Preferences "wrapper" around NSUserDefaults

Preferences.swift

protocol Preferences: class {
    var showNames: Bool { get set }
    func registerDefaults(registrar: Preferences -> Void)
}

private class DefaultPreferenceValues: Preferences {
    var showNames = false
    
    func registerDefaults(registrar: Preferences -> Void) {
        fatalError("The registrar cannot register defaults")
    }
}

extension NSUserDefaults: Preferences {
    
    var showNames: Bool {
        get { return boolForKey("showNames") }
        set { setBool(newValue, forKey: "showNames") }
    }
    
    func registerDefaults(registrar: Preferences -> Void) {
        let defaultValues = DefaultPreferenceValues()
        registrar(defaultValues)
        
        var defaultsToRegister = Dictionary<String, AnyObject>()
        let m = Mirror(reflecting: defaultValues)
        for (key, value) in m.children {
            if let key = key, let value = value as? AnyObject {
                defaultsToRegister[key] = value
            }
        }

        registerDefaults(defaultsToRegister)
    }
    
}

PreferencesUsage.swift

        preferences = NSUserDefaults.standardUserDefaults()
        preferences.registerDefaults {
            $0.showNames = true
        }