Along with Swift 4 (Foundation) now it is natively supported in both ways, JSON string to an object - an object to JSON string. Please see Apple's documentation here JSONDecoder() and here JSONEncoder()

JSON String to Object

let jsonData = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()
let myStruct = try! decoder.decode(myStruct.self, from: jsonData)

Swift Object to JSONString

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try! encoder.encode(myStruct)
print(String(data: data, encoding: .utf8)!)

UPDATE: Codable protocol introduced in Swift 4 should be sufficient for most of the JSON parsing cases. Below answer is for people who are stuck in previous versions of Swift and for legacy reasons

EVReflection :

• This works of reflection principle. This takes less code and also supports NSDictionary, NSCoding, Printable, Hashable and Equatable

Example:

    class User: EVObject { # extend EVObject method for the class
       var id: Int = 0
       var name: String = ""
       var friends: [User]? = []
    }

    # use like below
    let json:String = "{\"id\": 24, \"name\": \"Bob Jefferson\", \"friends\": [{\"id\": 29, \"name\": \"Jen Jackson\"}]}"
    let user = User(json: json)

ObjectMapper :

• Another way is by using ObjectMapper. This gives more control but also takes a lot more code.

Example:

    class User: Mappable { # extend Mappable method for the class
       var id: Int?
       var name: String?

       required init?(_ map: Map) {

       }

       func mapping(map: Map) { # write mapping code
          name    <- map["name"]
          id      <- map["id"]
       }

    }

    # use like below
    let json:String = "{\"id\": 24, \"name\": \"Bob Jefferson\", \"friends\": [{\"id\": 29, \"name\": \"Jen Jackson\"}]}"
    let user = Mapper<User>().map(json)

I worked a bit on a smaller solution that doesn't require inheritance. But it hasn't been tested much. It's pretty ugly atm.

https://github.com/peheje/JsonSerializerSwift

You can pass it into a playground to test it. E.g. following class structure:

//Test nonsense data
class Nutrient {
    var name = "VitaminD"
    var amountUg = 4.2

    var intArray = [1, 5, 9]
    var stringArray = ["nutrients", "are", "important"]
}

class Fruit {
    var name: String = "Apple"
    var color: String? = nil
    var weight: Double = 2.1
    var diameter: Float = 4.3
    var radius: Double? = nil
    var isDelicious: Bool = true
    var isRound: Bool? = nil
    var nullString: String? = nil
    var date = NSDate()

    var optionalIntArray: Array<Int?> = [1, 5, 3, 4, nil, 6]
    var doubleArray: Array<Double?> = [nil, 2.2, 3.3, 4.4]
    var stringArray: Array<String> = ["one", "two", "three", "four"]
    var optionalArray: Array<Int> = [2, 4, 1]

    var nutrient = Nutrient()
}

var fruit = Fruit()
var json = JSONSerializer.toJson(fruit)

print(json)

prints

{"name": "Apple", "color": null, "weight": 2.1, "diameter": 4.3, "radius": null, "isDelicious": true, "isRound": null, "nullString": null, "date": "2015-06-19 22:39:20 +0000", "optionalIntArray": [1, 5, 3, 4, null, 6], "doubleArray": [null, 2.2, 3.3, 4.4], "stringArray": ["one", "two", "three", "four"], "optionalArray": [2, 4, 1], "nutrient": {"name": "VitaminD", "amountUg": 4.2, "intArray": [1, 5, 9], "stringArray": ["nutrients", "are", "important"]}}

This is not a perfect/automatic solution but I believe this is the idiomatic and native way to do such. This way you don't need any libraries or such.

Create an protocol such as:

/// A generic protocol for creating objects which can be converted to JSON
protocol JSONSerializable {
    private var dict: [String: Any] { get }
}

extension JSONSerializable {
    /// Converts a JSONSerializable conforming class to a JSON object.
    func json() rethrows -> Data {
        try JSONSerialization.data(withJSONObject: self.dict, options: nil)
    }
}

Then implement it in your class such as:

class User: JSONSerializable {
    var id: Int
    var name: String

    var dict { return ["id": self.id, "name": self.name]  }
}

Now:

let user = User(...)
let json = user.json()

Note: if you want json as a string, it is very simply to convert to a string: String(data: json, encoding .utf8)

Some of the above answers are completely fine, but I added an extension here, just to make it much more readable and usable.

extension Encodable {
    var convertToString: String? {
        let jsonEncoder = JSONEncoder()
        jsonEncoder.outputFormatting = .prettyPrinted
        do {
            let jsonData = try jsonEncoder.encode(self)
            return String(data: jsonData, encoding: .utf8)
        } catch {
            return nil
        }
    }
}

struct User: Codable {
     var id: Int
     var name: String
}

let user = User(id: 1, name: "name")
print(user.convertToString!)

