Flutter FCM onBackgroundMessage does not work for nested non-static method calls - firebase

Question

I'm using firebase_messaging 6.0.9 with flutter 1.12.13. In the repo's readme: https://pub.dev/packages/firebase_messaging#-readme-tab- it says to declare the onBackgroudMessage callback as a static or top level method. Which I did, but it doesn't work when this callback invokes a non-static method. The following example demonstrates this with a singleton class:
class NotificationService {
static NotificationService _instance;
final FirebaseMessaging _firebase;
static NotificationService get instance => _instance;
NotificationService._internal() : this._firebase = FirebaseMessaging();
factory NotificationService() {
if (_instance == null) {
_instance = NotificationService._internal();
_instance._firebase.configure(
onBackgroundMessage: NotificationService.staticHandler
);
}
return _instance;
}
static Future<dynamic> staticHandler(Map<String, dynamic> msg) {
print("Static Func >>> $msg"); // Successfully prints
return NotificationService.instance.instanceFunc(msg); // Fails here, complaining that it's being invoked on null.
}
Future<dynamic> instanceFunc(Map<String, dynamic> msg) {
print("Instance Func >>> $msg");
}
void myVarFunc() {
print("This is my var func");
}
}
in main.dart, the notification service factory constructor is called:
import 'package:myProject/services/notification/notification_service.dart';
run(MyApp());
class MyApp extends StatelessWidget {
final NotificationService _ns = NotificationService();
NotificationService.instance.myVarFunc(); // Prints successfully.
.......
.......
.......
}
The invocation of instanceFunc fails, saying it's being called on null. The following are the logs:
I/flutter ( 6935): Static Func >>> {data: {title: Title_is_here, message: Message_is_here}}
I/flutter ( 6935): Unable to handle incoming background message.
I/flutter ( 6935): NoSuchMethodError: The method 'instanceFunc' was called on null.
I/flutter ( 6935): Receiver: null
I/flutter ( 6935): Tried calling: instanceFunc(_LinkedHashMap len:1)
I'm not really sure if this is right way to handle this scenario. Since I'm new to Dart and Flutter, my knowledge is pretty limited. I can't declare everything static and work, that's not good design IMO. I'm probably missing something here.

Answer 1

There are some reason of not getting callback on onBackgroundMessage:-
onBackgroundMessage didn't worked on iOS so you have to implement platform check
For example:-
_firebaseMessaging.configure(
onMessage: (Map<String, dynamic> message) async {
printDebug("onMessage foreground: $message");
},
onBackgroundMessage: Platform.isIOS ? null : _myBackgroundMessageHandler,
onLaunch: (Map<String, dynamic> message) async {
printDebug("onLaunch Kill: $message");
},
onResume: (Map<String, dynamic> message) async {
print("onResume Background: $message");
},
);
static Future<dynamic> _myBackgroundMessageHandler(
Map<String, dynamic> message) async {
print("onBackgroundMessage: $message");
return Future<void>.value();
}
And You have to make sure that your Notification payload didn't contain the notification key because if notification key is exist in payload then notification directly handled by your system. So you have to remove the notification key from payload to get callback on onBackgroundMessage.
Note:- If you remove the notification key then notification didn't rendered in systems notification tray. For this you can you local notification.