In Flutter, how do we use Firebase Messaging onBackgroundMessage to create a notification, using flutter_local_notifications?

We are working on an encrypted chat application where we use Firebase Messaging for data notifications. Some client-side logic needs to be done upon receiving a data notification, before showing an actual notification to the user. For example, a phone number will have to be translated to a local contact name. This translation is done by lookup with a map that is already available globally.

The data notifications are received just fine and the onBackgroundMessage callback is called as well. However, it seems impossible to access any kind of state from the onBackgroundMessage function. For example, printing the phone number of the logged in user returns null.
Printing this same global variable from the onMessage callback works just fine.

Running flutter_local_notifications from onMessage works fine, but again, does not work at all from onBackgroundMessage as 'no implementation could be found for the method .show()'. At the moment, it claims that flutterLocalNotificationsPlugin is null, which it isn't really.

It seems to us that onBackgroundMessage has no access to anything the app provides, as soon as the app is backgrounded. Something has to be done to make some of the scope/context available to the background process. For now, that would mainly be the flutter_local_notifications plugin in its entirety, as well as the local contacts list to translate phone number to name.

Has anyone got any idea how to do this?

Here is some of the code:

FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin;
final _chatRepository = ChatRepository();
Future<dynamic> backgroundMessageHandler(Map<String, dynamic> message) async {
if(message.containsKey('data')) {
await _showNotification(message);
return Future<void>.value();
}
}
Future _showNotification(message) async {
List<String> numbers = [];
numbers.add(message['data']['sender']);
var name = await _chatRepository.translatePhoneNumbersToChatName(numbers);
var androidPlatformChannelSpecifics = new AndroidNotificationDetails(
'channel id', 'channel name', 'channel description',
importance: Importance.Max, priority: Priority.High);
var iOSPlatformChannelSpecifics = new IOSNotificationDetails();
var platformChannelSpecifics = new NotificationDetails(
androidPlatformChannelSpecifics, iOSPlatformChannelSpecifics);
await flutterLocalNotificationsPlugin.show(
0,
name,
message['data']['body'],
platformChannelSpecifics,
payload: message['data']['body'],
);
}
class NotificationHandler {
final FirebaseMessaging fcm = FirebaseMessaging();
StreamSubscription iosSubscription;
String deviceToken = "";
Future<void> initialize() async {
flutterLocalNotificationsPlugin = new FlutterLocalNotificationsPlugin();
var initializationSettingsAndroid =
new AndroidInitializationSettings('@mipmap/ic_launcher');
var initializationSettingsIOS = new IOSInitializationSettings(onDidReceiveLocalNotification: onDidReceiveLocalNotification);
var initializationSettings = new InitializationSettings(initializationSettingsAndroid, initializationSettingsIOS);
flutterLocalNotificationsPlugin.initialize(initializationSettings, onSelectNotification: onClickNotification);
fcm.configure(
onMessage: (Map<String, dynamic> message) async {
if(message.containsKey('data')) {
print(message);
_showNotification(message);
}
},
onBackgroundMessage: Platform.isIOS
? null
: backgroundMessageHandler,
onLaunch: (Map<String, dynamic> message) async {
if(message.containsKey('data')) {
print(message);
_showNotification(message);
}
},
onResume: (Map<String, dynamic> message) async {
if(message.containsKey('data')) {
print(message);
_showNotification(message);
}
},
);
_updateDeviceToken();
}
.
.
.

Of course, the initialize above is called early on in the application lifecycle.

class NotificationHandler {
static final flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin(); // make it a static field of the class
// ...
}
Future _showNotification(message) async {
// ...
await NotificationHandler.flutterLocalNotificationsPlugin.show( // access it
// ...
);
}

Hope this works for you.

This plugin explains it all better than I could, but it just so happens that the background is a completely different isolate/context and thus it has no access to any plugins if they use an old (pre Flutter 12) API.
https://pub.dev/packages/android_alarm_manager#flutter-android-embedding-v1

Embedding v1 requires you to register any plugins that you want to access from the background. Doing this makes it flutter_local_notifications work properly.

Unfortunately, FCM docs are heavily lacking.