Allow passing raw string (without dumping) to `ServerSentEvent`

[netanel-haber]

🚀 Feature Request
Hey! It would help me if I could pass a string to ServerSentEvent that won't be dumped but will be sent as is [so it won't be wrapped in quotes) - right now my workaround is creating a special class for said strings so I can hardcodingly return a raw string via overriding JsonSettings.dumps.

[netanel-haber]

Can I try contributing a PR to allow this?

[BennyThink]

I would love this feature!

[RobertoPrevato]

Hi @netanel-haber and @BennyThink
Sorry for replying so late. I needed a vacation from GitHub for private reasons.
You had a very good point and a PR would have been welcomed here. But since so much time passed, I implemented this feature today and I am handling this PR.

I won't release today a new version, but it will be soon anyway.

Example (I just tested locally with https://github.com/Neoteroi/BlackSheep-Examples/tree/main/server-sent-events):

import asyncio
import os
from collections.abc import AsyncIterable

from blacksheep import Application, Request, get
from blacksheep.server.process import is_stopping
from blacksheep.server.sse import TextServerSentEvent  # <---

app = Application(show_error_details=True)
app.serve_files("static")


# Enable the signal handler to detect when the application is stopping.
os.environ["APP_SIGNAL_HANDLER"] = "1"


@get("/events")
async def events_handler(request: Request) -> AsyncIterable[TextServerSentEvent]:  # <---
    i = 0

    while True:
        if await request.is_disconnected():
            print("The request is disconnected!")
            break

        if is_stopping():
            print("The application is stopping!")
            break

        i += 1
        yield TextServerSentEvent(f"Hello World {i}")  # <---

        try:
            await asyncio.sleep(1)
        except asyncio.exceptions.CancelledError:
            break


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app, port=44777)

[netanel-haber]

Thanks ❤️

[RobertoPrevato]

You're welcome :)
I still have some doubts on whether to leave a built-in implementation to serve plain text, though. Because serving plain text using server-sent events has an intrinsic foot-gun, as \n\n is used to communicate the end of data. The foot-gun is described, for instance, in this article.
I would never use that option, unless it was strictly required by some external component I cannot control.

[UPDATE]

I will include automatic escape of \r\n in the TextServerSentEvent class (the same happens when using JSON). This ensures that messages don't break the SSE encoding. It also requires handling escaped new lines on the client. Example:

example

(function () {
    var eventSource = new EventSource("/events");

    eventSource.onmessage = function(event) {
        console.log(event);
        var element = document.getElementById("message");
        element.innerHTML = event.data.replace(/\\n/g, "<br />"); // <---------
        console.log("Data:", event.data);
    }

    eventSource.onerror = function(err) {
        console.error("EventSource failed:", err);
    }
})()

[herumes]

You're welcome :)
I still have some doubts on whether to leave a built-in implementation to serve plain text, though. Because serving plain text using server-sent events has an intrinsic foot-gun, as \n\n is used to communicate the end of data. The foot-gun is described, for instance, in this article.
I would never use that option, unless it was strictly required by some external component I cannot control.

[UPDATE]

I will include automatic escape of \r\n in the TextServerSentEvent class (the same happens when using JSON). This ensures that messages don't break the SSE encoding. It also requires handling escaped new lines on the client. Example:

example

(function () {
    var eventSource = new EventSource("/events");

    eventSource.onmessage = function(event) {
        console.log(event);
        var element = document.getElementById("message");
        element.innerHTML = event.data.replace(/\\n/g, "<br />"); // <---------
        console.log("Data:", event.data);
    }

    eventSource.onerror = function(err) {
        console.error("EventSource failed:", err);
    }
})()

Can I ask why \r\n\r\n rather than \n\n? Isn't SSE specifications typically \n\n (which if not done correctly leaves room for possibility of failed parsing from stricter SSE processing)? Please correct me if I'm wrong! Thanks, for SSE fixes and all of your work. BlackSheep is great.

[RobertoPrevato]

Hi @herumes
Thank you for your kind words! You are right, the code that writes SSE events should use \n instead of \r\n.
I will include this fix in the next release. I create a dedicated issue for this.