on_deliver_hook.erl

%% @hidden
-module(on_deliver_hook).
-include("vernemq_dev_int.hrl").

-callback on_deliver(UserName      :: username(),
                     SubscriberId  :: subscriber_id(),
                     QoS           :: qos(),
                     Topic         :: topic(),
                     Payload       :: payload(),
                     IsRetain      :: flag(),
                     Properties    :: deliver_properties()) ->
                                                   ok
                                                 | {ok, Payload    :: payload()}
                                                 | {ok, Modifiers  :: [msg_modifier()]}
                                                 | next.

-type deliver_properties() ::
        #{
          p_payload_format_indicator => unspecified | utf8,
          p_message_expiry_interval => seconds(),
          p_topic_alias => 1..65535,
          p_response_topic => topic(),
          p_correlation_data => binary(),
          p_user_property => nonempty_list(user_property()),
          p_subscription_id => [subscription_id()],
          p_content_type => utf8string()
         }.

-type msg_modifier() ::
        %% Rewrite the topic of the message.
        {topic, topic()}

        %% Rewrite the payload of the message.
      | {payload, payload()}.


-export_type([msg_modifier/0]).