Sample usage of Swift 5.1 dynamicMemberLookup that supports KeyPath.

DynamicUserDefaults.swift

import Foundation

@dynamicMemberLookup
final class DynamicUserDefaults {

    static var standard: DynamicUserDefaults {
        return DynamicUserDefaults(.standard)
    }

    private let keys = Keys()
    private let defaults: UserDefaults

    init(_ defaults: UserDefaults) {
        self.defaults = defaults
    }

    subscript<T>(dynamicMember keyPath: KeyPath<Keys, Key<T>>) -> T? {
        get {
            let key = keys[keyPath: keyPath]
            return defaults.object(forKey: key.rawKey) as? T
        }
        set {
            let key = keys[keyPath: keyPath]
            defaults.set(newValue, forKey: key.rawKey)
        }
    }
}

extension DynamicUserDefaults {

    struct Keys {}

    struct Key<T> {
        let rawKey: String
    }
}

extension DynamicUserDefaults.Keys {

    var username: DynamicUserDefaults.Key<String> {
        return .init(rawKey: "user-name")
    }
}

let ud = DynamicUserDefaults.standard
print(ud.username) // nil
ud.username = "Michael J. Fox"
print(ud.username) // Optional("Michael J. Fox")