Flutter- FCM A background message could not be handled in Dart as no onBackgroundMessage handler has been registered

Question

I wanted to handle the background message of the FCM in the Flutter, but I got an error when FCM sent a message to the app in the background mode and made an error in the log.

pubspec.yaml:

firebase_core: ^0.7.0
firebase_messaging: ^8.0.0-dev.14

MainActivity.kt :

package com.mydomain.myproject

import io.flutter.embedding.android.FlutterActivity

class MainActivity: FlutterActivity() {
}

Application.kt :

package com.mydomain.myproject

import com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin
import io.flutter.app.FlutterApplication
import io.flutter.plugin.common.PluginRegistry
import io.flutter.plugin.common.PluginRegistry.PluginRegistrantCallback
import io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingBackgroundExecutor
import io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingBackgroundService
import io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingPlugin
import io.flutter.plugins.pathprovider.PathProviderPlugin


class MyApplication : FlutterApplication(), PluginRegistrantCallback {

    override
    fun onCreate() {
        super.onCreate()
        FlutterFirebaseMessagingBackgroundService.setPluginRegistrant(this)
        FlutterFirebaseMessagingBackgroundExecutor.setPluginRegistrant(this)
    }

    override
    fun registerWith(registry: PluginRegistry) {
        PathProviderPlugin.registerWith(registry.registrarFor("io.flutter.plugins.pathprovider.PathProviderPlugin"))
        FlutterLocalNotificationsPlugin.registerWith(registry.registrarFor("com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin"))
        FlutterFirebaseMessagingPlugin.registerWith(registry.registrarFor("plugins.flutter.io/firebase_messaging"))
    }
}

Code of using fcm: I just called the _initFcm() in the initState() of the main app class. My purpose is when the app is in the background, the background FCM method must trigger, and I must save data of message in the preferences to do something with that when the app opens.

void _initFcm() {
    Firebase.initializeApp();
    FirebaseMessaging.onMessageOpenedApp.listen((message) {
      Logger.log("_messaging onMessageOpenedApp: ${message}");
    });
    FirebaseMessaging.instance.getInitialMessage().then((value) {
      Logger.log("_messaging getInitialMessage: ${value}");
    });
    FirebaseMessaging.onMessage.listen((RemoteMessage message) {
      RemoteNotification notification = message.notification;
      Map<String, dynamic> data = message.data;
      Logger.log("_messaging onMessage: ${message}");
      Fcm.showNotification(notification.body, notification.title);
      String type = data['type'];
      if (type == "view") {
        String notifPath = data['subject'];
        notificationProvider.addPath(notifPath);
        Logger.log('new notification added to notificationList: ${notifPath}');
      }
    });
    FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
      Logger.log("_messaging onMessageOpenedApp: $message");
    });
    FirebaseMessaging.onBackgroundMessage((message) {
      Logger.log("_messaging onBackgroundMessage: $message");
      return;
    });
  }

Run log:

[ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'toRawHandle' was called on null.
E/flutter ( 5030): Receiver: null
E/flutter ( 5030): Tried calling: toRawHandle()
E/flutter ( 5030): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
E/flutter ( 5030): #1      MethodChannelFirebaseMessaging.registerBackgroundMessageHandler (package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart:181:42)
E/flutter ( 5030): #2      FirebaseMessagingPlatform.onBackgroundMessage= (package:firebase_messaging_platform_interface/src/platform_interface/platform_interface_messaging.dart:107:14)
E/flutter ( 5030): #3      FirebaseMessaging.onBackgroundMessage (package:firebase_messaging/src/messaging.dart:103:31)
E/flutter ( 5030): #4      _ChatrAppState._initFcm (package:parsian_chatr/app/ui/chatr_app.dart:90:23)
E/flutter ( 5030): #5      _ChatrAppState.initState (package:parsian_chatr/app/ui/chatr_app.dart:69:5)
E/flutter ( 5030): #6      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4765:58)
E/flutter ( 5030): #7      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4601:5)
E/flutter ( 5030): #8      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3569:14)
E/flutter ( 5030): #9      Element.updateChild (package:flutter/src/widgets/framework.dart:3327:18)
E/flutter ( 5030): #10     RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1252:16)
E/flutter ( 5030): #11     RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1223:5)
E/flutter ( 5030): #12     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1165:17)
E/flutter ( 5030): #13     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2683:19)
E/flutter ( 5030): #14     RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1164:13)
E/flutter ( 5030): #15     WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:974:7)
E/flutter ( 5030): #16     WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:955:7)
E/flutter ( 5030): #17     _rootRun (dart:async/zone.dart:1182:47)
E/flutter ( 5030): #18     _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter ( 5030): #19     _CustomZone.runGuarded (dart:async/zone.dart:997:7)
E/flutter ( 5030): #20     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
E/flutter ( 5030): #21     _rootRun (dart:async/zone.dart:1190:13)
E/flutter ( 5030): #22     _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter ( 5030): #23     _CustomZone.bindCallback.<anonymous closure> (dart:async/zone.dart:1021:23)
E/flutter ( 5030): #24     Timer._createTimer.<anonymous closure> (dart:async-patch/timer_patch.dart:18:15)
E/flutter ( 5030): #25     _Timer._runTimers (dart:isolate-patch/timer_impl.dart:397:19)
E/flutter ( 5030): #26     _Timer._handleMessage (dart:isolate-patch/timer_impl.dart:428:5)
E/flutter ( 5030): #27     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)
E/flutter ( 5030): 

Answer 1

Try this

In the file containing MainApp class add a handler. Remember to add it outside the class.

Future<void> firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  print("Handling a background message");
}

and replace

FirebaseMessaging.onBackgroundMessage((message) {
      Logger.log("_messaging onBackgroundMessage: $message");
      return;
    });

with

FirebaseMessaging.onBackgroundMessage(firebaseMessagingBackgroundHandler);

Reference: https://firebase.flutter.dev/docs/messaging/usage/#background-messages

There are a few things to keep in mind about your background message handler:

It must not be an anonymous function. It must be a top-level function (e.g. not a class method which requires initialization).

Answer 2

In addition to mzafer answer,

add the below function definition on top of the file. [ outside of any class if exists ]

 Future<void> firebaseMessagingBackgroundHandler(RemoteMessage message) async {
       print("Handling a background message");
 } 

This worked for me.