格式化我的TextField中的Binding<String>

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class MyFormPage extends StatefulWidget {
  @override
  _MyFormPageState createState() => _MyFormPageState();
}

class _MyFormPageState extends State<MyFormPage> {
  TextEditingController _controller = TextEditingController();
  final _formatter = FilteringTextInputFormatter.digitsOnly; // 仅允许输入数字

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Form Page'),
      ),
      body: Center(
        child: TextField(
          controller: _controller,
          inputFormatters: [_formatter],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          String formattedText = _controller.text.replaceAllMapped(
              RegExp(r'\B(?=(\d{3})+(?!\d))'), ','); // 每三位添加一个逗号
          _controller.text = formattedText;
        },
        child: Icon(Icons.format_align_left),
      ),
    );
  }
}