如果处理后台消息，firebase_messaging flutter 插件会在启动时崩溃

我综合了firebase_messaging v5.1.6 https://pub.dev/packages/firebase_messaging使用我的 flutter 应用程序，如自述文件中提到的“处理后台消息”选项。

这是我的文件的外观。

我的应用程序.kt

class MyApplication : FlutterApplication(), PluginRegistrantCallback {
    override fun onCreate() {
        super.onCreate()
        FlutterFirebaseMessagingService.setPluginRegistrant(this)
    }

    override fun registerWith(registry: PluginRegistry) {
        GeneratedPluginRegistrant.registerWith(registry)
    }
}

firebase_util.dart

import 'package:firebase_analytics/firebase_analytics.dart';
import 'package:firebase_analytics/observer.dart';
import 'package:firebase_messaging/firebase_messaging.dart';

final kFirebaseAnalyticsObserver =
    FirebaseAnalyticsObserver(analytics: FirebaseUtil().analytics);

Future<dynamic> kFirebaseMessagingBackgroundMessageHandler(
    Map<String, dynamic> message) async {
  if (message.containsKey('data')) {
    // Handle data message
    final dynamic data = message['data'];
  }

  if (message.containsKey('notification')) {
    // Handle notification message
    final dynamic notification = message['notification'];
  }
}

class FirebaseUtil {
  static final FirebaseUtil _firebaseUtil = FirebaseUtil._internal();

  factory FirebaseUtil() {
    return _firebaseUtil;
  }

  FirebaseUtil._internal();

  final analytics = FirebaseAnalytics();
  final messaging = FirebaseMessaging();

  void messagingInit() {
    messaging.requestNotificationPermissions();
    messaging.configure(
      onMessage: (Map<String, dynamic> message) async {
        print("onMessage: $message");
      },
      onBackgroundMessage: kFirebaseMessagingBackgroundMessageHandler,
      onLaunch: (Map<String, dynamic> message) async {
        print("onLaunch: $message");
      },
      onResume: (Map<String, dynamic> message) async {
        print("onResume: $message");
      },
    );
  }
}

I call FirebaseUtil().messagingInit() in initState根应用程序小部件的。

这是应用程序启动时收到的错误日志。

E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252): Failed to handle method call
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252): java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.app.Activity.getApplicationContext()' on a null object reference
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at com.alternadom.wifiiot.WifiIotPlugin.<init>(WifiIotPlugin.java:65)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at com.alternadom.wifiiot.WifiIotPlugin.registerWith(WifiIotPlugin.java:76)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.plugins.GeneratedPluginRegistrant.registerWith(GeneratedPluginRegistrant.java:40)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at <<package_name>>.MyApplication.registerWith(MyApplication.kt:18)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService.startBackgroundIsolate(FlutterFirebaseMessagingService.java:164)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin.onMethodCall(FirebaseMessagingPlugin.java:134)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:222)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.embedding.engine.dart.DartMessenger.handleMessageFromDart(DartMessenger.java:96)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at io.flutter.embedding.engine.FlutterJNI.handlePlatformMessage(FlutterJNI.java:656)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at android.os.MessageQueue.nativePollOnce(Native Method)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at android.os.MessageQueue.next(MessageQueue.java:326)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at android.os.Looper.loop(Looper.java:160)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at android.app.ActivityThread.main(ActivityThread.java:6669)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
E/MethodChannel#plugins.flutter.io/firebase_messaging( 6252):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
...
E/flutter ( 6252): [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: PlatformException(error, Attempt to invoke virtual method 'android.content.Context android.app.Activity.getApplicationContext()' on a null object reference, null)
E/flutter ( 6252): #0      StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:569:7)
E/flutter ( 6252): #1      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:316:33)
E/flutter ( 6252): <asynchronous suspension>
E/flutter ( 6252): #2      FirebaseMessaging.configure (package:firebase_messaging/firebase_messaging.dart:118:16)
E/flutter ( 6252): #3      FirebaseUtil.messagingInit (package:<<package_name>>/util/firebase_util.dart:37:15)
E/flutter ( 6252): #4      _MainPageState.initState (package:<<package_name>>/pages/main.dart:29:20)
E/flutter ( 6252): #5      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4068:58)
E/flutter ( 6252): #6      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3919:5)
E/flutter ( 6252): #7      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3101:14)
E/flutter ( 6252): #8      Element.updateChild (package:flutter/src/widgets/framework.dart:2904:12)
E/flutter ( 6252): #9      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3961:16)
E/flutter ( 6252): #10     Element.rebuild (package:flutter/src/widgets/framework.dart:3738:5)
E/flutter ( 6252): #11     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2348:33)
E/flutter ( 6252): #12     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:760:20)
E/flutter ( 6252): #13     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:280:5)
E/flutter ( 6252): #14     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1033:15)
E/flutter ( 6252): #15     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:975:9)
E/flutter ( 6252): #16     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:891:5)
E/flutter ( 6252): #17     _rootRun (dart:async/zone.dart:1124:13)
E/flutter ( 6252): #18     _CustomZone.run (dart:async/zone.dart:1021:19)
E/flutter ( 6252): #19     _CustomZone.runGuarded (dart:async/zone.dart:923:7)
E/flutter ( 6252): #20     _invoke (dart:ui/hooks.dart:249:10)
E/flutter ( 6252): #21     _drawFrame (dart:ui/hooks.dart:207:3)

$ 扑动医生 -v

[✓] Flutter (Channel stable, v1.9.1+hotfix.2, on Linux, locale en_IN)
    • Flutter version 1.9.1+hotfix.2 at /home/harsh/flutter
    • Framework revision 2d2a1ffec9 (3 weeks ago), 2019-09-06 18:39:49 -0700
    • Engine revision b863200c37
    • Dart version 2.5.0

 [✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /home/harsh/Android/Sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • Java binary at: /home/harsh/android-studio/jre/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[✓] Android Studio (version 3.5)
    • Android Studio at /home/harsh/android-studio
    • Flutter plugin version 39.0.3
    • Dart plugin version 191.8423
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] Connected device (1 available)
    • Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

• No issues found!

我仔细检查了README并检查了实现。我觉得不错。不知道出了什么问题？插件有什么bug吗？

如果我没有通过，这个确切的代码就可以工作onBackgroundMessage论证中FirebaseMessaging.configure method.

您正在使用的插件之一（在本例中WifiIoTPlugin) 抛出空异常。主要是因为该插件是为仅前台场景而设计的。firebase_messaging是一个也在后台工作的插件，因此这就是冲突。

一个相似的issue https://github.com/flutter/flutter/issues/26003使用时引起的android_alarm_manager and google_maps_flutter一起作为android_alarm_manager.

通过在没有活动时不注册插件可以快速解决此问题。检查这个请求请求 https://github.com/flutter/plugins/pull/1255用于解决上述问题。

导致问题的插件（仅前台插件）代码应如下所示：

public static void registerWith(Registrar registrar) {
        if (registrar.activity() == null) {
            // When a background flutter view tries to register the plugin, the registrar has no activity.
            // We stop the registration process as this plugin is foreground only.
            return;
        }
        final ForegroundPlugin foregroundPlugin = new ForegroundPlugin(registrar.activity()); 
        ...
}