Flutter -如何在文本字段中为每个字符加下划线？

import 'package:flutter/material.dart';

class UnderlineTextField extends StatefulWidget {
  @override
  _UnderlineTextFieldState createState() => _UnderlineTextFieldState();
}

class _UnderlineTextFieldState extends State<UnderlineTextField> {
  String _inputText = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Underline TextField"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Stack(
              alignment: Alignment.bottomCenter,
              children: [
                TextField(
                  onChanged: (value) {
                    setState(() {
                      _inputText = value;
                    });
                  },
                ),
                RichText(
                  text: TextSpan(
                    children: List.generate(
                      _inputText.length,
                      (index) => TextSpan(
                        text: _inputText[index],
                        style: TextStyle(
                          decoration: TextDecoration.underline,
                        ),
                      ),
                    ),
                  ),
                ),
              ],
            ),
            SizedBox(height: 20),
            Text("输入的文本：$_inputText"),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: UnderlineTextField(),
  ));
}