((Action)delegate()

            {

                if (isBussy) { return; }

                isBussy = true;

                while (true)

                {

                    //code long time task.................

                    Dispatcher.BeginInvoke((Action)(delegate()

            {

                btnStart.Content = DateTime.Now.ToString();

            }

             ));

                    Thread.Sleep(300);

                }

                isBussy = false;

            }).BeginInvoke(null, null);