将文本字段大写，但仅当我在flutter中单击按钮时

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Text Field Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController _textEditingController = TextEditingController();
  String _uppercaseText = '';

  void _convertToUppercase() {
    setState(() {
      _uppercaseText = _textEditingController.text.toUpperCase();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Text Field Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextField(
              controller: _textEditingController,
              decoration: InputDecoration(
                labelText: 'Enter text',
              ),
            ),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: _convertToUppercase,
              child: Text('Convert to Uppercase'),
            ),
            SizedBox(height: 20),
            Text(
              'Uppercase Text: $_uppercaseText',
              style: TextStyle(fontSize: 18),
            ),
          ],
        ),
      ),
    );
  }
}