如何在Flutter中更改` `BottomNavigationBar()`中项目之间的间距？

import 'package:flutter/material.dart';

class CustomBottomNavigationBar extends BottomNavigationBar {
  CustomBottomNavigationBar({
    Key key,
    @required List<BottomNavigationBarItem> items,
    int currentIndex = 0,
    ValueChanged<int> onTap,
    Color backgroundColor,
    double elevation = 8.0,
    BottomNavigationBarType type,
    Color fixedColor,
    IconThemeData iconTheme,
    TextStyle selectedItemStyle,
    double iconSize = 24.0,
    bool showSelectedLabels = true,
    bool showUnselectedLabels = true,
  }) : super(
          key: key,
          items: items,
          currentIndex: currentIndex,
          onTap: onTap,
          backgroundColor: backgroundColor,
          elevation: elevation,
          type: type,
          fixedColor: fixedColor,
          iconTheme: iconTheme,
          selectedItemColor: selectedItemStyle?.color,
          unselectedItemColor: selectedItemStyle?.color,
          selectedFontSize: selectedItemStyle?.fontSize,
          unselectedFontSize: selectedItemStyle?.fontSize,
          iconSize: iconSize,
          showSelectedLabels: showSelectedLabels,
          showUnselectedLabels: showUnselectedLabels,
        );

  @override
  Widget build(BuildContext context) {
    final BottomNavigationBar parent = super.build(context);
    return Padding(
      padding: EdgeInsets.symmetric(vertical: 8.0), // 调整垂直间距
      child: parent,
    );
  }
}

CustomBottomNavigationBar(
  items: [
    BottomNavigationBarItem(
      icon: Icon(Icons.home),
      label: 'Home',
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.search),
      label: 'Search',
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.person),
      label: 'Profile',
    ),
  ],
  currentIndex: _selectedIndex,
  onTap: _onItemTapped,
)