Add world delta streaming and viewer smoothing

apps/viewer/src/api.ts

@@ -1,4 +1,4 @@
-import type { WorldSnapshot } from "./contracts";
+import type { WorldDelta, WorldSnapshot } from "./contracts";
 
 export async function fetchWorldSnapshot(signal?: AbortSignal) {
   const response = await fetch("/api/world", { signal });
@@ -8,6 +8,24 @@ export async function fetchWorldSnapshot(signal?: AbortSignal) {
   return response.json() as Promise<WorldSnapshot>;
 }
 
+export function openWorldStream(
+  afterSequence: number,
+  handlers: {
+    onDelta: (delta: WorldDelta) => void;
+    onOpen?: () => void;
+    onError?: () => void;
+  },
+) {
+  const stream = new EventSource(`/api/world/stream?afterSequence=${afterSequence}`);
+  stream.addEventListener("open", () => handlers.onOpen?.());
+  stream.addEventListener("error", () => handlers.onError?.());
+  stream.addEventListener("world-delta", (event) => {
+    const message = event as MessageEvent<string>;
+    handlers.onDelta(JSON.parse(message.data) as WorldDelta);
+  });
+  return stream;
+}
+
 export async function resetWorld() {
   const response = await fetch("/api/world/reset", {
     method: "POST",