Reduce Boiler Plate code for UserDefault using PropertyWrapper

UserDefault.swift

// Strongly Typed key
struct Key: RawRepresentable {
    let rawValue: String
}

extension Key: ExpressibleByStringLiteral {
    init(stringLiteral: String) {
        rawValue = stringLiteral
    }
}

extension Key {
    static let isFirstLaunch: Key = "isFirstLaunch"
}

struct DefaultKey {
    @UserDefault(.isFirstLaunch, defaultValue: true)
    static var isFirstLaunch: Bool
}


// User Default Property Wrapper

// The marker protocol
protocol DefaultsSerializable {}

extension Data: DefaultsSerializable {}
extension String: DefaultsSerializable {}
extension Date: DefaultsSerializable {}
extension Bool: DefaultsSerializable {}
extension Int: DefaultsSerializable {}
extension Double: DefaultsSerializable {}
extension Float: DefaultsSerializable {}

// Every element must be a property-list type
extension Array: DefaultsSerializable where Element: DefaultsSerializable {}
extension Dictionary: DefaultsSerializable where Key == String, Value: DefaultsSerializable {}

@propertyWrapper
struct UserDefault<T> where T: DefaultsSerializable {
    let key: Key
    let defaultValue: T

    init(_ key: Key, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }

    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key.rawValue) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key.rawValue)
        }
    }
}