为什么在授予用户权限时，UIUserNotificationType.None 在当前设置中返回 true？

我正在编写一种方法来检查当前用户设置是否包含某些通知类型。

当检查当前设置是否包含 UIUserNotificationsType.None 时，无论授予权限还是拒绝权限，它都会返回 true。有谁知道这是为什么吗？

func registerForAllNotificationTypes()
{
    registerNotificationsForTypes([.Badge, .Alert, .Sound])
}

func registerNotificationsForTypes(types:UIUserNotificationType)
{
    let settings = UIUserNotificationSettings.init(forTypes:types, categories: nil)
    UIApplication.sharedApplication().registerUserNotificationSettings(settings)
}

func isRegisteredForAnyNotifications() -> Bool
{
    let currentSettings = UIApplication.sharedApplication().currentUserNotificationSettings()
    print(currentSettings)
    print((currentSettings?.types.contains(.Alert))!)
    print((currentSettings?.types.contains(.Badge))!)
    print((currentSettings?.types.contains(.Sound))!)
    print((currentSettings?.types.contains(.None))!)

    return (currentSettings?.types.contains(.Alert))! //Just testing .Alert for now
}

当权限开启时：

Optional(<UIUserNotificationSettings: 0x7fabdb719360; types: (UIUserNotificationTypeAlert UIUserNotificationTypeBadge UIUserNotificationTypeSound);>) true true true true

当权限关闭时：

Optional(<UIUserNotificationSettings: 0x7f96d9f52140; types: (none);>) false false false true

有趣的是，它只是确认 0 包含 0 :) 看一下 UIUserNotificationsType 的枚举定义：https://developer.apple.com/library/prerelease/ios/documentation/UIKit/Reference/UIUserNotificationSettings_class/index.html#//apple_ref/c/tdef/UIUserNotificationType https://developer.apple.com/library/prerelease/ios/documentation/UIKit/Reference/UIUserNotificationSettings_class/index.html#//apple_ref/c/tdef/UIUserNotificationType

struct UIUserNotificationType : OptionSetType {
    init(rawValue rawValue: UInt)
    static var None: UIUserNotificationType { get }
    static var Badge: UIUserNotificationType { get }
    static var Sound: UIUserNotificationType { get }
    static var Alert: UIUserNotificationType { get }
}

但它在 Objective-C 中更清晰可见：

typedef enum UIUserNotificationType : NSUInteger {
   UIUserNotificationTypeNone    = 0,
   UIUserNotificationTypeBadge   = 1 << 0,
   UIUserNotificationTypeSound   = 1 << 1,
   UIUserNotificationTypeAlert   = 1 << 2,
} UIUserNotificationType;