[docs] Worthwhile mentioning how to use with subscribeToMore? (+ possible caveats)

[s-h-a-d-o-w]

I was scratching my head for quite a while about the behavior of this (based on the full example) compared to simply publishing directly on mutations with something like graphql-yoga's pubsub.

Seeing how I suppose neither using the apollo-client, nor subscribeToMore is particularly rare, I wondered whether it would be worth mentioning in the README that if one uses it, one has to skip broadcasting the initial snapshot. (I keep track of it with an isInitialSnapshot in the handler.)

This has also made me realize that in a scenario where you have e.g. 1000 documents in that collection and hundreds of people per hour loading up the page, that's a lot of unnecessary load and traffic. Maybe this would also be good to point out?

[psteinroe]

I struggled with the same. One key information is that registerHandler gets called every time someone subscribes.

This is how it works for me:

ps.registerHandler(events.MESSAGE_ADDED, broadcast => {
  let isInitialSnapshot = true;
  firestore.collectionGroup('Messages').onSnapshot(snapshot => {
    if (isInitialSnapshot) {
      isInitialSnapshot = false;
      return;
    }
    snapshot
      .docChanges()
      .filter(change => change.type === 'added')
      .map(async item => {
        return broadcast({
          chatId: item.doc.ref.parent.parent.id,
          message: item.doc.data(),
        });
      });
  });
});