feat: Advertise WebSocket via Updates-Via header.

src/ldp/http/metadata/WebSocketMetadataWriter.ts

@@ -0,0 +1,25 @@
+import type { HttpResponse } from '../../../server/HttpResponse';
+import { addHeader } from '../../../util/HeaderUtil';
+import { MetadataWriter } from './MetadataWriter';
+
+/**
+ * A {@link MetadataWriter} that advertises a WebSocket through the Updates-Via header.
+ */
+export class WebSocketMetadataWriter extends MetadataWriter {
+  private readonly socketUrl: string;
+
+  public constructor({ hostname = 'localhost', port = 80, protocol = 'ws:' }:
+  { hostname?: string; port?: number; protocol?: string }) {
+    super();
+    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);
+  }
+}