Flutter:如何通过按钮以编程方式更改文本字段中文本的对齐方式？

import 'package:flutter/material.dart';

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

class _MyHomePageState extends State<MyHomePage> {
  TextAlign _textAlign = TextAlign.left;

  void _changeTextAlignment() {
    setState(() {
      _textAlign = _textAlign == TextAlign.left ? TextAlign.right : TextAlign.left;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Change Text Alignment'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextFormField(
              textAlign: _textAlign,
              decoration: InputDecoration(
                labelText: 'Enter text',
              ),
            ),
            SizedBox(height: 20),
            RaisedButton(
              onPressed: _changeTextAlignment,
              child: Text('Change Alignment'),
            ),
          ],
        ),
      ),
    );
  }
}

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