Firebase Messaging - iOS background notification onBackgroundMessage() not triggering

[RyanMannix]

Hello, I have looked through many discussions and issues here, and I see that there are a lot of users experiencing difficulties with iOS background messages. I have a problem getting onBackgroundMessage() to trigger when receiving a notification when app is minimized. Thank you in advance for any help 🙏

I have a golang FCM server which is sending notifications to FCM, which is then sending them out to user devices. This process works correctly - notifications are delivered successfully.

Flutter app (iOS/Android) is running firebase_messaging: ^14.4.0
Golang FCM server is running firebase.google.com/go/v4 v4.11.0
Simulator - iOS version 16.4

I have some logic (saving notification to local storage) that I'd like to trigger upon receipt of that notification. This works 100% for android. For iOS, it works when notification is received in the foreground. If the app is minimized (not swipe closed) it does not trigger the onBackgroundMessage(myBackgroundMessageHandler) method so it never writes to file.

Background Handler looks like this:

@pragma('vm:entry-point')
Future<void> myBackgroundMessageHandler(RemoteMessage message) async {
  await Firebase.initializeApp(options: DevFirebaseOptions.currentPlatform);
  await setupLocalNotifications();
  debugPrint('In the background!');
  debugPrint('Notification payload: ${message.data}');
  var a = message.contentAvailable;
  var b = message.messageType;
  var c = message.messageId;
  var d = message.data;
  var e = message.notification;
  debugPrint("a: $a, b: $b, c: $c, d: $d, e: $e");
  if (message.notification != null) {
    CustomNotification note = CustomNotification(
        id: message.notification!.hashCode,
        title: message.notification!.title ?? "",
        body: message.notification!.body ?? "",
        payload: message.data,
        time: DateTime.timestamp().toString());

    await writeNotificationToFile(note);
  }
}

I can't really dig into that notification to see what the values are (since firebase.onBackgroundMessage(myBackgroundMessageHandler) is never actually called from iOS, but in android the output is like this:
a: false, b: null, c: 0:1688586103618334%9238a8c09238a8c0, d: {path: /friends}, e: Instance of 'RemoteNotification'
a in this case is ContentAvailable, which I'm explicitly setting for APNS in the go server (below), so in theory iOS would look identical just true for ContentAvailable.

This is what my golang server is sending:

client, err := app.App.Messaging(ctx)
if err != nil {
	log.Fatalf("error getting Messaging client: %v\n", err)
}

message := &messaging.MulticastMessage{
	Notification: &messaging.Notification{
		Title: n.Title,
		Body:  n.Body,
	},

	Data: n.Payload,

	Android: &messaging.AndroidConfig{
		Notification: &messaging.AndroidNotification{
			ChannelID: "high_importance_channel",
			Priority:  messaging.PriorityHigh,
		},
	},

	APNS: &messaging.APNSConfig{
		Payload: &messaging.APNSPayload{
			Aps: &messaging.Aps{
				Sound:            "default",
				ContentAvailable: true,
			},
		},
	},

	Tokens: n.FcmTokens,
}

batchResponse, err := client.SendMulticast(ctx, message)

So I believe that the ContentAvailable value must be set to true. I'm missing something else about the formatting of the notification values for the onBackgroundMessage() to be triggered. If anyone can help it'd be greatly appreciate.

[RyanMannix]

I've gone through the "Submit a background notification" issue template, and the only thing I haven't done is tested on a physical device... so it's possible that's the reason. Will be able to test on a hard device early next week.

[MuhammedEminK]

I 've a same problem. Did you solve it?