UserDefaults+PropertyWrapper.swift

import Foundation

@propertyWrapper
struct UserDefault<Value> {
    private let key: String
    private let defaultValue: Value
    private let defaults: UserDefaults

    init(key: String, defaultValue: Value, defaults: UserDefaults = .standard) {
        self.key = key
        self.defaultValue = defaultValue
        self.defaults = defaults
    }

    var wrappedValue: Value {
        get {
            (defaults.object(forKey: key) as? Value) ?? defaultValue
        }
        set {
            defaults.set(newValue, forKey: key)
        }
    }
}

final class A {
    @UserDefault(key: "foo", defaultValue: nil, defaults: .standard)
    var foo: String?

    @UserDefault(key: "bar", defaultValue: nil, defaults: .standard)
    var bar: String

    private func fooType() {
        print(type(of: self.foo))   // Optional<String>
    }

    private func barType() {
        print(type(of: self.bar))   // Optional<String>
    }
}