feat: setOnBackgroundMessage() is now a Future<void> that will await for the function to complete.

lib/src/platform/src/method_call_handler.dart

@@ -94,7 +94,7 @@ class AblyMethodCallHandler {
   }
 
   Future<Object?> _onPushBackgroundMessage(RemoteMessage remoteMessage) async {
-    _pushNotificationEvents.handleBackgroundMessage(remoteMessage);
+    await _pushNotificationEvents.handleBackgroundMessage(remoteMessage);
     return null;
   }
 

lib/src/platform/src/push_notification_events_internal.dart

@@ -81,9 +81,14 @@ class PushNotificationEventsInternal implements PushNotificationEvents {
 
   /// @nodoc
   /// Handles a RemoteMessage passed from the platform side.
-  void handleBackgroundMessage(RemoteMessage remoteMessage) {
+  Future<void> handleBackgroundMessage(RemoteMessage remoteMessage) async {
     if (_onBackgroundMessage != null) {
-      _onBackgroundMessage!(remoteMessage);
+      final onBackgroundMessageResult = _onBackgroundMessage!(remoteMessage);
+      // ^^^ _onBackgroundMessage() can return void or Future<void>
+      if (onBackgroundMessageResult is Future) {
+        // if it returns a Future we await on it.
+        await onBackgroundMessageResult;
+      }
     } else {
       // ignore:avoid_print
       print('Received RemoteMessage but no handler was set. '

lib/src/push_notifications/src/push_notification_events.dart

@@ -4,7 +4,7 @@ import 'package:ably_flutter/ably_flutter.dart';
 
 /// Type definition for a callback invoked when background push message
 /// is received.
-typedef BackgroundMessageHandler = void Function(RemoteMessage message);
+typedef BackgroundMessageHandler = dynamic Function(RemoteMessage message);
 
 /// Push Notification events, such as message arriving or notification tap.
 abstract class PushNotificationEvents {