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packages/dev-server/src/hmr.ts

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+/**
+ * HMR hub — tracks connected browser HMR sockets and broadcasts update events.
+ *
+ * Each open page holds one WebSocket to `/__wrnexus/hmr`. The in-process file
+ * watcher (see index.ts) classifies a change and broadcasts a typed message:
+ *
+ *   { type: "css" }     -> the browser hot-swaps the stylesheet (no reload)
+ *   { type: "reload" }  -> the browser asks for fresh HTML over the HMR socket
+ *
+ * Server-logic changes (pages/api/middleware/realtime) are NOT broadcast here:
+ * they require a fresh process, so the child exits and the supervisor respawns
+ * it. The browser then reconnects and performs a soft DOM morph automatically.
+ */
+
+export type HmrMessage = { type: "css"; version: number } | { type: "reload"; version: number };
+
+/** Minimal shape of a Bun ServerWebSocket we rely on. */
+interface Socket {
+  send(data: string): unknown;
+}
+
+export class HmrHub {
+  private sockets = new Set<Socket>();
+  private version = 0;
+
+  add(ws: Socket): void {
+    this.sockets.add(ws);
+  }
+
+  remove(ws: Socket): void {
+    this.sockets.delete(ws);
+  }
+
+  broadcast(message: HmrMessage): void {
+    const payload = JSON.stringify(message);
+    for (const ws of this.sockets) {
+      try {
+        ws.send(payload);
+      } catch {
+        this.sockets.delete(ws);
+      }
+    }
+  }
+
+  get size(): number {
+    return this.sockets.size;
+  }
+
+  css(): void {
+    this.broadcast({ type: "css", version: ++this.version });
+  }
+
+  reload(): void {
+    this.broadcast({ type: "reload", version: ++this.version });
+  }
+}