require 'gtk2'

w = Gtk::Window.new
w.signal_connect('destroy') { Gtk.main_quit }

pb = Gtk::ProgressBar.new

t = Thread.new do
  fraction = 0.0
  while true
    print "."
    sleep(1)
    fraction = (fraction + 0.1)
    fraction -= 1 while fraction > 1
    Gtk.idle_add do
      pb.fraction = fraction
      false
    end
  end
end
w << pb

w.show_all
Gtk.main