Add docs for sse middleware

docs/middleware/README.md

@@ -25,6 +25,7 @@ To accelerate your development, the Flamego core team and the community have bui
 - [i18n](i18n.md) for providing internationalization and localization.
 - [captcha](captcha.md) for generating and validating captcha images.
 - [hcaptcha](hcaptcha.md) for providing [hCaptcha](https://www.hcaptcha.com/) verification.
+- [sse](sse.md) for providing [Server-sent events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events).
 
 ::: tip
 If you notice any middleware that is missing from the list, please don't hesitate to [send a pull request to this page](https://github.com/flamego/flamego.dev/edit/main/docs/middleware/README.md)!

docs/middleware/sse.md

@@ -0,0 +1,81 @@
+---
+prev:
+  text: Middleware
+  link: ../middleware
+---
+
+# sse
+
+The sse middleware provides [Server-sent events](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events) integration for [Flame instances](../core-concepts.md#instances).
+
+You can read source code of this middleware on [GitHub](https://github.com/flamego/sse) and API documentation on [pkg.go.dev](https://pkg.go.dev/github.com/flamego/sse?tab=doc).
+
+## Installation
+
+The minimum requirement of Go is **1.18**.
+
+```:no-line-numbers
+go get github.com/flamego/sse
+```
+
+## Usage examples
+
+:::: code-group
+::: code-group-item main.go
+```go:no-line-numbers{28}
+package main
+
+import (
+	"math/rand"
+	"net/http"
+	"time"
+
+	"github.com/flamego/flamego"
+	"github.com/flamego/sse"
+	"github.com/flamego/template"
+)
+
+var bulletins = []string{"Hello Flamego!", "Flamingo? No, Flamego!", "Most powerful routing syntax", "Slim core but limitless extensibility"}
+
+type bulletin struct {
+	Data        string
+	PublishedAt time.Time
+}
+
+func main() {
+	f := flamego.Classic()
+	f.Use(template.Templater(), flamego.Renderer())
+
+	f.Get("/", func(ctx flamego.Context, t template.Template) {
+		t.HTML(http.StatusOK, "index")
+	})
+
+	f.Get("/bulletin", sse.Bind(bulletin{}), func(msg chan<- *bulletin) {
+		for {
+			select {
+			case <-time.Tick(1 * time.Second):
+				msg <- &bulletin{
+					Data:        bulletins[rand.Intn(len(bulletins))],
+					PublishedAt: time.Now(),
+				}
+			}
+		}
+	})
+
+	f.Run()
+}
+```
+:::
+::: code-group-item templates/index.html
+```html:no-line-numbers
+<script src="https://cdn.jsdelivr.net/npm/way-js@0.2.1/dist/way.js"></script>
+<p><b><span way-data="data"></span></b>[<span way-data="published-at"></span>]</p>
+<script>
+  let es = new EventSource("/bulletin");
+  es.onmessage = (evt) => {
+    let bulletin = JSON.parse(evt.data);
+    way.set('data', bulletin.Data)
+    way.set('published-at', new Date(bulletin.PublishedAt).toLocaleString())
+  };
+</script>
+```