how to be notified when an application with dbus service lauchs or quits?

[cbx]

Hi,all!
I want to know if there is any way to get notified when an application
lauchs or quits.May be there exists something like a signal?

Say,after my program connect to the dbus,if Rhythmbox(as an example) lauchs,
my program can receive an signal.And then if rhythmbox closes later,another
signal will be sent.

I tried to use org.freedesktop.Dbus.NameAcquired, but it seems this signal
is not for this purpose.

Thank you!
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