Sample code for my blog post Reflection and Mirror in Swift http://www.vadimbulavin.com/2018-03-09-reflection-and-mirror-in-swift/

Reflection and Mirror in Swift - JSON serialization.swift

import Foundation

protocol JSONSerializable {
    func toJSON() throws -> Any?
}

enum CouldNotSerializeError: Error {
    case noImplementation(source: Any, type: String)
    case undefinedKey(source: Any, type: String)
}

extension JSONSerializable {

    func toJSON() throws -> Any? {
        let mirror = Mirror(reflecting: self)

        guard !mirror.children.isEmpty else { return self }

        var result: [String: Any] = [:]

        for child in mirror.children {
            if let value = child.value as? JSONSerializable {
                if let key = child.label {
                    result[key] = try value.toJSON()
                } else {
                    throw CouldNotSerializeError.undefinedKey(source: self, type: String(describing: type(of: child.value)))
                }
            } else {
                throw CouldNotSerializeError.noImplementation(source: self, type: String(describing: type(of: child.value)))
            }
        }

        return result
    }
}

struct Order {
    let uid = UUID()
    let itemsCount = 1
    let isDeleted = false
    let name = "A cup"
    let subtitle: String? = nil
    let category = Category(name: "Cups")
}

struct Category {
    let name: String
}

extension String: JSONSerializable {}
extension Int: JSONSerializable {}
extension Bool: JSONSerializable {}
extension UUID: JSONSerializable {}
extension Order: JSONSerializable {}
extension Category: JSONSerializable {}

extension Optional: JSONSerializable {
    func toJSON() throws -> Any? {
        if let x = self {
            guard let value = x as? JSONSerializable else {
                throw CouldNotSerializeError.noImplementation(source: x, type: String(describing: type(of: x)))
            }
            return try value.toJSON()
        }
        return nil
    }
}

do {
    try Order().toJSON()
} catch {
    print(error)
}