Decode based on a type field

TypeCoder.swift

import Foundation

/// Simple example of decoding different values based on a "type" field

let json = Data("""
    [{
            "type": "user",
            "name": "Alice"
        },
        {
            "type": "document",
            "path": "/etc/passwd"
        }
    ]
""".utf8)

struct User: Codable {
    let name: String
}

struct Document: Codable {
    let path: String
}

enum Item {
    case user(User)
    case document(Document)
}

extension Item: Codable {
    enum CodingKeys: CodingKey {
        case type
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try container.decode(String.self, forKey: .type)

        // The important point here is that you can extract a second container with a different
        // CodingKey from the same decoder.
        switch type {
        case "user": self = .user(try User(from: decoder))
        case "document": self = .document(try Document(from: decoder))
        default: throw DecodingError.dataCorruptedError(forKey: .type, in: container,
                                                        debugDescription: "Unknown type")
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
        case .user(let user):
            // And similarly, you can just keep encoding other stuff into the current encoder.
            try container.encode("user", forKey: .type)
            try user.encode(to: encoder)
        case .document(let document):
            try container.encode("document", forKey: .type)
            try document.encode(to: encoder)
        }
    }
}

let items = try JSONDecoder().decode([Item].self, from: json)
print(items)

let output = try JSONEncoder().encode(items)
print(String(data: output, encoding: .utf8)!)