DefaultRawValueDecodable.swift

import Foundation

protocol DefaultRawValueDecodable: RawRepresentable, Decodable {

    static var defaultValue: Self { get }
}

extension DefaultRawValueDecodable where RawValue: Decodable {

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let stringValue = try container.decode(RawValue.self)

        self = Self(rawValue: stringValue) ?? Self.defaultValue
    }
}

import XCTest

class DefaultRawValueDecodableTests: XCTestCase {

    func testDecode_actualValue() throws {
        let json = """
        {
         "enumProperty": "existing value"
        }
        """
        let decoder = JSONDecoder()

        let decodedValue = try decoder.decode(Wrapper.self, from: json.data(using: .utf8)!)

        XCTAssertEqual(decodedValue.enumProperty, TestDefaultRawValueDecodable.existingValue, "actual value should be decoded")
    }

    func testDecode_defaultValue() throws {
        let json = """
        {
         "enumProperty": "some other value"
        }
        """
        let decoder = JSONDecoder()

        let decodedValue = try decoder.decode(Wrapper.self, from: json.data(using: .utf8)!)

        XCTAssertEqual(decodedValue.enumProperty, TestDefaultRawValueDecodable.unknown, "default value should be decoded")
    }

    enum TestDefaultRawValueDecodable: String, DefaultRawValueDecodable {
        case existingValue = "existing value"
        case unknown

        static var defaultValue: TestDefaultRawValueDecodable = .unknown
    }

    struct Wrapper: Decodable {
        let enumProperty: TestDefaultRawValueDecodable
    }
}