Advanced Codable Patterns

enum ContentBlock: Decodable {
    case text(String)
    case image(URL)

    enum CodingKeys: String, CodingKey { case type, content, url }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        switch try values.decode(String.self, forKey: .type) {
        case "text": self = .text(try values.decode(String.self, forKey: .content))
        case "image": self = .image(try values.decode(URL.self, forKey: .url))
        default:
            throw DecodingError.dataCorruptedError(
                forKey: .type, in: values,
                debugDescription: "Unknown content type"
            )
        }
    }
}

struct LossyArray<Element: Decodable>: Decodable {
    let elements: [Element]

    init(from decoder: Decoder) throws {
        var values = try decoder.unkeyedContainer()
        var decoded: [Element] = []
        while !values.isAtEnd {
            if let value = try? values.decode(Element.self) {
                decoded.append(value)
            } else {
                _ = try? values.decode(DiscardedValue.self)
            }
        }
        elements = decoded
    }
}

private struct DiscardedValue: Decodable {}

struct UserID: Codable, Hashable {
    let rawValue: String

    init(from decoder: Decoder) throws {
        rawValue = try decoder.singleValueContainer().decode(String.self)
    }

    func encode(to encoder: Encoder) throws {
        var value = encoder.singleValueContainer()
        try value.encode(rawValue)
    }
}

let values = try decoder.container(keyedBy: CodingKeys.self)
theme = try values.decodeIfPresent(String.self, forKey: .theme) ?? "system"