Using NSUserDefaults in Swift 4.

Defaults.swift

import Foundation

public let Defaults = UserDefaults.standard

open class DefaultsKeys {
    fileprivate init() {}
}

open class DefaultsKey<ValueType>: DefaultsKeys {
    public let _key: String

    public init(_ key: String) {
        self._key = key
        super.init()
    }
}

extension UserDefaults {

    public subscript<T: Codable>(key: DefaultsKey<T>) -> T? {
        get {
            guard let data = object(forKey: key._key) as? Data else { return nil }

            let decoder = JSONDecoder()
            let dictionary = try! decoder.decode([String: T].self, from: data)
            return dictionary["top"]
        }
        set {
            guard let value = newValue else { return set(nil, forKey: key._key) }

            let encoder = JSONEncoder()
            let data = try! encoder.encode(["top": value])
            set(data, forKey: key._key)
        }
    }
}


// Usage
extension DefaultsKeys {
    static let accessToken = DefaultsKey<AccessToken>("access_token")
}

guard let token = Defaults[.accessToken] else { return }