onOldMessagesReceived

Called once per batch of the topic's stored messages after subscribing.

Only messages published with PubSubPublishOptions.setPersist(true) are stored. A topic with none fires no history callback at all, so do not wait on the last-batch flag to decide a topic is empty.

Each call carries part of the history — append to what you hold rather than replacing it. PubSubHistoryInfo.isLast marks the final batch.

PubSubSubscribeOptions changes only how much history is replayed, never how it is delivered.