如何在Flutter中的TextForm上添加上标或下标字符串？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo'),
        ),
        body: Center(
          child: Padding(
            padding: EdgeInsets.all(16.0),
            child: TextFormField(
              decoration: InputDecoration(
                labelText: '示例',
              ),
              validator: (value) {
                if (value.isEmpty) {
                  return '请输入内容';
                }
                return null;
              },
              onChanged: (value) {
                // do something
              },
              keyboardType: TextInputType.text,
              // 在TextFormField中添加上标或下标字符串
              inputFormatters: [
                // 上标字符串
                SubscriptFormatter(),
                // 下标字符串
                SuperscriptFormatter(),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

// 上标字符串样式
class SuperscriptFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    String text = newValue.text;
    String newText = '';

    for (int i = 0; i < text.length; i++) {
      newText += String.fromCharCode(text.codeUnitAt(i) + 8304);
    }

    return TextEditingValue(
      text: newText,
      selection: newValue.selection,
      composing: newValue.composing,
    );
  }
}

// 下标字符串样式
class SubscriptFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
      TextEditingValue oldValue, TextEditingValue newValue) {
    String text = newValue.text;
    String newText = '';

    for (int i = 0; i < text.length; i++) {
      newText += String.fromCharCode(text.codeUnitAt(i) + 8320);
    }

    return TextEditingValue(
      text: newText,
      selection: newValue.selection,
      composing: newValue.composing,
    );
  }
}