Swift获取UUID

func get_uuid() -> String{
    //不存在则生成一个新的并保存
    let uuid_ref = CFUUIDCreate(nil)
    let uuid_string_ref = CFUUIDCreateString(nil , uuid_ref)
    let uuid = uuid_string_ref! as String
    UserDefaults.standard.set(uuid, forKey: "donkey")
    return uuid
}