Adds a Swift extension to make UserDefaults more consistent to work with.

1. UserDefaults+Key.swift

//
//  UserDefaults.swift
//  
//  Created by Shaps Benkau on 24/05/2018.
//  Copyright © 2018 152percent Ltd. All rights reserved.
//

import Foundation

#if os(iOS)
import UIKit
public typealias SystemColor = UIColor
#else
import Cocoa
public typealias SystemColor = NSColor
#endif

public extension UserDefaults {

    func set<T>(_ value: T?, forKey key: Key) {
        set(value, forKey: key.rawValue)
    }

    func value<T>(forKey key: Key) -> T? {
        return value(forKey: key.rawValue) as? T
    }

    func register(defaults: [Key: Any]) {
        let mapped = Dictionary(uniqueKeysWithValues: defaults.map { (key, value) -> (String, Any) in
            if let color = value as? SystemColor {
                return (key.rawValue, NSKeyedArchiver.archivedData(withRootObject: color))
            } else if let url = value as? URL {
                return (key.rawValue, url.absoluteString)
            } else {
                return (key.rawValue, value)
            }
        })

        register(defaults: mapped)
    }

}

public extension UserDefaults {

    subscript<T>(key: Key) -> T? {
        get { return value(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> SystemColor? {
        get { return color(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> URL? {
        get { return url(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> Bool {
        get { return bool(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> Int {
        get { return integer(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> Double {
        get { return double(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> Float {
        get { return float(forKey: key) }
        set { set(newValue, forKey: key) }
    }

    subscript(key: Key) -> CGFloat {
        get { return CGFloat(float(forKey: key) as Float) }
        set { set(newValue, forKey: key) }
    }

}

public extension UserDefaults {

    func bool(forKey key: Key) -> Bool {
        return bool(forKey: key.rawValue)
    }

    func integer(forKey key: Key) -> Int {
        return integer(forKey: key.rawValue)
    }

    func float(forKey key: Key) -> Float {
        return float(forKey: key.rawValue)
    }

    func float(forKey key: Key) -> CGFloat {
        return CGFloat(float(forKey: key) as Float)
    }

    func double(forKey key: Key) -> Double {
        return double(forKey: key.rawValue)
    }

    func url(forKey key: Key) -> URL? {
        return string(forKey: key).flatMap { URL(string: $0) }
    }

    func date(forKey key: Key) -> Date? {
        return object(forKey: key.rawValue) as? Date
    }

    func string(forKey key: Key) -> String? {
        return string(forKey: key.rawValue)
    }

    func set(_ url: URL?, forKey key: Key) {
        set(url?.absoluteString, forKey: key.rawValue)
    }

    func set(_ color: SystemColor?, forKey key: Key) {
        guard let color = color else {
            set(nil, forKey: key.rawValue)
            return
        }

        let data = NSKeyedArchiver.archivedData(withRootObject: color)
        set(data, forKey: key.rawValue)
    }

    func color(forKey key: Key) -> SystemColor? {
        return data(forKey: key.rawValue)
            .flatMap { NSKeyedUnarchiver.unarchiveObject(with: $0) as? SystemColor }
    }

}

public extension UserDefaults {

    struct Key: Hashable, RawRepresentable, ExpressibleByStringLiteral {

        public var rawValue: String

        public init(rawValue: String) {
            self.rawValue = rawValue
        }

        public init(stringLiteral value: String) {
            self.rawValue = value
        }

    }

}

2. Example.swift

let defaults = UserDefaults.standard
defaults.register(defaults: [.defaultFontName: "Menlo"])

var fontName = defaults[.defaultFontName]
print(fontName) // "Menlo"

defaults[.defaultFontName] = "Arial"
fontName = defaults[.defaultFontName]
print(fontName) // "Arial"