Implement EventStream for live schedule updates

app/api/events/route.ts

@@ -0,0 +1,69 @@
+import { Schedule } from "@/app/schedule/types";
+
+function sendMessage(
+	stream: WritableStream<string>,
+	message: string,
+) {
+	const writer = stream.getWriter();
+	writer.ready
+		.then(() => writer.write(message))
+		.catch(console.error)
+		.finally(() => writer.releaseLock())
+	;
+}
+
+let streams = new Set<WritableStream<string>>();
+let lastBroadcastData: string | null = null;
+let lastBroadcastId = 0;
+export function broadcastUpdate(schedule: Schedule) {
+	const id = Date.now();
+	const data = JSON.stringify(schedule);
+	lastBroadcastId = id;
+	lastBroadcastData = data;
+	const message = `id: ${id}\nevent: update\ndata: ${data}\n\n`
+	for (const stream of streams) {
+		sendMessage(stream, message);
+	}
+}
+
+setInterval(() => {
+	for (const stream of streams) {
+		sendMessage(stream, "data: keepalive\n\n");
+	}
+}, 10e3)
+
+export async function GET(request: Request) {
+	const encoder = new TextEncoder();
+	const source = request.headers.get("x-forwarded-for");
+	console.log(`starting event stream for ${source}`)
+	const stream = new TransformStream<string, Uint8Array>({
+		transform(chunk, controller) {
+			controller.enqueue(encoder.encode(chunk));
+		},
+		flush(controller) {
+			console.log(`finished event stream for ${source}`);
+			streams.delete(stream.writable);
+		},
+		// @ts-expect-error experimental API
+		cancel(reason) {
+			console.log(`cancelled event stream for ${source}`);
+			streams.delete(stream.writable);
+		}
+	})
+	streams.add(stream.writable);
+	if (lastBroadcastId) {
+		sendMessage(
+			stream.writable,
+			`id: ${lastBroadcastData}\nevent: update\ndata: ${lastBroadcastData}\n\n`
+		);
+	}
+	return new Response(
+		stream.readable,
+		{
+			headers: {
+				"Access-Control-Allow-Origin": "*",
+				"Content-Type": "text/event-stream",
+			}
+		}
+	);
+}