I have the following code for HomePage class:

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {

  //inicializamos algunos valores

  late final FirebaseMessaging _messaging;
   late int totalNotificationCounter;
  PushNotification? _notificationInfo;

  //register Notification
  void registerNotification() async {

    await Firebase.initializeApp();
    //instance for Firebase Messaging
    _messaging = FirebaseMessaging.instance;

    //hay tres estados de los permisos para recibir la notificacion
    //not determined (null), granted (true), declined (false)

    NotificationSettings settings = await _messaging.requestPermission(
      alert: true,
      badge: true,
      provisional: false,
      sound: true
    );

    if (settings.authorizationStatus == AuthorizationStatus.authorized){
      print("User granted the permission");

      FirebaseMessaging.onMessage.listen((RemoteMessage message) {
        PushNotification notification = PushNotification(
          title: message.notification!.title,
          body: message.notification!.body,
          dataTitle: message.data['title'],
          dataBody: message.data['body']
        );

        setState(() {
          totalNotificationCounter   ;
          _notificationInfo = notification;
        });

      });
    }


  }
@override
  void initState() {
    // TODO: implement initState
    totalNotificationCounter = 0;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("PushNotification"),),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: const [
            Text("Flutter Push Notification",textAlign: TextAlign.center,
            style: TextStyle(color: Colors.black,fontSize: 20),),
            //add notification badge to count total # of notifications received
            NotificationBadge(totalNotification: totalNotificationCounter),
          ],
        ),
      ),
    );
  }
}

And here the code for NotificationBadge class:

class NotificationBadge extends StatelessWidget {

   final int totalNotification;
  const NotificationBadge({Key? key,  required this.totalNotification}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: 40,
        height: 40,
        decoration: const BoxDecoration(
          color: Colors.orange,
          shape: BoxShape.circle,
        ),
      child: Center(
        child: Padding(
          padding: EdgeInsets.all(8),
          child: Text("$totalNotification",style: TextStyle(color: Colors.white,fontSize: 20),),
        ),
      ),
    );
  }
}

There is an issue at line

NotificationBadge(totalNotification: totalNotificationCounter),

from the first class.

This is the error shown:

A value of type 'Null' can't be assigned to a parameter of type 'int' in a const constructor. (Documentation)  Try using a subtype, or removing the keyword 'const'.
Invalid constant value. 
The values in a const list literal must be constants. (Documentation)  Try removing the keyword 'const' from the list literal

I have tried removing the const from the seconde class, but other errors are shown.

CodePudding user response：

The cause of the issue is totalNotificationCounter not being initialized and totalNotification parameter needed in NotificationBadge() is non-nullable. You may want consider setting a default value to totalNotificationCounter and only display the Widget if the value is greater than 0.

i.e.

Column(
  children: [
    Text(),
    // Return empty Widget if totalNotificationCounter is 0
    totalNotificationCounter > 0 ? 
        NotificationBadge(totalNotification: totalNotificationCounter) : SizedBox(),
  ]
)

CodePudding user response：

in your initState add super.initState();

  @override
  void initState() {
    super.initState();
    totalNotificationCounter = 0;
  }