在items我是使用返回值的形式，老师您看下代码，和老师直接写在里边的执行效率有区别么？

import 'package:flutter/material.dart';
import 'package:flutter_demo/fragment/HomeFragment.dart';
import 'package:flutter_demo/fragment/MyFragment.dart';
import 'package:flutter_demo/fragment/SearchFragment.dart';
import 'package:flutter_demo/fragment/ShootFragment.dart';

class MainActivity extends StatefulWidget {
  @override
  _MainActivityState createState() => _MainActivityState();
}

class _MainActivityState extends State<MainActivity> {
  //定义显示的Page
  final List<Widget> _pages = [
    HomeFragment(),
    SearchFragment(),
    ShootFragment(),
    MyFragment()
  ];

  //定义显示的导航文本
  final List<String> _titles = ['首页', '搜索', '旅拍', '我的'];

  //定义显示的图标
  final List<IconData> _icons = [
    Icons.home,
    Icons.search,
    Icons.camera_alt,
    Icons.person
  ];

  //定义默认颜色
  final Color _defaultColor = Colors.grey; //默认颜色
  final Color _checkColor = Colors.blue; //选中颜色

  //定义默认显示页面
  int _currentPage = 0;

  //定义PageView控制器
  final PageController _controller = PageController(initialPage: 0);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: PageView(
        children: _pages,
        controller: _controller,
      ),
      bottomNavigationBar: BottomNavigationBar(
        items: _navItemLists(),
        type: BottomNavigationBarType.fixed,
        onTap: (index) {
          _controller.jumpToPage(index);
          setState(() {
            _currentPage = index;
          });
        },
      ),
    );
  }

  List<BottomNavigationBarItem> _navItemLists() {

    List<BottomNavigationBarItem> _items = [];

    _titles.asMap().forEach((i, v) {
      _items.add(
        BottomNavigationBarItem(
            title: Text(
              v,
              style: TextStyle(
                  color: _currentPage != i ? _defaultColor : _checkColor),
            ),
            icon: Icon(
              _icons[i],
              color: _currentPage != i ? _defaultColor : _checkColor,
            ),
            activeIcon: Icon(
              _icons[i],
              color: _currentPage != i ? _defaultColor : _checkColor,
            )),
      );
    });
    return _items;
  }
}