flutter – firebase_messaging onMessage stream never fires after UIScene migration on iOS

After migrating a Flutter iOS app to use UISceneDelegation (via FlutterSceneDelegate in Info.plist), FirebaseMessaging.onMessage in Dart never fires for foreground messages. Background banners appear correctly via APNs, but the Dart layer receives nothing — no onMessage, no onMessageOpenedApp logs, and no getInitialMessage data.

What works:

• FCM token is generated and non-null

• Push notification banners appear when the app is backgrounded

• didRegisterForRemoteNotificationsWithDeviceToken fires correctly

• UNUserNotificationCenter willPresent fires correctly (confirmed via Swift print)

• Messaging.messaging().appDidReceiveMessage(userInfo) is called successfully from willPresent

What breaks:

• FirebaseMessaging.onMessage never fires in Dart

• FirebaseMessaging.onMessageOpenedApp never fires in Dart

• No Dart-side logs appear for any incoming notification

• Root Cause (Diagnosed): When using FlutterImplicitEngineDelegate, plugins are registered via: func didInitializeImplicitFlutterEngine(\_ engineBridge: FlutterImplicitEngineBridge) { GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry) }, After GeneratedPluginRegistrant.register runs, FLTFirebaseMessagingPlugin is expected to set itself as Messaging.messaging().delegate. However, printing the delegate immediately after registration reveals: print(Messaging.messaging().delegate) // → nil

This did not occur before the UIScene migration, when GeneratedPluginRegistrant.register(with: self) was called in didFinishLaunchingWithOptions and FlutterAppDelegate (which lives for the app’s lifetime) held the plugins strongly.

This is my appdelegate.swift file:

import Flutter
import Firebase
import UserNotifications
import flutter_local_notifications
@main
@objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    FirebaseApp.configure()
    UNUserNotificationCenter.current().delegate = self
    let authOptions: UNAuthorizationOptions = [.alert, .badge, .sound]
    UNUserNotificationCenter.current().requestAuthorization(options: authOptions) { _, _ in }
    application.registerForRemoteNotifications()
//     GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
  func didInitializeImplicitFlutterEngine(_ engineBridge: FlutterImplicitEngineBridge) {
  // This is required to make any communication available in the action isolate.
      FlutterLocalNotificationsPlugin.setPluginRegistrantCallback { (registry) in
          GeneratedPluginRegistrant.register(with: registry)
      }
      GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
    }
  override func application(_ application: UIApplication,
                            didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    Messaging.messaging().apnsToken = deviceToken
//     print("✅ APNs token: \(deviceToken.map { String(format: "%02x", $0) }.joined())")
    super.application(application, didRegisterForRemoteNotificationsWithDeviceToken: deviceToken)
  }
//   override func application(_ application: UIApplication,
//                             didReceiveRemoteNotification userInfo: [AnyHashable : Any],
//                             fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
//     Messaging.messaging().appDidReceiveMessage(userInfo)
//     completionHandler(.newData)
//   }
}

And this is my info.plist file:

    
    UIApplicationSceneManifest
     
      UIApplicationSupportsMultipleScenes
      
      UISceneConfigurations
      
      UIWindowSceneSessionRoleApplication
        
          
            UISceneClassName
            UIWindowScene
            UISceneDelegateClassName
            FlutterSceneDelegate
            UISceneConfigurationName
            flutter
            UISceneStoryboardFile
            Main
          
        
       
     
       CFBundleDevelopmentRegion
       $(DEVELOPMENT_LANGUAGE)
       CFBundleDisplayName
       Bongo Payments
       CFBundleExecutable
       $(EXECUTABLE_NAME)
       CFBundleIdentifier
       $(PRODUCT_BUNDLE_IDENTIFIER)
       CFBundleInfoDictionaryVersion
       6.0
       CFBundleName
       bongo_pay
       CFBundlePackageType
       APPL
       CFBundleShortVersionString
       $(FLUTTER_BUILD_NAME)
       CFBundleSignature
       ????
       UIBackgroundModes
       
          remote-notification
       
       FirebaseAppDelegateProxyEnabled
       
       NSAppTransportSecurity
       
          NSAllowsArbitraryLoads
          
       
       CFBundleVersion
       $(FLUTTER_BUILD_NUMBER)
       NSLocationWhenInUseUsageDescription
       We need your location to provide better service.
       NSLocationAlwaysAndWhenInUseUsageDescription
       This app uses notifications
       NSLocationAlwaysUsageDescription
       We need your location even when the app is in the background.
       NSUserTrackingUsageDescription
       This app uses notifications
       LSRequiresIPhoneOS
       
       UILaunchStoryboardName
       LaunchScreen
       UIMainStoryboardFile
       Main
       NSFaceIDUsageDescription
       This app requires Face ID authentication to secure your personal data and ensure only you can access your account.
       NSCameraUsageDescription
       We need access to your camera to take photos.
       NSPhotoLibraryUsageDescription
       We need access to your photo library to upload images.
       NSAppleMusicUsageDescription
        This app does not use Apple Music but requires limited media library access for system features.
       UISupportedInterfaceOrientations
       
          UIInterfaceOrientationPortrait
          UIInterfaceOrientationLandscapeLeft
          UIInterfaceOrientationLandscapeRight
       
       UISupportedInterfaceOrientations~ipad
       
          UIInterfaceOrientationPortrait
          UIInterfaceOrientationPortraitUpsideDown
          UIInterfaceOrientationLandscapeLeft
          UIInterfaceOrientationLandscapeRight
       
       CADisableMinimumFrameDurationOnPhone
       
       UIApplicationSupportsIndirectInputEvents
       
       io.flutter.embedded_views_preview
       
       NSMicrophoneUsageDescription
       We need access to your microphone for video or audio capture in web pages.
       NSPhotoLibraryAddUsageDescription
       We need access to save images to your photo library.
       UIFileSharingEnabled
       
       LSSupportsOpeningDocumentsInPlace
       
       UIStatusBarHidden