StringCoded.swift

import Foundation

@propertyWrapper
struct StringCoded<WrappedValue: LosslessStringConvertible>: Codable {
    var wrappedValue: WrappedValue

    init(wrappedValue: WrappedValue) {
        self.wrappedValue = wrappedValue
    }

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

        guard let value = WrappedValue(stringValue) else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Unable to initialize an \(WrappedValue.self) using decoded value '\(stringValue)'."
            )
        }

        self.init(wrappedValue: value)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(wrappedValue.description)
    }
}

StringCodedOptional.swift

import Foundation

@propertyWrapper
struct StringCodedOptional<Wrapped: LosslessStringConvertible>: Codable {
    var wrappedValue: Wrapped?

    init(wrappedValue: Wrapped? = nil) {
        self.wrappedValue = wrappedValue
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let stringValue = try container.decode(String.self)
        let value = Wrapped(stringValue)a
        self.init(wrappedValue: value)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        if let wrappedValue = wrappedValue {
            try container.encode(String(describing: wrappedValue))
        } else {
            try container.encode("")
        }
    }
}