/// <summary>
        /// 建立 AuthenticationTokenCreateContext
        /// </summary>
        /// <param name="context"></param>
        /// <returns></returns>
        public async Task CreateAsync(AuthenticationTokenCreateContext context)
        {
            var clientid = context.Ticket.Properties.Dictionary["as:client_id"];

            if (string.IsNullOrEmpty(clientid))
            {
                return;
            }

            //產成的新Token的唯一標別碼,在此使用Guid(有需要再改其它演算法)
            var refreshTokenId = Guid.NewGuid().ToString("n");

            //更新Token的生存時間值,該值將被用於確定新Token有多長的有效期
            using (AuthBLL _auth = new AuthBLL())
            {
                var refreshTokenLifeTime = context.OwinContext.Get <string>("as:clientRefreshTokenLifeTime");

                //有值延長為2倍,若為null則預設60分鐘
                double doubleRefreshTokenLifeTime   = 60;
                bool   isDoubleRefreshTokenLifeTime = double.TryParse(refreshTokenLifeTime, out doubleRefreshTokenLifeTime);
                if (!isDoubleRefreshTokenLifeTime)
                {
                    doubleRefreshTokenLifeTime = 60;
                }
                else
                {
                    doubleRefreshTokenLifeTime = doubleRefreshTokenLifeTime * 2;
                }

                var token = new RefreshToken()
                {
                    Id        = Helper.GetHash(refreshTokenId),
                    ClientId  = clientid,
                    Subject   = context.Ticket.Identity.Name,
                    IssuedUtc = DateTime.UtcNow,

                    //有效期限(分鐘,有需要再改)
                    ExpiresUtc = DateTime.UtcNow.AddMinutes(doubleRefreshTokenLifeTime)
                };

                context.Ticket.Properties.IssuedUtc  = token.IssuedUtc;
                context.Ticket.Properties.ExpiresUtc = token.ExpiresUtc;

                //在資料庫中以序列化形式儲存
                token.ProtectedTicket = context.SerializeTicket();

                //儲存到RefreshTokens資料表中
                var result = await _auth.AddRefreshToken(token);

                if (result)
                {
                    //建立暫存refreshToken用
                    var refreshTokenProperties = new AuthenticationProperties(context.Ticket.Properties.Dictionary)
                    {
                        IssuedUtc  = context.Ticket.Properties.IssuedUtc,
                        ExpiresUtc = DateTime.UtcNow.AddMinutes(doubleRefreshTokenLifeTime)
                    };

                    var refreshTokenTicket = new AuthenticationTicket(context.Ticket.Identity, refreshTokenProperties);
                    _refreshTokens.TryAdd(refreshTokenId, refreshTokenTicket);

                    //回傳新的Token
                    context.SetToken(refreshTokenId);
                }
            }
        }