如何判断flutter应用在android中是否处于活动状态

import android.app.Activity;
import android.app.Application;
import android.os.Bundle;

public class MainActivity extends FlutterActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        getApplication().registerActivityLifecycleCallbacks(new Application.ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            }

            @Override
            public void onActivityStarted(Activity activity) {
            }

            @Override
            public void onActivityResumed(Activity activity) {
                // Flutter应用处于活动状态
            }

            @Override
            public void onActivityPaused(Activity activity) {
                // Flutter应用处于非活动状态
            }

            @Override
            public void onActivityStopped(Activity activity) {
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
            }
        });
    }
}

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

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  final MethodChannel _lifecycleChannel = MethodChannel('lifecycle_channel');

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    switch (state) {
      case AppLifecycleState.resumed:
        // Flutter应用处于活动状态
        _lifecycleChannel.invokeMethod('onResumed');
        break;
      case AppLifecycleState.paused:
        // Flutter应用处于非活动状态
        _lifecycleChannel.invokeMethod('onPaused');
        break;
      default:
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Flutter App'),
        ),
        body: Center(
          child: Text('Hello, Flutter!'),
        ),
      ),
    );
  }
}