main.dart

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

void main() {
  const isCat = true;
  runApp(
    Provider<Animal>(
      create: (context) {
        debugPrint('create');
        return isCat ? Cat(name: 'Whiskers', meows: 'meow') : Dog(name: 'Fido', barks: 'woof');
      },
      builder: (context, _) => MaterialApp(
        home: Scaffold(
          appBar: AppBar(
            title: const Text('Hello World'),
          ),
          body: Column(
            children: [
              Visibility(
                visible: (context.read<Animal>() is Dog),
                replacement: const CatWidget(),
                child: const DogWidget(),
              ),
              const CommonWidget(),
            ],
          ),
        ),
      ),
    ),
  );
}

class CommonWidget extends StatelessWidget {
  const CommonWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return Text(
      context.read<Animal>().name,
    );
  }
}

class DogWidget extends StatelessWidget {
  const DogWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return Text(
      (context.read<Animal>() as Dog).barks,
    );
  }
}

class CatWidget extends StatelessWidget {
  const CatWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return Text(
      (context.read<Animal>() as Cat).meows,
    );
  }
}

abstract class Animal {
  Animal({required this.name});
  String name;
}

class Dog extends Animal {
  Dog({
    required super.name,
    required this.barks,
  });

  String barks;
}

class Cat extends Animal {
  Cat({
    required super.name,
    required this.meows,
  });

  String meows;
}