如何在Flutter中监听platformBrightness的变化？

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

Brightness _currentBrightness;

@override
void initState() {
  super.initState();
  _currentBrightness = MediaQuery.of(context).platformBrightness;
  SystemChannels.platform.invokeMethod('SystemNavigator.userAgent');
  SystemChannels.lifecycle.setMessageHandler((msg) {
    if (msg == AppLifecycleState.resumed.toString()) {
      _handleBrightnessChange();
    }
    return null;
  });
}

void _handleBrightnessChange() {
  setState(() {
    _currentBrightness = MediaQuery.of(context).platformBrightness;
  });
  // 在这里可以根据亮度模式进行相应的操作或调用相关函数
}

@override
Widget build(BuildContext context) {
  return MaterialApp(
    theme: _currentBrightness == Brightness.light ? lightTheme : darkTheme,
    home: Scaffold(
      body: Center(
        child: Text(
          _currentBrightness == Brightness.light ? 'Light Mode' : 'Dark Mode',
          style: TextStyle(fontSize: 20),
        ),
      ),
    ),
  );
}