from dbus , how do i find if app went down

I am writing lib which will monitor apps and notify/callback higher level
 if apps went down.
How can I achieve this?
I tried doing this using propertieschanged signal and reading substate
property and that msg does not have old value and new value in the msg.
I noticed that when app go down i get two signals and hence two substates -
stop and stop-term.
So it is difficult to call registered callbacks only when substate changes
from running->stop.
Do i need to maintain current state of app in my lib?
is there any other easier way for this ??
Thanks

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