public class Yoba
    {
        private readonly MainWindow _window;
        private Timer timer;

        public Yoba(MainWindow window)
        {
            _window = window;

            timer = new Timer(1000);
            timer.Elapsed += TimerOnElapsed;
            timer.Enabled = true;
        }

        private void TimerOnElapsed(object sender, ElapsedEventArgs elapsedEventArgs)
        {
            _window.Dispatcher.Invoke(UpdateTextBox);
        }

        private void UpdateTextBox()
        {
            _window.tb.Text += "yoba, eto ti?" + Environment.NewLine;
        }
    }