我的textFormField在左边插入新值，如何只在右边插入

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  final TextEditingController _textEditingController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextFormField(
              controller: _textEditingController,
              decoration: InputDecoration(
                labelText: '输入框',
              ),
            ),
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            String currentValue = _textEditingController.text;
            String newValue = '新值';

            String updatedValue = currentValue + newValue;
            _textEditingController.text = updatedValue;
            _textEditingController.notifyListeners();
          },
          child: Icon(Icons.add),
        ),
      ),
    );
  }
}