【Flutter・Dart】abstract instanceを使用した抽象クラスの理解を深める

// a.dart
abstract class Vehicle {
  void moveForward(int meters);
}

import 'a.dart';

// Error: `Vehicle` can't be instantiated because
// it is marked as `abstract`.
Vehicle myVehicle = Vehicle();

// Can be extended.
class Car extends Vehicle {
  int passengers = 4;

  @override
  void moveForward(int meters) {
    // ...
  }
}

// Can be implemented.
class MockVehicle implements Vehicle {
  @override
  void moveForward(int meters) {
    // ...
  }
}

// a.dart
interface class Vehicle {
  void moveForward(int meters) {
    // ...
  }
}

import 'a.dart';

// Can be constructed.
Vehicle myVehicle = Vehicle();

// ERROR: `Vehicle` can't be extended in a different library because
// it is marked with `interface`.
class Car extends Vehicle {
  int passengers = 4;
  // ...
}

// Can be implemented.
class MockVehicle implements Vehicle {
  @override
  void moveForward(int meters) {
    // ...
  }
}

// FirebaseのAuthの状態を取得
class FirebaseAuthRepositoryImpl implements AuthRepository {
  @override
  Stream<User?> authStateChanges() {
    return FirebaseAuth.instance.authStateChanges();
  }
}

// FirebaseAuthのData層とPresentation層の間のインターフェース
abstract interface class AuthRepository {
  Stream<User?> authStateChanges();
}

// 中継役を型としたProvider
final authRepositoryProvider = Provider<AuthRepository>((ref) {
  return FirebaseAuthRepositoryImpl();
});

// 実際のxx_page.dartから呼ぶ出すためのStreamProvider(上記Providerを使用)
final authStateProvider = StreamProvider<User?>((ref) {
  return ref.watch(authRepositoryProvider).authStateChanges();
});