How to keep track of chat messages seen by user

Question

I am building a simple Chat App using Firebase and Ionic. I am trying to keep track of number of messages that are unread for different groups that the user is member of (like WhatsApp).

I have the messages linked to $scope through $firebaseArray

$scope.messages = $firebaseArray(<ref to message path in firebase>);

$scope.messages is iterated over and shown in the view. When a new message arrives it automatically shows up in the group being viewed. I want to keep track of messages not viewed. I am not quite sure of the best way to keep track of which messages the user has seen. I see suggestions here to keep track of timestamp when a user last visited a group but am wondering if there is a better/simpler solution to this.

The things that come to mind that I need to do are -

1. Setup some sort of monitoring for all groups the user is member of not just the one being viewed by the user. If user is member of 50 groups I need to monitor all 50 of them individually (wonder what, if any, performance implications that has). Here I will need to setup a child_added monitoring for each of the groups the user is member of
2. Track timestamp for last time a user was in a group
3. As the user switches groups, change the $firebaseArray to monitor the new group and cancel the $firebaseArray setup for the previous. Change the child_added monitoring for the new group to the group being viewed previously
4. If the user moves to another part of the app where he is not looking at any group messages (perhaps in a setting section) then again be aware of that and monitor all groups - remove the firebeaseArray from the group last being viewed and add a child_added monitoring when user moves away

If the user goes to another application and pushes the Chat application in the background then I am a little unsure of what to do

1. How does the code determine that the group whose messages the user was looking at is no longer active and new messages coming in now need to be counted
2. I am assuming I need to monitor for the App Pause and Resume events to address that. Is there a better way?

Trying to keep my code simple but feels like monitoring for unread messages is a significant chunk of work. Thanks for any ideas or pointers to sample code or to simplify this.

-S