fix(splash): reuse latest chat message preview

src/app/chat/hooks/use-splash-latest-message-sync.ts

@@ -0,0 +1,53 @@
+"use client";
+
+import { useEffect, useRef } from "react";
+
+import type { LoginStatus } from "@/data/dto/auth";
+import type { UiMessage } from "@/data/dto/chat";
+import { resolveSplashLatestMessageCacheIdentity } from "@/lib/chat/splash_latest_message";
+import { getLatestSplashMessagePreview } from "@/lib/chat/splash_latest_message_preview";
+import { useSplashLatestMessageCache } from "@/providers/splash-latest-message-provider";
+
+export interface UseSplashLatestMessageSyncInput {
+  historyLoaded: boolean;
+  loginStatus: LoginStatus;
+  messages: readonly UiMessage[];
+}
+
+export function useSplashLatestMessageSync({
+  historyLoaded,
+  loginStatus,
+  messages,
+}: UseSplashLatestMessageSyncInput): void {
+  const cache = useSplashLatestMessageCache();
+  const hasObservedInitialSnapshotRef = useRef(false);
+  const identityPromiseRef = useRef<Promise<string | null> | null>(null);
+
+  useEffect(() => {
+    hasObservedInitialSnapshotRef.current = false;
+    identityPromiseRef.current = null;
+  }, [loginStatus]);
+
+  useEffect(() => {
+    if (!historyLoaded) return;
+
+    if (!hasObservedInitialSnapshotRef.current) {
+      hasObservedInitialSnapshotRef.current = true;
+      return;
+    }
+
+    const preview = getLatestSplashMessagePreview(messages);
+    let cancelled = false;
+
+    identityPromiseRef.current ??= resolveSplashLatestMessageCacheIdentity();
+    void identityPromiseRef.current.then((identity) => {
+      if (!cancelled && identity) {
+        cache.set(identity, preview);
+      }
+    });
+
+    return () => {
+      cancelled = true;
+    };
+  }, [cache, historyLoaded, messages]);
+}