Sample code of using Default to decode a property to the default value

Default.swift

import UIKit

protocol DefaultValue {
    associatedtype Value: Decodable
    static var defaultValue: Value { get }
}

@propertyWrapper
struct Default<T: DefaultValue> {
    var wrappedValue: T.Value
}

extension Default: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        wrappedValue = (try? container.decode(T.Value.self)) ?? T.defaultValue
    }
}

extension KeyedDecodingContainer {
    func decode<T>(
        _ type: Default<T>.Type,
        forKey key: Key
    ) throws -> Default<T> where T: DefaultValue {
        try decodeIfPresent(type, forKey: key) ?? Default(wrappedValue: T.defaultValue)
    }
}

struct Video: Decodable {

    enum State: String, Decodable, DefaultValue {
        case streaming
        case archived
        case unknown

        static let defaultValue = Video.State.unknown
    }

    let id: Int
    let title: String

    @Default.False var commentEnabled: Bool
    @Default.True var publicVideo: Bool

    @Default<State> var state: State
}

extension Bool {
    enum False: DefaultValue {
        static let defaultValue = false
    }
    enum True: DefaultValue {
        static let defaultValue = true
    }
}

extension Default {
    typealias True = Default<Bool.True>
    typealias False = Default<Bool.False>
}


let json = #"{"id": 12345, "title": "My First Video", "state": "reserved"}"#
let value = try! JSONDecoder().decode(Video.self, from: json.data(using: .utf8)!)
print(value)