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Note: In this page, references to modules refer to Gradle modules and not to Dagger modules.

A project with multiple Gradle modules is known as a multi-module project. In a multi-module project that ships as a single APK with no feature modules, it's common to have an app module that can depend on most modules of your project and a base or core module that the rest of the modules usually depend on. The app module typically contains your Application class, whereas the base module contains all common classes shared across all modules in your project.

The app module is a good place to declare your application component (for example, ApplicationComponent in the image below) that can provide objects that other components might need as well as the singletons of your app. As an example, classes like OkHttpClient , JSON parsers, accessors for your database, or SharedPreferences objects that may be defined in the core module, will be provided by the ApplicationComponent defined in the app module.

In the app module, you could also have other components with shorter lifespans. An example could be a UserComponent with user-specific configuration (like a UserSession ) after a log in.

In the different modules of your project, you can define at least one subcomponent that has logic specific to that module as seen in figure 1.

Figure 1. Example of a Dagger graph in a multi-module project

For example, in a login module, you could have a LoginComponent scoped with a custom @ModuleScope annotation that can provide objects common to that feature such as a LoginRepository . Inside that module, you can also have other components that depend on a LoginComponent with a different custom scope, for example @FeatureScope for a LoginActivityComponent or a TermsAndConditionsComponent where you can scope more feature-specific logic such as ViewModel objects.

For other modules such as Registration , you would have a similar setup.

A general rule for a multi-module project is that modules of the same level shouldn't depend on each other. If they do, consider whether that shared logic (the dependencies between them) should be part of the parent module. If so, refactor to move the classes to the parent module; if not, create a new module that extends the parent module and have both of the original modules extend the new module.

As a best practice, you would generally create a component in a module in the following cases:

You need to perform field injection, as with LoginActivityComponent .

You need to scope objects, as with LoginComponent .

If neither of these casses apply and you need to tell Dagger how to provide objects from that module, create and expose a Dagger module with @Provides or @Binds methods if construction injection is not possible for those classes.

Implementation with Dagger subcomponents

The Using Dagger in Android apps doc page covers how to create and use subcomponents. However, you cannot use the same code because feature modules don't know about the app module. As an example, if you think about a typical Login flow and the code we have in the previous page, it doesn't compile any more:

Kotlin class LoginActivity: Activity() { ... override fun onCreate(savedInstanceState: Bundle?) { // Creation of the login graph using the application graph loginComponent = (applicationContext as MyDaggerApplication) .appComponent.loginComponent().create() // Make Dagger instantiate @Inject fields in LoginActivity loginComponent.inject(this) ... } } Java public class LoginActivity extends Activity { ... @Override protected void onCreate(Bundle savedInstanceState) { // Creation of the login graph using the application graph loginComponent = ((MyApplication) getApplicationContext()) .appComponent.loginComponent().create(); // Make Dagger instantiate @Inject fields in LoginActivity loginComponent.inject(this); ... } }

The reason is that the login module doesn't know about MyApplication nor appComponent . To make it work, you need to define an interface in the feature module that provides a FeatureComponent that MyApplication needs to implement.

In the following example, you can define a LoginComponentProvider interface that provides a LoginComponent in the login module for the Login flow:

Kotlin interface LoginComponentProvider { fun provideLoginComponent(): LoginComponent } Java public interface LoginComponentProvider { public LoginComponent provideLoginComponent(); }

Now, the LoginActivity will use that interface instead of the snippet of code defined above:

Kotlin class LoginActivity: Activity() { ... override fun onCreate(savedInstanceState: Bundle?) { loginComponent = (applicationContext as LoginComponentProvider) .provideLoginComponent() loginComponent.inject(this) ... } } Java public class LoginActivity extends Activity { ... @Override protected void onCreate(Bundle savedInstanceState) { loginComponent = ((LoginComponentProvider) getApplicationContext()) .provideLoginComponent(); loginComponent.inject(this); ... } }

Now, MyApplication needs to implement that interface and implement the required methods:

Kotlin class MyApplication: Application(), LoginComponentProvider { // Reference to the application graph that is used across the whole app val appComponent = DaggerApplicationComponent.create() override fun provideLoginComponent(): LoginComponent { return appComponent.loginComponent().create() } } Java public class MyApplication extends Application implements LoginComponentProvider { // Reference to the application graph that is used across the whole app ApplicationComponent appComponent = DaggerApplicationComponent.create(); @Override public LoginComponent provideLoginComponent() { return appComponent.loginComponent.create(); } }

This is how you can use Dagger subcomponents in a multi-module project. With feature modules, the solution is different due to the way modules depend on each other.

Component dependencies with feature modules

With feature modules, the way modules usually depend on each other is inverted. Instead of the app module including feature modules, the feature modules depend on the app module. See figure 2 for a representation of how modules are structured.

Figure 2. Example of a Dagger graph in a project with feature modules

In Dagger, components need to know about their subcomponents. This information is included in a Dagger module added to the parent component (like the SubcomponentsModule module in Using Dagger in Android apps).

Unfortunately, with the reversed dependency between the app and the feature module, the subcomponent is not visible from the app module because it's not in the build path. As an example, a LoginComponent defined in a login feature module cannot be a subcomponent of the ApplicationComponent defined in the app module.

