onMessageDrop

Called when the SDK detects that the server did not deliver every live message of the topic, reporting how many were lost.

Fired on the main thread. Two situations produce it: the server reports a range it had to discard because this subscriber fell behind (setMaxQueue), or delivery resumes after a shed and the SDK sees the message sequence jump — which also covers messages skipped by setNewMessageLimit or by the server's own per-window ceiling. The count is coalesced: one callback may represent many messages.

Treat it as a hint that something was missed, not as a complete loss signal. Losses are only detectable once delivery resumes, so messages shed at the very end of a burst, or while the pubsub connection was reconnecting, are never reported. Under DROP the server does not track sequence numbers at all and this never fires.

To recover the missing messages, subscribe to the topic again and read its history — which works only for messages published with setPersist enabled. Default: no-op.

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