//This will print like the following:

{
  "id" : 1,
  "name" : "name"
}

struct User:Codable{
 var id:String?
 var name:String?
 init(_ id:String,_ name:String){
   self.id  = id
   self.name = name
 }
}

Now just make your object like this

let user = User("1","pawan")

do{
      let userJson =  try JSONEncoder().encode(parentMessage) 
            
    }catch{
         fatalError("Unable To Convert in Json")      
    }

Then reconvert from json to Object

let jsonDecoder = JSONDecoder()
do{
   let convertedUser = try jsonDecoder.decode(User.self, from: userJson.data(using: .utf8)!)
 }catch{
   
 }

2021 | SWIFT 5.1 | Results solution

Data sample:

struct ConfigCreds: Codable { // Codable is important!
    // some params
}

Solution usage sample:

var configCreds = ConfigCreds()
var jsonStr: String = ""

// Object -> JSON
configCreds
   .asJson()
   .onSuccess { jsonStr = $0 }
   .onFailure { _ in // any failure code }

// JSON -> object of type "ConfigCreds"
someJsonString
    .decodeFromJson(type: ConfigCreds.self)
    .onSuccess { configCreds = $0 }
    .onFailure { _ in // any failure code }

Back-End code:

@available(macOS 10.15, *)
public extension Encodable {
    func asJson() -> Result<String, Error>{
        JSONEncoder()
            .try(self)
            .flatMap{ $0.asString() }
    }
}

public extension String {
    func decodeFromJson<T>(type: T.Type) -> Result<T, Error> where T: Decodable {
        self.asData()
            .flatMap { JSONDecoder().try(type, from: $0) }
    }
}

///////////////////////////////
/// HELPERS
//////////////////////////////

@available(macOS 10.15, *)
fileprivate extension JSONEncoder {
    func `try`<T : Encodable>(_ value: T) -> Result<Output, Error> {
        do {
            return .success(try self.encode(value))
        } catch {
            return .failure(error)
        }
    }
}

fileprivate extension JSONDecoder {
    func `try`<T: Decodable>(_ t: T.Type, from data: Data) -> Result<T,Error> {
        do {
            return .success(try self.decode(t, from: data))
        } catch {
            return .failure(error)
        }
    }
}

fileprivate extension String {
    func asData() -> Result<Data, Error> {
        if let data = self.data(using: .utf8) {
            return .success(data)
        } else {
            return .failure(WTF("can't convert string to data: \(self)"))
        }
    }
}

fileprivate extension Data {
    func asString() -> Result<String, Error> {
        if let str = String(data: self, encoding: .utf8) {
            return .success(str)
        } else {
            return .failure(WTF("can't convert Data to string"))
        }
    }
}

fileprivate func WTF(_ msg: String, code: Int = 0) -> Error {
    NSError(code: code, message: msg)
}

internal extension NSError {
    convenience init(code: Int, message: String) {
        let userInfo: [String: String] = [NSLocalizedDescriptionKey:message]
        self.init(domain: "FTW", code: code, userInfo: userInfo)
    }
}

Not sure if lib/framework exists, but if you would like to do it automatically and you would like to avoid manual labour :-) stick with MirrorType ...

class U {

  var id: Int
  var name: String

  init(id: Int, name: String) {
    self.id = id
    self.name = name
  }

}

extension U {

  func JSONDictionary() -> Dictionary<String, Any> {
    var dict = Dictionary<String, Any>()

    let mirror = reflect(self)

    var i: Int
    for i = 0 ; i < mirror.count ; i++ {
      let (childName, childMirror) = mirror[i]

      // Just an example how to check type
      if childMirror.valueType is String.Type {
        dict[childName] = childMirror.value
      } else if childMirror.valueType is Int.Type {
        // Convert to NSNumber for example
        dict[childName] = childMirror.value
      }
    }

    return dict
  }

}

Take it as a rough example, lacks proper conversion support, lacks recursion, ... It's just MirrorType demonstration ...

P.S. Here it's done in U, but you're going to enhance NSManagedObject and then you'll be able to convert all NSManagedObject subclasses. No need to implement this in all subclasses/managed objects.