abstract class FirestoreAdapter<VH : RecyclerView.ViewHolder>(
    private val query: Query
) : RecyclerView.Adapter<VH>(), EventListener<QuerySnapshot> {
    // Override class members here.
}

override fun onEvent(
    documentSnapshots: QuerySnapshot?,
    exception: FirebaseFirestoreException?
) {
    // Dispatch the event
    for (change in documentSnapshots!!.documentChanges) {
        // Snapshot of the changed document
        when (change.type) {
            // New Document was added to set of documents matching query.
            DocumentChange.Type.ADDED -> onDocumentAdded(change)
            // New document within the query was modified.
            DocumentChange.Type.MODIFIED -> onDocumentModified(change)
            // Removed, deleted, no longer matches the query.
            DocumentChange.Type.REMOVED -> onDocumentRemoved(change)
        }
    }
}

private val snapshots = ArrayList<DocumentSnapshot>()

protected open fun onDocumentAdded(change: DocumentChange) {
    snapshots.add(change.newIndex, change.document)
    notifyItemInserted(change.newIndex)
}

protected open fun onDocumentModified(change: DocumentChange) {
    if (change.oldIndex == change.newIndex) {
        // Item changed but remained in same position
        snapshots[change.oldIndex] = change.document
        notifyItemChanged(change.oldIndex)
    } else {
        // Item changed and changed position
        snapshots.removeAt(change.oldIndex)
        snapshots.add(change.newIndex, change.document)
        notifyItemMoved(change.oldIndex, change.newIndex)
    }
}

protected open fun onDocumentRemoved(change: DocumentChange) {
    snapshots.removeAt(change.oldIndex)
    notifyItemRemoved(change.oldIndex)
}

var registration: ListenerRegistration? = null

open fun startListening() {
    if (registration == null) {
        registration = query.addSnapshotListener(this)
    }
}

open fun stopListening() {
    if (registration != null) {
        registration!!.remove()
        registration = null
    }

    snapshots.clear()
}

open fun startListening() {
    if (registration == null) {
        registration = query.addSnapshotListener(this)
    }
}

open fun stopListening() {
    if (registration != null) {
        registration!!.remove()
        registration = null
    }

    snapshots.clear()
}