This function sets the online status to true under user node. And sets to false when user exits the app or gets disconnected with the internet.

FirebaseOnlineOfflinePresence.kt

  /**
     * This function sets the online status to true under user node. And sets to false when
     * user exits the app.
     */
    fun setOnlinePresence() {
        FirebaseDatabase.getInstance().reference.child(".info/connected")
        var onlineRef = FirebaseDatabase.getInstance().reference.child(".info/connected")
        var currentUserRef = FirebaseDatabase.getInstance().reference.child("/users/" + sCurrentUser?.uid + "/online");
        var lastSeenRef = FirebaseDatabase.getInstance().reference.child("/users/" + sCurrentUser?.uid + "/last_seen_online");

        onlineRef?.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(dataSnapshot: DataSnapshot) {
                Log.d("", "DataSnapshot:" + dataSnapshot)
                if (dataSnapshot.getValue(Boolean::class.java)!!) {
                    currentUserRef?.onDisconnect()?.setValue(false)
                    lastSeenRef?.onDisconnect()?.setValue(Calendar.getInstance().timeInMillis.toString())
                    currentUserRef?.setValue(true)
                }
            }

            override fun onCancelled(databaseError: DatabaseError) {
                Log.d("", "DatabaseError:" + databaseError)
            }
        })
    }