Server-Sent Events (SSE) encoding helpers.

Pairs naturally with the {loop, ...} handler return: subscribe to a pubsub topic in handle/1, then call roadrunner_sse:event/1,2,3 from handle_info/3 to format each event before emitting it via the Push fun.

Example handler:

-behaviour(roadrunner_handler).
-export([handle/1, handle_info/3]).

handle(Req) ->
    ok = my_pubsub:subscribe(self(), notifications),
    Headers = [
        {~"content-type", ~"text/event-stream"},
        {~"cache-control", ~"no-cache"}
    ],
    {{loop, 200, Headers, undefined}, Req}.

handle_info({notification, Body}, Push, State) ->
    _ = Push(roadrunner_sse:event(~"notify", Body)),
    {ok, State};
handle_info(close, Push, State) ->
    _ = Push(roadrunner_sse:comment(~"bye")),
    {stop, State}.

Per the SSE spec (WHATWG HTML §9.2):

• Each event ends with a blank line (\n\n).
• data: may repeat — newlines in the payload split it into multiple data: lines, all reassembled into one event by the client.
• event: is optional; an unnamed event dispatches as the generic message event in the browser.
• id: lets the client resume from the last event seen.
• retry: N tells the client to wait N milliseconds before reconnecting.
• A line starting with : is a comment — useful for keep-alives over proxies that close idle connections.