如何向Dropdownbutton添加字符串

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('DropdownButton Example'),
        ),
        body: Center(
          child: DropdownButton<String>(
            items: <String>[
              'Option 1',
              'Option 2',
              'Option 3',
              'Option 4',
            ].map((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
            onChanged: (String newValue) {
              // 当用户选择一个选项时，会触发这个回调函数
              print('Selected: $newValue');
            },
          ),
        ),
      ),
    );
  }
}