「派生型プロパティを Decodable で扱う」 https://hitorigoto.zumuya.com/200716_decodableProtocolProperty にインスパイヤされたコード。いじってたらPropertyWrapperがなくなってしまった…

PolymorphicDecodable.swift

// 汎用コード

struct CustomCodingKey: CodingKey, ExpressibleByStringLiteral {
    let stringValue: String
    let intValue: Int? = nil
    init?(stringValue: String) { self.stringValue = stringValue }
    init?(intValue: Int) { return nil }
    init(stringLiteral value: String) { stringValue = value }
    init(_ value: String) { stringValue = value }
}
protocol PolymorphicDecodable: Decodable {
    static var hintKeyName: String { get }
    init?(hint: String, decoder: Decoder, container: KeyedDecodingContainer<CustomCodingKey>) throws
}
extension PolymorphicDecodable {
    static var hintKeyName: String { "type" }
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CustomCodingKey.self)
        let hintKey = CustomCodingKey(Self.hintKeyName)
        let hint = try container.decode(String.self, forKey: hintKey)
        guard let v = try Self(hint: hint, decoder: decoder, container: container) else {
            throw DecodingError.dataCorruptedError(
                forKey: hintKey,
                in: container,
                debugDescription: "unknown type hint: \(hint)"
            )
        }
        self = v
    }
}

// 以下 ドメイン固有コード

struct BoolParameter: Decodable {
    let defaultValue: Bool
}
struct NumberParameter: Decodable {
    let defaultValue: CGFloat
    let minValue: CGFloat
    let maxValue: CGFloat
}
enum BoolOrNumberParameter: PolymorphicDecodable {
    case bool(BoolParameter)
    case number(NumberParameter)

    init?(hint: String, decoder: Decoder, container: KeyedDecodingContainer<CustomCodingKey>) throws {
        switch hint {
        case "bool": self = .bool(try BoolParameter(from: decoder))
        case "number": self = .number(try NumberParameter(from: decoder))
        default: return nil
        }
    }
}

struct Bird: Decodable {
    let eggs: [String]
    let destination: URL
}
enum Animal: PolymorphicDecodable {
    case dog(name: String)
    case cat(name: String, cuteFactor: Double)
    case bird(Bird)

    static var hintKeyName: String { "kind" }
    init?(hint: String, decoder: Decoder, container: KeyedDecodingContainer<CustomCodingKey>) throws {
        switch hint {
        case "dog": self = .dog(name: try container.decode(String.self, forKey: "name"))
        case "cat": self = .cat(name: try container.decode(String.self, forKey: "name"),
                                cuteFactor: try container.decode(Double.self, forKey: "cute_factor"))
        case "bird": self = .bird(try Bird(from: decoder))
        default: return nil
        }
    }
}
struct Root: Decodable {
    let parameter: BoolOrNumberParameter
    let pets: [Animal]
}

let root = try JSONDecoder().decode(Root.self, from: """
{
    "parameter": {
        "type": "number",
        "defaultValue": 1.5,
        "minValue": 0.0,
        "maxValue": 2.0
    },
    "pets": [
        { "kind": "dog", "name": "pochi" },
        { "kind": "cat", "name": "tama", "cute_factor": 99.99 },
        { "kind": "bird", "eggs": [ "uzura", "温泉", "ホビロン" ], "destination": "https://www.youtube.com/watch?v=8kQZHYbZkLs" }
    ]
}
""".data(using: .utf8)!)
dump(root)
//▿ __lldb_expr_89.Root
//▿ parameter: __lldb_expr_89.BoolOrNumberParameter.number
//  ▿ number: __lldb_expr_89.NumberParameter
//    - defaultValue: 1.5
//    - minValue: 0.0
//    - maxValue: 2.0
//▿ pets: 3 elements
//  ▿ __lldb_expr_89.Animal.dog
//    ▿ dog: (1 element)
//      - name: "pochi"
//  ▿ __lldb_expr_89.Animal.cat
//    ▿ cat: (2 elements)
//      - name: "tama"
//      - cuteFactor: 99.99
//  ▿ __lldb_expr_89.Animal.bird
//    ▿ bird: __lldb_expr_89.Bird
//      ▿ eggs: 3 elements
//        - "uzura"
//        - "温泉"
//        - "ホビロン"
//      ▿ destination: https://www.youtube.com/watch?v=8kQZHYbZkLs
//        - _url: https://www.youtube.com/watch?v=8kQZHYbZkLs #0
//          - super: NSObject

※ なおJSONはあくまで例であり、

• 情報に意味はありません
• そもそもこういうレスポンス設計はおすすめしません

{
    "pets": [
        { "kind": "dog", "name": "pochi" },
        { "kind": "cat", "name": "tama", "cute_factor": 99.99 },
        { "kind": "bird", "eggs": [ "uzura", "温泉", "ホビロン" ], "destination": "https://www.youtube.com/watch?v=8kQZHYbZkLs" }
    ]
}

ではなく

{
    "pets": [
        { "kind": "dog", "dog": { "name": "pochi" } },
        { "kind": "cat", "cat": { "name": "tama", "cute_factor": 99.99 } },
        { "kind": "bird", "bird": { "eggs": [ "uzura", "温泉", "ホビロン" ], "destination": "https://www.youtube.com/watch?v=8kQZHYbZkLs" } }
    ]
}

のように、オブジェクト種別のヒントと具体的なデータはnestさせてオブジェクトごと切り分けることを強く勧めます