app.tsx

function usePrefetch() {
  return useQuery("A", () => fetchA());
}

function InnerA({isLoading, data}: ReturnType<typeof usePrefetch>) {
  useEffect(() => {
    if (data) sendEvent('fetchA', data);
  }, [data]);
  if (isLoading) return <p>Loading</p>;
  return <div> {data} <B /> </div>;
}

function A() {
  const inner = <InnerA {...usePrefetch()} />;
  return useIsPrefetch() ? <B /> : inner;
}