Associated.swift

import Foundation

enum TargetEnum: Decodable {
    case first(SomeObject)
    case second(OtherObject)

    enum CodingKeys: CodingKey {
        case type
    }

    enum Unassociated: String, Decodable {
        case first
        case second
    }

    var unassociated: Unassociated {
        switch self {
        case .first:
            return .first
        case .second:
            return .second
        }
    }

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

        switch type {
        case .first:
            try self = .first(SomeObject(from: decoder))
        case .second:
            try self = .second(OtherObject(from: decoder))
        }
    }
}

struct SomeObject: Decodable {
    let name: String
}

struct OtherObject: Decodable {
    let number: Int
}

let data = """
[{
  "type": "first",
  "name": "Test"
}, {
  "type": "second",
  "number": 10
}]
""".data(using: .utf8)!

let decoded = try! JSONDecoder().decode([TargetEnum].self, from: data)
print(decoded)
print(decoded.filter { $0.unassociated == .first })

/* OUTPUT:

 [__lldb_expr_66.TargetEnum.first(__lldb_expr_66.SomeObject(name: "Test")),
  __lldb_expr_66.TargetEnum.second(__lldb_expr_66.OtherObject(number: 10))]
 [__lldb_expr_1.TargetEnum.first(__lldb_expr_1.SomeObject(name: "Test"))]

 */