您应该何时使用 useMemo 和 useCallback，它们解决了什么问题？

function Table({ rows, onSelect }) {
  // (a) expensive computation — recompute only when `rows` changes
  const sorted = useMemo(() => rows.slice().sort(byName), [rows]);

  // (b) stable function identity — so a React.memo child doesn't re-render,
  //     and effects depending on it don't re-fire
  const handleSelect = useCallback(id => onSelect(id), [onSelect]);

  return <Grid rows={sorted} onSelect={handleSelect} />;
}

const x = useMemo(() => a + b, [a, b]); // ❌ pointless — adding is cheaper than memoizing