在flutter中有没有办法在下拉列表中的一个项目中添加更多的项目？

import 'package:flutter/material.dart';

class MyDropdown extends StatefulWidget {
  @override
  _MyDropdownState createState() => _MyDropdownState();
}

class _MyDropdownState extends State<MyDropdown> {
  String _selectedItem;

  List<String> _items = [
    '项目1',
    '项目2',
    '项目3',
  ];

  @override
  Widget build(BuildContext context) {
    return DropdownButtonFormField(
      value: _selectedItem,
      items: _items.map((String item) {
        return DropdownMenuItem(
          value: item,
          child: Text(item),
        );
      }).toList(),
      onChanged: (String selectedItem) {
        setState(() {
          _selectedItem = selectedItem;
        });
      },
      decoration: InputDecoration(
        labelText: '选择项目',
      ),
    );
  }
}