wip notif handle when in background; TODO binding

lib/notifications.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 
 import 'package:collection/collection.dart';
+import 'package:firebase_messaging/firebase_messaging.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_local_notifications/flutter_local_notifications.dart';
@@ -47,7 +48,8 @@ class NotificationService {
     // TODO(#324) defer notif setup if user not logged into any accounts
     //   (in order to avoid calling for permissions)
 
-    ZulipBinding.instance.firebaseMessagingOnMessage.listen(_onRemoteMessage);
+    ZulipBinding.instance.firebaseMessagingOnMessage.listen(_onForegroundMessage);
+    FirebaseMessaging.onBackgroundMessage(_onBackgroundMessage);
 
     NotificationDisplayManager._init();
 
@@ -78,8 +80,27 @@ class NotificationService {
   }
 }
 
-void _onRemoteMessage(FirebaseRemoteMessage message) {
+void _onForegroundMessage(FirebaseRemoteMessage message) {
   assert(debugLog("notif message: ${message.data}"));
+  _onRemoteMessage(message);
+}
+
+Future<void> _onBackgroundMessage(FirebaseRemoteMessage message) async {
+  // This callback will run in a separate isolate from the rest of the app.
+  // See docs:
+  //   https://firebase.flutter.dev/docs/messaging/usage/#background-messages
+  assert(() {
+    debugLogEnabled = true;
+    return true;
+  }());
+  LiveZulipBinding.ensureInitialized();
+  NotificationDisplayManager._init();
+
+  assert(debugLog("notif message in background: ${message.data}"));
+  _onRemoteMessage(message);
+}
+
+void _onRemoteMessage(FirebaseRemoteMessage message) {
   final data = FcmMessage.fromJson(message.data);
   switch (data) {
     case MessageFcmMessage(): NotificationDisplayManager._onMessageFcmMessage(data, message.data);