A struct storing the liveliness changed status.
The total number of currently active publishers that write the topic read by the subscriber.
This count increases when a newly matched publisher asserts its liveliness for the first time or when a publisher previously considered to be not alive reasserts its liveliness. The count decreases when a publisher considered alive fails to assert its liveliness and becomes not alive, whether because it was deleted normally or for some other reason
The total count of current publishers that write the topic read by the subscriber that are no longer asserting their liveliness.
This count increases when a publisher considered alive fails to assert its liveliness and becomes not alive for some reason other than the normal deletion of that publisher. It decreases when a previously not alive publisher either reasserts its liveliness or is deleted normally
The change in the alive_count since the last time the listener was called or the status was read.
The change in the not_alive_count since the last time the listener was called or the status was read.