Error Handling from widget with exception manager

error_manage_widget_with_exception.dart

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

class User {
  final String? name;
  final int? age;

  const User(this.name, this.age);

  bool isEmpty() {
    return name == null || age == null;
  }

  MapEntry<String, int> get nameAndAge {
    if (isEmpty()) CustomException.instance.throwException('User not found');
    return MapEntry(name!, age!);
  }
}

class HomeView extends StatefulWidget {
  const HomeView({super.key});

  @override
  State<HomeView> createState() => _HomeViewState();
}

class _HomeViewState extends State<HomeView> with _HomeViewMixin {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('HomeView'),
      ),
      body: _users == null
          ? const Center(
              child: CircularProgressIndicator(),
            )
          : _UserListView(users: _users!),
    );
  }
}

class _UserListView extends StatelessWidget {
  const _UserListView({required this.users});

  final List<User> users;

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: users.length,
      itemBuilder: (context, index) {
        return UserCard(user: users[index]);
      },
    );
  }
}

mixin _HomeViewMixin on State<HomeView> {
  List<User>? _users;

  Future<void> fetchUser() async {
    final backendResponse = List<User?>.generate(
      10,
      (index) {
        if (index.isOdd) return null;
        return User('User $index', index);
      },
    );
    await Future.delayed(Durations.short1);

    /// 1
    _users = backendResponse
        .where((element) => element != null)
        .cast<User>()
        .toList();

    setState(() {});
  }

  @override
  void initState() {
    super.initState();
    fetchUser();
  }
}

class UserCard extends StatelessWidget {
  const UserCard({super.key, required this.user});
  final User user;

  /// yasi 30dan buyukse sol taraf yesil olacak kirmizi
  /// title olarak adi gozukecek
  /// age gozukecek
  @override
  Widget build(BuildContext context) {
    final latestUserData = user.nameAndAge;
    return ListTile(
      title: Text(latestUserData.key),
      subtitle: Text(latestUserData.value.toString()),
      leading: Container(
        width: 50,
        height: 50,
        color: latestUserData.value > 30 ? Colors.green : Colors.red,
      ),
    );
  }
}

abstract class CustomException {
  void throwException(String message);
  static CustomException get instance =>
      kDebugMode ? DevelopmentException() : ProductionException();
}

class DevelopmentException implements CustomException {
  @override
  void throwException(String message) {
    throw Exception(message);
  }
}

class ProductionException implements CustomException {
  /// custom end point
  @override
  void throwException(String message) {}
}