How do I increase/decrease counter using FirebaseUI for Android without delay?

Question

I created something similar to a subscription/like counter using Firebase Real-time Database and Firebase's cloud functions (using a transaction):

// This is a cloud function that increases subs by 1
export const onSubscriberCreate = functions.database
.ref('/channels/{$ch_id}/subscribers/{$uid}')
.onCreate((snapshot, context) => {
    const countRef = snapshot.ref.parent.parent.child('subs_count')
    return countRef.transaction(count => {
        return count + 1
    })
})

Then, I used FirebaseUI (FirebaseRecyclerAdapter) for Android to populate a RecyclerView of channels. When the user presses a channel's "Subscribe" button, his id is being sent to /channels/{$ch_id}/subscribers/ which triggers the cloud function.

However, the cloud function is really slow (about 5 secs), so I want to "fake" the update of the counter displayed to the user even before the cloud function is executed (I tried it by changing the TextView):

channelRef.child("subscribers")
    .child(user.getUid()).setValue(true)
    .addOnSuccessListener(new OnSuccessListener<Void>() {
                @Override
                public void onSuccess(Void aVoid) {
                    subsInfo.setText(channel.getSubs_count() + 1) + " SUBSCRIBERS");
                }
            });

The problem is that the channel object is being updated twice (the user id on the subscribers' list and the counter increased) so that the information of the channel is being downloaded again and binded to the ViewHolder even before the server updates the counter, so that's not a good solution.

I thought about moving the transaction code into the client, is it really necessary? Is there a better solution?

Answer 1

The best thing I feel you should do is to move the subscriber's list from inside the channel node to somewhere out. This will also make your channel object weigh lesser and you can store/update number of subscribers easily inside the channel node. Now for every user, you are downloading the entire list of users everytime you want to download a channel's information. And you don't need a cloud function to update the number of subscribers. You can do that totally on the client side using Transactions.

root/channels/{$channel}/{channelName,numberOfSubscribers,etc}
root/subscribers/{&channel}/{$userId}

This is probably how you want your data structure should be unless you really want to get all the users list. If that's the case, you can just show the size of the list of subscribers inside the TextView where you are showing the number of subscribers.