Implementing SOLID Principles in Flutter Applications

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Mar 26, 2025

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The SOLID principles are fundamental guidelines in object-oriented programming that help create maintainable and scalable software. Let’s explore how to apply these principles in Flutter development.

1. Single Responsibility Principle (SRP)

This principle states that a class should have only one reason to change. In Flutter, we can apply this to widgets, services, and models.

Bad Practice ❌:

class UserWidget extends StatelessWidget {
  final User user;

  UserWidget({required this.user});

  Future<void> saveUser() async {
    // Database operations
    await DatabaseHelper.instance.insertUser(user);
    // API calls
    await ApiService.updateUser(user);
    // Local storage
    await SharedPreferences.getInstance()
      .setString('user_data', json.encode(user.toJson()));
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Column(
        children: [
          Text(user.name),
          Text(user.email),
          ElevatedButton(
            onPressed: saveUser,
            child: Text('Save User'),
          ),
        ],
      ),
    );
  }
}

Good Practice ✅:

// User Repository
class UserRepository {
  Future<void> saveUser(User user) async {
    await DatabaseHelper.instance.insertUser(user);
    await ApiService.updateUser(user);
    await _saveToLocalStorage(user);
  }

  Future<void> _saveToLocalStorage(User user) async {
    final prefs = await SharedPreferences.getInstance();
    await prefs.setString('user_data', json.encode(user.toJson()));
  }
}

// User Widget
class UserWidget extends StatelessWidget {
  final User user;
  final UserRepository userRepository;

  UserWidget({
    required this.user,
    required this.userRepository,
  });

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Column(
        children: [
          UserInfoDisplay(user: user),
          UserActionButtons(
            onSave: () => userRepository.saveUser(user),
          ),
        ],
      ),
    );
  }
}

2. Open/Closed Principle (OCP)

Classes should be open for extension but closed for modification.