Hexagonal design pattern and how it is used in android

The hexagonal design pattern, also known as Ports and Adapters or Hexagonal Architecture, is an architectural pattern that emphasizes the separation of concerns and modularity in software development. This pattern was introduced by Alistair Cockburn, and it helps create more maintainable and testable code by isolating the core business logic from external dependencies.

Key Concepts:

1. Core Business Logic:The heart of the application is the core business logic, representing the unique functionality and rules that make up the application's value.This logic is encapsulated in use cases, entities, and domain services.
2. Ports:Ports are interfaces that define how the core business logic interacts with the external world.They specify the operations or services that the application requires, such as retrieving data, storing data, or sending notifications.
3. Adapters:Adapters are implementations of the ports.They connect the core business logic to external systems, such as databases, UI frameworks, or third-party services.Adapters convert the generic operations defined by ports into specific calls to external systems.
4. Hexagonal Shape:The hexagonal shape metaphorically represents the different sides of the application.The core business logic is at the center, surrounded by ports on one side and adapters on the other.This design promotes a clear separation between the application's internal logic and its external interactions.

Advantages:

1. Testability:The core business logic can be thoroughly tested in isolation, without dependencies on external systems.Mocks or stubs can be easily used for testing the interactions with adapters.
2. Flexibility:Adapters can be swapped or replaced without affecting the core business logic.This flexibility makes it easier to adapt the application to new requirements or technologies.
3. Isolation of Concerns:The separation between core business logic and external systems results in cleaner and more maintainable code.Changes in the external systems do not impact the internal logic and vice versa.
4. Dependency Inversion:The hexagonal pattern follows the Dependency Inversion Principle, where high-level modules (core logic) do not depend on low-level modules (adapters); both depend on abstractions (ports).

Example Workflow:

1. Define Ports:Identify the operations or services needed by the core business logic.Create interfaces or abstract classes to represent these ports.
2. Implement Adapters:Create adapter classes that implement the defined ports.Adapters interact with databases, UI frameworks, or other external systems.
3. Implement Core Business Logic:Develop the core business logic using the defined ports.The core logic remains independent of specific external implementations.
4. Dependency Injection:Use dependency injection to inject the appropriate adapters into the core business logic.This can be done through constructor injection or using a dependency injection framework.
5. Connect to External Systems:Create the necessary connections between adapters and external systems.Adapters handle the details of communication with databases, services, etc.
6. Build User Interface:Create the user interface components that interact with the core business logic through ports.

The hexagonal design pattern is a powerful concept for building scalable, maintainable, and adaptable software systems. Its principles align with key software design principles such as SOLID, making it a valuable pattern for modern software development.

How it used in android ????????

Step 1: Identify Core Business Logic

Identify the core business logic of your application. This is the functionality that represents the unique value of your application. It could include use cases, domain entities, and business rules.

Step 2: Define Ports

Ports are interfaces that define how the core business logic interacts with the external world. In Android, ports could be interfaces or abstract classes that represent the operations that the application needs to perform, such as fetching data, storing data, or displaying information.

// Example: Port for fetching user data
public interface UserRepository {
    User getUserById(String userId);
    void saveUser(User user);
}
        

Step 3: Implement Adapters

Adapters are implementations of the ports. They are responsible for connecting the core business logic to external systems. For Android, adapters could be classes that interact with the database, network, or UI.

// Example: Adapter for UserRepository using SQLite database
public class SQLiteUserRepository implements UserRepository {
    // Implementation of UserRepository methods using SQLite database
    // ...
}        

Step 4: Implement Core Business Logic

Implement the core business logic using the ports. The core logic should be independent of the external systems it interacts with. It only relies on the interfaces defined by the ports.

public class UserUseCase {
    private final UserRepository userRepository;

    public UserUseCase(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User getUserDetails(String userId) {
        // Business logic
        return userRepository.getUserById(userId);
    }

    public void saveUserDetails(User user) {
        // Business logic
        userRepository.saveUser(user);
    }
}
        

Step 5: Dependency Injection

Use dependency injection to inject the appropriate adapters into the core business logic. This can be achieved through constructor injection or a dependency injection framework.

// Example: Using Dagger for dependency injection
@Module
public class AppModule {
    @Provides
    public UserRepository provideUserRepository() {
        return new SQLiteUserRepository();
    }
}

// In your application component
@Component(modules = AppModule.class)
public interface AppComponent {
    void inject(MyApplication application);
}        

Step 6: Connect to Android Framework

Finally, create the Android-specific parts of your application, such as Activities, Fragments, and Views. These components interact with the core business logic through the ports.

public class UserDetailsActivity extends AppCompatActivity {
    private UserUseCase userUseCase;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_user_details);

        // Dagger can be used to inject the UserUseCase with the appropriate UserRepository
        DaggerAppComponent.create().inject(this);

        // Use the userUseCase in your UI components
        // ...
    }
}        

By following these steps, you can create an Android application with a hexagonal architecture that separates the core business logic from external dependencies. This makes your codebase more modular, testable, and maintainable.