如何在flutter中增加DropdownButton中文本和箭头之间的空白？

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: DropdownExample(),
        ),
      ),
    );
  }
}

class DropdownExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      value: 'Select an option',
      icon: Icon(Icons.arrow_downward),
      iconSize: 24,
      elevation: 16,
      style: TextStyle(color: Colors.deepPurple),
      underline: Container(
        height: 2,
        color: Colors.deepPurpleAccent,
      ),
      onChanged: (String newValue) {
        print(newValue);
      },
      items: <String>['Option 1', 'Option 2', 'Option 3']
          .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
      // 自定义 DropdownButton 的样式
      itemHeight: 60, // 增加选项高度
      dropdownColor: Colors.white, // 设置下拉菜单背景颜色
      buttonHeight: 60, // 增加按钮高度
      iconAllowedMargins: EdgeInsets.only(left: 20, right: 20), // 增加图标左右边距
    );
  }
}