FEATURE: Swap HTML on Server Sent Events (take 5)

[benpate]

This PR implements the new SSE syntax proposed by @1cg, and fixes my previous PR that was built upon the wrong base branch I'll briefly summarize his spec here:

Swapping on Named Events

This PR adds a new swap on syntax to the hx-sse attribue that can be used to swap content into the DOM for any named SSE event.

  <div hx-sse="connect /event_stream">
    <div hx-sse="swap on chatter">
      ...
    </div>
  </div>

The swap on indicates that this element wants to listen for chatter messages and swap the body when they occur.

The hx-swap attribute can then be used to determine exactly how the swap is done, as with normal swaps.

Data-only Messages

In the case of a data-only message, the message type is message according to the SSE spec. Thus, the form swap on message should work the same as the named event implementation.

  <div hx-sse="connect /event_stream">
    <div hx-sse="swap on message">
      ...
    </div>
  </div>

Declaring and Using An SSE Endpoint On The Same Element

This PR supports both connecting and listening for an event (or events) in the same hx-sse declaration, by
allowing comma separated values:

  <div hx-sse="connect /event_stream, swap on message">
     ...
  </div>

Leave Triggers In Place

This PR does not change the way that SSE can be used to trigger additional events in HTMX. So the following, original code still works:

  <div hx-sse="connect /event_stream">
    <div hx-get="/chatroom" hx-trigger="sse:chatter">
      ...
    </div>
  </div>

Links

1. MDN SSE Article - https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events
2. SSE Spec - https://html.spec.whatwg.org/multipage/server-sent-events.html
3. Initial SSE improvement PR - FEATURE: Swap HTML on Server Sent Events (like WebSockets) #155
4. Second SSE improvement PR - FEATURE: Swap HTML on Server Sent Events (like WebSockets) improved version #185
5. Updated SSE syntax proposal - https://gist.github.com/1cg/5dd26530aeec92a9389b367747bbb5ee

[1cg]

OK, merged!

How about some docs?

[benpate]

Sure. I'll put those together tomorrow.