SingleValueEventExample.kt

 private val valueHandler = object: ValueEventListener {
    override fun onDataChange(snapshot: DataSnapshot?) {
        groups.value = snapshot?.children?.distinct() ?: emptyList()
        loadFinished()
    }

    override fun onCancelled(error: DatabaseError?) {
        groups.value = emptyList()
        loadFinished()
    }
}

firebase.getReference("types/$groupId").orderByChild("name").addListenerForSingleValueEvent(valueHandler)