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Middleware
sse

sse

The sse middleware provides Server-Sent Events for sending updates from Flame instances to web clients over an HTTP connection.

You can read source code of this middleware on GitHub and API documentation on pkg.go.dev.

Installation 

go get github.com/flamego/sse

Usage examples

The sse.Bind middleware configures the response as an event stream and injects a typed, send-only channel into the next handler. Pass a non-pointer value to Bind; a value of type T makes a chan<- *T available to the handler. Values sent through the channel are encoded as JSON event data.

The following example sends the current time to each connected client once per second:

package main

import (
	"time"

	"github.com/flamego/flamego"
	"github.com/flamego/sse"
)

type update struct {
	Time time.Time `json:"time"`
}

func main() {
	f := flamego.Classic()
	f.Get("/events", sse.Bind(update{}), func(c flamego.Context, events chan<- *update) {
		ticker := time.NewTicker(time.Second)
		defer ticker.Stop()

		for {
			select {
			case <-c.Request().Context().Done():
				return
			case now := <-ticker.C:
				select {
				case events <- &update{Time: now}:
				case <-c.Request().Context().Done():
					return
				}
			}
		}
	})
	f.Run()
}

Clients can consume the stream with the browser’s EventSource API:

<p id="time"></p>
<script>
  const events = new EventSource("/events");
  events.onmessage = (event) => {
    const update = JSON.parse(event.data);
    document.querySelector("#time").textContent = update.time;
  };
</script>

The route handler remains active for the lifetime of the connection. Always observe c.Request().Context().Done() and stop timers or other resources when the client disconnects.

The middleware periodically sends a ping to keep the connection alive. The default interval is 10 seconds and can be changed with sse.Options:

f.Get("/events",
	sse.Bind(update{}, sse.Options{
		PingInterval: 30 * time.Second,
	}),
	func(c flamego.Context, events chan<- *update) {
		// Produce events until the request context is canceled.
	},
)
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