Enh: Add support for Event Emitters

[juancarlosm]

When using pipelines to create agentic tasks, it will be great to update the user UI with "status" messages, like "executing function", "creating images", "buying tickets"... This can be done in open-webui function calling using "Event Emitters"

Is there any way to support this in pipeline?

[jeremykei]

Any guidance on how these emitters can be rigged on pipelines to begin with? It would appear we need to acquire an instance of the current event emitter on each invocation of pipe()...

What and where can the metadata be assembled assuming we can get an emitter instance from the backend?

[pfr-mf6]

This would be a wonderful feature. I'd love to see LangGraph agents use event emitters. I just wonder how the chat history (namely, the "thought process") will be saved. It would be nice to pull up an old chat and see the thoughts along with the messages.

[ezavesky]

Threading to #229 , #373, an examples from openweb.

It appears that you could get there via this streaming example with a little bit of work.

[mpangrazzi]

It seems that ATM the chat events like replace, status and message are handled from a websocket, which can be driven by e.g. the event emitter in Pipe(s), but too bad not in Pipelines, which instead use SSE events and supports only OpenAI-compatible message format.

It would be great if chat events can be streamed not only via internal WebSocket but via SSE as well, allowing any custom backend (and also Pipelines) to create nice custom UX while building agents.