public async Task<bool> AddRefreshToken(RefreshToken token)
        {

           var existingToken = _ctx.RefreshTokens.Where(r => r.Subject == token.Subject && r.ClientId == token.ClientId).SingleOrDefault();

           if (existingToken != null)
           {
             var result = await RemoveRefreshToken(existingToken);
           }
          
            _ctx.RefreshTokens.Add(token);

            return await _ctx.SaveChangesAsync() > 0;
        }
        public async Task CreateAsync(AuthenticationTokenCreateContext context)
        {
            var clientid = context.Ticket.Properties.Dictionary["as:client_id"];

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

            var refreshTokenId = Guid.NewGuid().ToString("n");

            using (AuthRepository _repo = new AuthRepository())
            {
                var refreshTokenLifeTime = context.OwinContext.Get<string>("as:clientRefreshTokenLifeTime"); 
               
                var token = new RefreshToken() 
                { 
                    Id = Helper.GetHash(refreshTokenId),
                    ClientId = clientid, 
                    Subject = context.Ticket.Identity.Name,
                    IssuedUtc = DateTime.UtcNow,
                    ExpiresUtc = DateTime.UtcNow.AddMinutes(Convert.ToDouble(refreshTokenLifeTime)) 
                };

                context.Ticket.Properties.IssuedUtc = token.IssuedUtc;
                context.Ticket.Properties.ExpiresUtc = token.ExpiresUtc;
                
                token.ProtectedTicket = context.SerializeTicket();

                var result = await _repo.AddRefreshToken(token);

                if (result)
                {
                    context.SetToken(refreshTokenId);
                }
             
            }
        }
 public async Task<bool> RemoveRefreshToken(RefreshToken refreshToken)
 {
     _ctx.RefreshTokens.Remove(refreshToken);
      return await _ctx.SaveChangesAsync() > 0;
 }