http4s + scalatags + htmx + fs2/SSE

main.scala

//> using scala 3.7
//> using dep org.http4s::http4s-ember-server:0.23.30
//> using dep org.http4s::http4s-dsl:0.23.30
//> using dep org.http4s::http4s-scalatags::0.25.2
//> using option -Wunused:all
import scalatags.Text.all.*
import cats.effect.*
import fs2.Stream
import org.http4s.*
import org.http4s.dsl.io.*
import org.http4s.ember.server.*
import org.http4s.implicits.*
import org.http4s.scalatags.*

import scala.concurrent.duration.{span as _, *}

object SSEApp extends IOApp.Simple:

  val indexPage = html(
    meta(charset := "utf-8"),
    head(
      script(src := "https://unpkg.com/[email protected]"),
      script(src := "https://unpkg.com/[email protected]"),
    ),
    body(
      h1("http4s/fs2 ❤️ HTMX + SSE"),
      div(
        id := "random-box",
        p(
          "Your UUID: ",
          span(
            attr("hx-ext") := "sse",
            attr("sse-connect") := "/events",
            attr("sse-swap") := "message",
          ),
        ),
      ),
    ),
  )

  val routes = HttpRoutes.of[IO] {
    case GET -> Root => Ok(indexPage)

    case GET -> Root / "events" =>
      Ok(Stream.fixedRateStartImmediately[IO](100.millis).evalMap { _ =>
        IO.randomUUID.map { uuid =>
          ServerSentEvent(data = Some(uuid.toString))
        }
      })
  }

  override def run: IO[Unit] =
    EmberServerBuilder
      .default[IO]
      .withHttpApp(routes.orNotFound)
      .build
      .evalTap { server =>
        IO.println(
          s"Server running at ${server.baseUri}"
        )
      }
      .useForever