Behaviour for supervised, stateful PubSub consumers.

macula:subscribe_callback/4 is the right tool for a stateless one-shot reaction (log a line, forward to another process): it spawns its own receiver process so a slow callback does not back-pressure the pool, but that process is not part of your supervision tree, has no init/1 for setup, and gives you nowhere to thread state across events.

macula_subscriber is the supervised alternative. Implement three callbacks; start_link/5,6 returns an ordinary gen_server pid you drop straight into your own supervision tree, and every event arrives as a Module:handle_event/4 call against state your module owns and threads itself.

Example

   -module(temperature_logger).
   -behaviour(macula_subscriber).
   -export([init/1, handle_event/4, terminate/2]).
  
   init(_Args) -> {ok, #{count => 0}}.
  
   handle_event(_Topic, #{value := V}, _Meta, State) ->
       Count = maps:get(count, State) + 1,
       io:format("reading ~p: ~p~n", [Count, V]),
       {noreply, State#{count := Count}}.
  
   terminate(_Reason, _State) -> ok.

   {ok, Pid} = macula_subscriber:start_link(temperature_logger, Pool,
       Realm, <<"sensors.temperature_v1">>, []).