Swift Invalid type in JSON write error using JSONSerialization.data

Question

I have an array with elements of a custom type. Here is the type :

public class RequestElemDataBody: Codable {
    public var name: String
    public var value: String

    public init(name: String, value: String) {
        self.name = name
        self.value = value
    }
}

This is how I declare my array :

var elementsInForm = [RequestElemDataBody]()

I use a function to convert this array to Data then to String :

func json(from object: [Any]) -> String? {
    guard let data = try? JSONSerialization.data(withJSONObject: object, options: []) else {
        return nil
    }
    return String(data: data, encoding: String.Encoding.utf8)
}

When executing I get this error message :

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid type in JSON write (Data.RequestElemDataBody)'

I don't know what is wrong with my custom type since it is Codable.

How can I parse my array with my function without it throwing an error ?

Answer 1

You should use JSONEncoder when serializing Codable.

The example from the documentation page:

struct GroceryProduct: Codable {
    var name: String
    var points: Int
    var description: String?
}

let pear = GroceryProduct(name: "Pear", points: 250, description: "A ripe pear.")

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

let data = try encoder.encode(pear)
print(String(data: data, encoding: .utf8)!)

/* Prints:
 {
   "name" : "Pear",
   "points" : 250,
   "description" : "A ripe pear."
 }
*/

Answer 2

Problem with your approach is that you try to encode the whole array. But your codable object is not an array at the moment.

Try passing single element to your method and return the string to work with it outside;

func json(from element: RequestElemDataBody) -> String {
...
}

Answer 3

As mentioned here the top level object can be Array or Dictionary and all objects can be instances of NSString, NSNumber, NSArray, NSDictionary, or NSNull. . In your case they are instances of an custom object.

So you can try converting the objects to dictionary first and then use JSONSerialization

And to convert to dictionary you can create an extension on Encodable

    extension Encodable {

    var dict : [String: Any]? {
        guard let data = try? JSONEncoder().encode(self) else { return nil }
        guard let json = try? JSONSerialization.jsonObject(with: data, options: []) as? [String:Any] else { return nil }
        return json
    }
}

and while calling function you can use ArrayName.compactMap({$0.dict})

Also you can use ObjectMapper library.

Reference from: How to convert a Swift object to a dictionary