Modifying private and immutable properties (let) in Codable instances

ModifyCodable.swift

import Foundation

extension Decodable where Self: Encodable {
    /// Creates a new instance and changes the value for the provided key.
    ///
    /// - Parameters:
    ///   - key: The key path to the property that you want to modify.
    ///    Use period to separate levels and [] for indexes.
    ///    Examples: "id", "name.firstName", "children[2].name.firstName"
    ///
    ///   - value: The new value for the provided key.
    ///
    ///  - Returns: A new instance with modified property
    func modifying<T>(_ key: String, to value: T) throws -> Self {
        let originalData = try JSONEncoder().encode(self)
        var object = try JSONSerialization.jsonObject(with: originalData, options: [])

        let keyComponents = keyComponents(from: key)
        object = modify(object, keyComponents: keyComponents, value: value)

        let data = try JSONSerialization.data(withJSONObject: object, options: [])
        return try JSONDecoder().decode(Self.self, from: data)
    }

    private func modify<T>(_ object: Any, keyComponents: [KeyComponent], value: T) -> Any {
        var keyComponents = keyComponents
        while !keyComponents.isEmpty {
            let keyComponent = keyComponents.removeFirst()
            if var array = object as? [Any], case .index(let index) = keyComponent, index < array.count {
                if keyComponents.isEmpty {
                    array[index] = value
                } else {
                    var nextObject = array[index]
                    nextObject = modify(nextObject, keyComponents: keyComponents, value: value)
                    array[index] = nextObject
                }
                return array
            } else if var dictionary = object as? [String: Any], case .key(let key) = keyComponent {
                if keyComponents.isEmpty {
                    dictionary[key] = value
                } else if var nextObject = dictionary[key] {
                    nextObject = modify(nextObject, keyComponents: keyComponents, value: value)
                    dictionary[key] = nextObject
                }
                return dictionary
            }
        }
        return object
    }

    private func keyComponents(from key: String) -> [KeyComponent] {
        let indexSearch = /(.+)\[(\d+)\]/
        return key.components(separatedBy: ".").flatMap { keyComponent in
            if let result = try? indexSearch.wholeMatch(in: keyComponent), let index = Int(result.2) {
                return [KeyComponent.key(String(result.1)), .index(index)]
            }
            return [.key(keyComponent)]
        }
    }
}

private enum KeyComponent {
    case key(String)
    case index(Int)
}

Usage:

// Simple

struct File: Codable {
    let name: String
    let size: Int
}

let file = File(name: "image.png", size: 256)

let modifiedFile = try file
    .modifying("name", to: "video.mp4")
    .modifying("size", to: 512)

dump(modifiedFile)

/*
 File
  - name: "video.mp4"
  - size: 512
 */

// Complex

struct Name: Codable {
    let firstName: String
    let lastName: String
}

struct User: Codable {
    let id: Int
    let name: Name
    let children: [User]
}

let userA = User(
    id: 1,
    name: Name(firstName: "A1", lastName: "A1L"),
    children: [
        User(id: 11, name: Name(firstName: "A11", lastName: "A11L"), children: [
            User(id: 111, name: Name(firstName: "A111", lastName: "A111L"), children: [])
        ]),
        User(id: 12, name: Name(firstName: "A12", lastName: "A12L"), children: [])
    ]
)

let userB = try userA
    .modifying("name.firstName", to: "B1")
    .modifying("id", to: 2)
    .modifying("children[1].name.lastName", to: "B12L")
    .modifying("children[0].children[0].name.firstName", to: "B111")

dump(userB)
/*
 User
   - id: 2
   ▿ name: Name
     - firstName: "B1"
     - lastName: "A1L"
   ▿ children: 2 elements
     ▿ User
       - id: 11
       ▿ name: Name
         - firstName: "A11"
         - lastName: "A11L"
       ▿ children: 1 element
         ▿ User
           - id: 111
           ▿ name: Name
             - firstName: "B111"
             - lastName: "A111L"
           - children: 0 elements
     ▿ User
       - id: 12
       ▿ name: Name
         - firstName: "A12"
         - lastName: "B12L"
       - children: 0 elements
 */