change: Move WebSocketAdvertiser to middleware.

src/server/middleware/WebSocketAdvertiser.ts

@@ -0,0 +1,31 @@
+import { addHeader } from '../../util/HeaderUtil';
+import { HttpHandler } from '../HttpHandler';
+import type { HttpResponse } from '../HttpResponse';
+
+interface WebSocketSettings {
+  hostname?: string;
+  port?: number;
+  protocol?: string;
+}
+
+/**
+ * Handler that advertises a WebSocket through the Updates-Via header.
+ */
+export class WebSocketAdvertiser extends HttpHandler {
+  private readonly socketUrl: string;
+
+  public constructor(settings: WebSocketSettings = {}) {
+    super();
+    const { hostname = 'localhost', port = 80, protocol = 'ws:' } = settings;
+    const secure = /^(?:https|wss)/u.test(protocol);
+    const socketUrl = new URL(`${secure ? 'wss' : 'ws'}://${hostname}:${port}/`);
+    if (socketUrl.hostname !== hostname) {
+      throw new Error(`Invalid hostname: ${hostname}`);
+    }
+    this.socketUrl = socketUrl.href.slice(0, -1);
+  }
+
+  public async handle({ response }: { response: HttpResponse }): Promise<void> {
+    addHeader(response, 'updates-via', this.socketUrl);
+  }
+}