on
Called once per live batch with all of its messages, when the server has batching on for the room. It is a server-side setting, not a subscribe option, so a build must work with and without it.
This does not replace per-message delivery: every message in the batch also arrives on onMessageReceived. Handle the messages in one callback or the other, never both, or you will count each message twice.
Override it to amortise per-message work — one database transaction or analytics flush per batch instead of one per message.