Flutter -在焦点上更改TextField中文本的颜色？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('TextField Focus Color Change')),
        body: FocusColorChangeExample(),
      ),
    );
  }
}

class FocusColorChangeExample extends StatefulWidget {
  @override
  _FocusColorChangeExampleState createState() => _FocusColorChangeExampleState();
}

class _FocusColorChangeExampleState extends State<FocusColorChangeExample> {
  final FocusNode _focusNode = FocusNode();
  Color _textColor = Colors.black;

  @override
  void initState() {
    super.initState();
    _focusNode.addListener(_updateTextColor);
  }

  @override
  void dispose() {
    _focusNode.removeListener(_updateTextColor);
    _focusNode.dispose();
    super.dispose();
  }

  void _updateTextColor() {
    setState(() {
      _textColor = _focusNode.hasFocus ? Colors.red : Colors.black;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: TextField(
        focusNode: _focusNode,
        style: TextStyle(color: _textColor),
        decoration: InputDecoration(labelText: 'Enter text'),
      ),
    );
  }
}