PropertyWrapper: NestedKey - deal with en/de-coding values within nested data

_Usage.swift

let json = """
{
    "name": {
        "first": "Ian",
        "last": "Keen"
    },
    "age": 42,
    "settings": {
        "permissions": {
            "admin": true
        }
    }
}
"""

struct User: Codable {
    private enum CodingKeys: String, NestedCodingKey {
        case firstName = "name.first"
        case lastName = "name.last"
        case age
        case isAdmin = "setting.permissions.admin"
    }

    @NestedKey var firstName: String
    @NestedKey var lastName: String
    var age: Int
    @NestedKey var isAdmin: Bool
}

NestedKey.swift

public protocol NestedCodingKey: CodingKey {
    var nestedKeys: [String] { get }
}

@propertyWrapper
public struct NestedKey<T: Codable>: Codable {
    public var wrappedValue: T

    public init(wrappedValue: T) {
        self.wrappedValue = wrappedValue
    }

    public init(from decoder: Decoder) throws {
        let codingKey = decoder.codingPath.last!

        guard let key = codingKey as? NestedCodingKey else {
            throw DecodingError.keyNotFound(codingKey, .init(codingPath: decoder.codingPath, debugDescription: "CodingKeys must conform to NestedCodingKey"))
        }

        let last = key.nestedKeys.last!
        let container = try key.nestedKeys.dropFirst().dropLast().reduce(decoder.container(keyedBy: AnyCodingKey.self)) { container, key in
            return try container.nestedContainer(keyedBy: AnyCodingKey.self, forKey: AnyCodingKey(key))
        }

        self.wrappedValue = try container.decode(T.self, forKey: AnyCodingKey(last))
    }

    public func encode(to encoder: Encoder) throws {
        let codingKey = encoder.codingPath.last!

        guard let key = codingKey as? NestedCodingKey else {
            throw EncodingError.invalidValue(codingKey, .init(codingPath: encoder.codingPath, debugDescription: "CodingKeys must conform to NestedCodingKey"))
        }

        let last = key.nestedKeys.last!
        var container = encoder.container(keyedBy: AnyCodingKey.self)

        for key in key.nestedKeys.dropFirst().dropLast() {
            container = container.nestedContainer(keyedBy: AnyCodingKey.self, forKey: .init(key))
        }
        
        try container.encode(wrappedValue, forKey: AnyCodingKey(last))
    }
}

extension NestedKey: Equatable where T: Equatable { }

extension KeyedDecodingContainer {
    public func decode<T>(_ type: NestedKey<T?>.Type, forKey key: KeyedDecodingContainer<K>.Key) throws -> NestedKey<T?> {
        return NestedKey<T?>(wrappedValue: try? decode(NestedKey<T>.self, forKey: key).wrappedValue)
    }
}

extension NestedCodingKey where Self: RawRepresentable, RawValue == String {
    public init?(stringValue: String) { self.init(rawValue: stringValue) }
    public init?(intValue: Int) { fatalError() }

    public var intValue: Int? { nil }
    public var stringValue: String { nestedKeys.first! }

    public var nestedKeys: [String] { rawValue.components(separatedBy: ".") }
}

AnyCodingKey : https://gist.github.com/IanKeen/3d226854c8c59a17e151a0022b71f6bb

Original inspiration from @AliSoftware : https://gist.github.com/AliSoftware/89b275d7259d23ebf12d377b6ffe15cd

👋