group_listener.kt

    private fun addGroupListener() {
        val fireStore= FirebaseFirestore.getInstance()
        val userCollection= fireStore.collection("Users")
        val groupCollection= fireStore.collection("Groups")
        userCollection.document(myUserId).addSnapshotListener { snapshot, error ->
                if (error == null) {
                    val groups = snapshot?.get("groups")
                    val groupsIds =
                        if (groups == null) ArrayList() else groups as ArrayList<String>

                    for(id in groupsIds)
                        groupCollection.document(id)
                            .get().addOnSuccessListener { snapshot->
                                val group=snapshot.toObject(Group::class.java)
                                groupDao.insertGroup(group)
                            }
                }
            }
    }