如何使用PyGI使用Wnck获取Windows列表？

from gi.repository import Gtk, Wnck

while Gtk.events_pending():
    Gtk.main_iteration()
#... in my app class...
    screen = Wnck.Screen.get_default()
    wins = screen.get_windows()

wins == []

screen.get_windows()

screen.force_update()

In [1]: from gi.repository import Gtk, Wnck

In [2]: Gtk.main_iteration()
Out[2]: True

In [3]: screen = Wnck.Screen.get_default()

In [4]: screen.force_update()

In [5]: screen.get_windows()
Out[5]: 
[<Window object at 0x167bd20 (WnckWindow at 0x195d0e0)>,
 <Window object at 0x167bf00 (WnckWindow at 0x195d740)>,
 <Window object at 0x167bf50 (WnckWindow at 0x195d850)>]

Gtk.main_iteration_do(False)

Gtk.main_iteration()