如何在TextField中将占位符文本居中

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Center Placeholder Text in TextField'),
        ),
        body: Center(
          child: Padding(
            padding: EdgeInsets.all(16.0),
            child: TextFormField(
              controller: _controller,
              decoration: InputDecoration(
                labelText: 'Enter text',
                labelStyle: TextStyle(
                  fontSize: 18.0,
                  fontWeight: FontWeight.bold,
                ),
                hintText: 'Placeholder text',
                hintStyle: TextStyle(
                  fontSize: 18.0,
                  fontWeight: FontWeight.bold,
                ),
              ),
              textAlign: TextAlign.center,
            ),
          ),
        ),
      ),
    );
  }
}