Dependency Injection (DI) is a design pattern used to implement IoC (Inversion of Control), allowing the creation of dependent objects outside of a class and providing those objects to a class in various ways. Dagger is a popular DI framework for Java and Android that helps manage dependencies efficiently.

1. Dependency Injection (DI): A technique where an object receives other objects it depends on.
2. Inversion of Control (IoC): A principle where the control of object creation and management is transferred from the class itself to an external entity.
3. Dagger: A fully static, compile-time dependency injection framework for Java and Android.

• Simplifies Dependency Management: Automatically handles the creation and provision of dependencies.
• Improves Testability: Makes it easier to inject mock dependencies for testing.
• Enhances Code Maintainability: Reduces boilerplate code and makes the codebase cleaner and more modular.

1. Add Dagger Dependencies: Add the following dependencies to your build.gradle file.

   dependencies {
       implementation 'com.google.dagger:dagger:2.x'
       kapt 'com.google.dagger:dagger-compiler:2.x'
   }
   

2. Enable Annotation Processing: Ensure that annotation processing is enabled in your project.

   android {
       ...
       kapt {
           generateStubs = true
       }
   }
   

A module is a class annotated with @Module that provides dependencies.

A component is an interface annotated with @Component that connects modules and the classes that request injection.

Use the @Inject annotation to request dependencies in your classes.

1. Define the Repository:

   public class UserRepository {
       private final ApiService apiService;
   
       @Inject
       public UserRepository(ApiService apiService) {
           this.apiService = apiService;
       }
   
       public LiveData<User> getUser(int userId) {
           // Implementation to fetch user data
       }
   }
   

2. Create the ViewModel:

   public class UserViewModel extends ViewModel {
       private final UserRepository userRepository;
   
       @Inject
       public UserViewModel(UserRepository userRepository) {
           this.userRepository = userRepository;
       }
   
       public LiveData<User> getUser(int userId) {
           return userRepository.getUser(userId);
       }
   }
   

3. Define the Module:

   @Module
   public class AppModule {
       @Provides
       public ApiService provideApiService() {
           return new Retrofit.Builder()
                   .baseUrl("https://api.example.com")
                   .addConverterFactory(GsonConverterFactory.create())
                   .build()
                   .create(ApiService.class);
       }
   
       @Provides
       public UserRepository provideUserRepository(ApiService apiService) {
           return new UserRepository(apiService);
       }
   }
   

4. Create the Component:

   @Component(modules = {AppModule.class})
   public interface AppComponent {
       void inject(UserViewModel userViewModel);
   }
   

5. Inject the ViewModel:

   public class MainActivity extends AppCompatActivity {
       @Inject
       UserViewModel userViewModel;
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
   
           // Initialize Dagger
           DaggerAppComponent.create().inject(this);
   
           // Use the injected UserViewModel instance
           // ...
       }
   }
   

1. Create a LoggingService class that provides logging functionality.
2. Define a Dagger module that provides an instance of LoggingService.
3. Create a Dagger component that includes the module.
4. Inject LoggingService into an Activity and use it to log a message.

1. LoggingService:

   public class LoggingService {
       public void log(String message) {
           Log.d("LoggingService", message);
       }
   }
   

2. Module:

   @Module
   public class LoggingModule {
       @Provides
       public LoggingService provideLoggingService() {
           return new LoggingService();
       }
   }
   

3. Component:

   @Component(modules = {LoggingModule.class})
   public interface LoggingComponent {
       void inject(MainActivity mainActivity);
   }
   

4. Activity:

   public class MainActivity extends AppCompatActivity {
       @Inject
       LoggingService loggingService;
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
   
           // Initialize Dagger
           DaggerLoggingComponent.create().inject(this);
   
           // Use the injected LoggingService instance
           loggingService.log("Hello, Dagger!");
       }
   }
   

• Forgetting to Annotate with @Inject: Ensure that fields, constructors, or methods that need injection are annotated with @Inject.
• Not Including Modules in Components: Make sure all necessary modules are included in the component.
• Circular Dependencies: Be cautious of circular dependencies, which can cause runtime errors.

In this section, you learned about Dependency Injection and how to use Dagger to manage dependencies in your Android projects. You set up Dagger, created modules and components, and injected dependencies into your classes. This knowledge will help you write cleaner, more maintainable, and testable code. In the next section, you will learn about Unit Testing and UI Testing to ensure the reliability of your applications.