Add writeSSE / stream at the Context level for increased DX with SSEs

[mirshko]

What is the feature you are proposing?

I opened this discussion a while back (https://github.com/orgs/honojs/discussions/3472) to see if there were ways around the current streamSSE while loop for callback based SSE sending.

For a project I'm working on the SDK requires a onBlock callback that is called when new data comes in, to which I want to be able to send that data to a client with SSEs.

See:

app.get("/sse", async (c) => {
  const target = c.sendEvents();

  const unwatch = publicClient.watchBlocks( 
    { onBlock: block => target.dispatchMessage(block); }
  )
  
  target.addEventListener("close", (evt) => {
    unwatch()
  });
  
  // ...
})

This model is heavily based on the Oak approach seen here: https://oakserver.github.io/oak/sse.html.

If we could have a similar api approach for a Hono ServerSentEventTarget baked into Context, that would open this connection and keep it open until otherwise closed (in a client disconnect event) that would be ideal.

I couldn't find a way to implement my above code with the current streamSSE since it requires a while loop to remain open.

@maou-shonen mentioned https://github.com/honojs/middleware/tree/main/packages/event-emitter however, it only works for inside the app, but its API is very ideal I would imagine for this feature.