Due to the order of events in MediaTrackReceiver and friends, SubscribedTrack
will be closed before the track is removed from RoomTrackManager.
Because of this, when a track is unpublished, it's possible to be subscribed
to the track as it's closing.
By introducing a closing state, we'd prevent accidental subscription to
closing tracks.
UpdateSubscription had a shortcoming where when it couldn't find the
participant, it ignored the request.
This PR further removes the reliance of current publisher state from
subscribers.
- SubscribeToTrack only takes in a trackID
- Introduced RoomTrackManager to maintain all published tracks to a room
- Added TrackUnpublished event to clearly indicate when a track has been removed
- SubscribeRequested event no longer include information about the publisher
David Zhao