How can I retrieve the online users who are in a particular subscription?

For example, in /room/:roomId route, users need Meteor.subscribe('room', roomId). There are 5 users are currently subscribing it. How can I retrieve the 5 users userIds on server side?

Actually I could find what I need in Meteor.default_server.sessions , however it’s not reactive. I had to fetch the data every 5 seconds. What I’m looking for is a hook function like onSubscriptionStart.

Anyone has any good ideas about retrieving online users’ ids who are in a particular subscription?