFailure("JSON string creation failed.")
            return nil
        }

        print("JSON string: \(jsonString)")
        return jsonString
    }
}

How to use it:

let exampleDict: [String: Any] = [
        "Key1" : "stringValue",         // type: String
        "Key2" : boolValue,             // type: Bool
        "Key3" : intValue,              // type: Int
        "Key4" : customTypeInstance,    // type: e.g. struct Person: Codable {...}
        "Key5" : customClassInstance,   // type: e.g. class Human: NSObject, NSCoding {...}
        // ... 
    ]

    if let jsonString = JSONStringEncoder().encode(exampleDict) {
        // Successfully created JSON string.
        // ... 
    } else {
        // Failed creating JSON string.
        // ...
    }

Note: If you are adding instances of your custom types (structs) into the dictionary make sure your types conform to the Codable protocol and if you are adding objects of your custom classes into the dictionary make sure your classes inherit from NSObject and conform to the NSCoding protocol.

Swift 5 - 6/30/21

Adopt the Codable protocol (similar to interface in other programming languages)

struct ConfigRequestBody: Codable {
var systemid: String
var password: String
var request: String = "getconfig"

init(systemID: String, password: String){
    self.systemid = systemID
    self.password = password
 }
}

Create instance of struct/class that you want to turn into JSON:

let requestBody = ConfigRequestBody(systemID: systemID, password: password)

Encode the object into JSON using a JSONEncoder. Here I print the string representation so you can see the result:

    let encoder = JSONEncoder()
    encoder.outputFormatting = .prettyPrinted
    do {
        let result = try encoder.encode(requestBody)
        // RESULT IS NOW JSON-LIKE DATA OBJECT
        if let jsonString = String(data: result, encoding: .utf8){
            // JSON STRING
            print("JSON \(jsonString)")
        }
    } catch {
        print("Your parsing sucks \(error)")
        return nil
    }
}

This worked for me... Swift 2

static func checkUsernameAndPassword(username: String, password: String) -> String?{
    let para:NSMutableDictionary = NSMutableDictionary()
        para.setValue("demo", forKey: "username")
        para.setValue("demo", forKey: "password")
       // let jsonError: NSError?
    let jsonData: NSData
    do{
        jsonData = try NSJSONSerialization.dataWithJSONObject(para, options: NSJSONWritingOptions())
        let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding) as! String
        print("json string = \(jsonString)")
        return jsonString

    } catch _ {
        print ("UH OOO")
        return nil
    }
}

Check out https://github.com/peheje/JsonSerializerSwift

Use case:

//Arrange your model classes
class Object {
  var id: Int = 182371823
  }
class Animal: Object {
  var weight: Double = 2.5
  var age: Int = 2
  var name: String? = "An animal"
  }
class Cat: Animal {
  var fur: Bool = true
}

let m = Cat()

//Act
let json = JSONSerializer.toJson(m)

//Assert
let expected = "{\"fur\": true, \"weight\": 2.5, \"age\": 2, \"name\": \"An animal\", \"id\": 182371823}"
stringCompareHelper(json, expected) //returns true

Currently supports standard types, optional standard types, arrays, arrays of nullables standard types, array of custom classes, inheritance, composition of custom objects.

Use SwiftyJSON to generate JSON string. Learn from the comments of the answer of Matt.

@2022/10

let savedData = ["Something": 1]

var json = JSON()

json.dictionaryObject = [ 
    "type_id": 1,
    "model_id": 1,
    "transfer": [
        "startDate": "10/04/2015 12:45",
        "endDate": "10/04/2015 16:00"
    ],
    "custom": savedData
]

let jsonStr = json.rawString()