Dagger has a mechanism called component dependencies that you can use to solve this issue. Instead of the child component being a subcomponent of the parent component, the child component is dependent on the parent component. With that, there is no parent-child relationship; now components depend on others to get certain dependencies. Components need to expose types from the graph for dependent components to consume them.

Note: This issue happens whenever you want to create a subcomponent of ApplicationComponent . If you need to create a regular gradle module that depends on a feature module and needs to create a component that depends on a component defined in that feature module, you can use subcomponents as usual.

For example: a feature module called login wants to build a LoginComponent that depends on the AppComponent available in the app Gradle module.

Below are definitions for the classes and the AppComponent that are part of the app Gradle module:

Kotlin // UserRepository's dependencies class UserLocalDataSource @Inject constructor() { ... } class UserRemoteDataSource @Inject constructor() { ... } // UserRepository is scoped to AppComponent @Singleton class UserRepository @Inject constructor( private val localDataSource: UserLocalDataSource, private val remoteDataSource: UserRemoteDataSource ) { ... } @Singleton @Component interface AppComponent { ... } Java // UserRepository's dependencies public class UserLocalDataSource { @Inject public UserLocalDataSource() {} } public class UserRemoteDataSource { @Inject public UserRemoteDataSource() { } } // UserRepository is scoped to AppComponent @Singleton public class UserRepository { private final UserLocalDataSource userLocalDataSource; private final UserRemoteDataSource userRemoteDataSource; @Inject public UserRepository(UserLocalDataSource userLocalDataSource, UserRemoteDataSource userRemoteDataSource) { this.userLocalDataSource = userLocalDataSource; this.userRemoteDataSource = userRemoteDataSource; } } @Singleton @Component public interface ApplicationComponent { ... }

In your login gradle module that includes the app gradle module, you have a LoginActivity that needs a LoginViewModel instance to be injected:

Kotlin // LoginViewModel depends on UserRepository that is scoped to AppComponent class LoginViewModel @Inject constructor( private val userRepository: UserRepository ) { ... } Java // LoginViewModel depends on UserRepository that is scoped to AppComponent public class LoginViewModel { private final UserRepository userRepository; @Inject public LoginViewModel(UserRepository userRepository) { this.userRepository = userRepository; } }

LoginViewModel has a dependency on UserRepository that is available and scoped to AppComponent . Let's create a LoginComponent that depends on AppComponent to inject LoginActivity :

Kotlin // Use the dependencies attribute in the Component annotation to specify the // dependencies of this Component @Component(dependencies = [AppComponent::class]) interface LoginComponent { fun inject(activity: LoginActivity) } Java // Use the dependencies attribute in the Component annotation to specify the // dependencies of this Component @Component(dependencies = AppComponent.class) public interface LoginComponent { void inject(LoginActivity loginActivity); }

LoginComponent specifies a dependency on AppComponent by adding it to the dependencies parameter of the component annotation. Because LoginActivity will be injected by Dagger, add the inject() method to the interface.

Note: LoginComponent is annotated with @Component and not with @Subcomponent as you did in the is annotated withand not withas you did in the Using Dagger in an Android app page

When creating a LoginComponent , an instance of AppComponent needs to be passed in. Use the component factory to do it:

Kotlin @Component(dependencies = [AppComponent::class]) interface LoginComponent { @Component.Factory interface Factory { // Takes an instance of AppComponent when creating // an instance of LoginComponent fun create(appComponent: AppComponent): LoginComponent } fun inject(activity: LoginActivity) } Java @Component(dependencies = AppComponent.class) public interface LoginComponent { @Component.Factory interface Factory { // Takes an instance of AppComponent when creating // an instance of LoginComponent LoginComponent create(AppComponent appComponent); } void inject(LoginActivity loginActivity); }

Now, LoginActivity can create an instance of LoginComponent and call the inject() method.

Kotlin class LoginActivity: Activity() { // You want Dagger to provide an instance of LoginViewModel from the Login graph @Inject lateinit var loginViewModel: LoginViewModel override fun onCreate(savedInstanceState: Bundle?) { // Gets appComponent from MyApplication available in the base Gradle module val appComponent = (applicationContext as MyApplication).appComponent // Creates a new instance of LoginComponent // Injects the component to populate the @Inject fields DaggerLoginComponent.factory().create(appComponent).inject(this) super.onCreate(savedInstanceState) // Now you can access loginViewModel } } Java public class LoginActivity extends Activity { // You want Dagger to provide an instance of LoginViewModel from the Login graph @Inject LoginViewModel loginViewModel; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); // Gets appComponent from MyApplication available in the base Gradle module AppComponent appComponent = ((MyApplication) getApplicationContext()).appComponent; // Creates a new instance of LoginComponent // Injects the component to populate the @Inject fields DaggerLoginComponent.factory().create(appComponent).inject(this); // Now you can access loginViewModel } }

LoginViewModel depends on UserRepository ; and for LoginComponent to be able to access it from AppComponent , AppComponent needs to expose it in its interface:

Kotlin @Singleton @Component interface AppComponent { fun userRepository(): UserRepository } Java @Singleton @Component public interface AppComponent { UserRepository userRepository(); }

The scoping rules with dependent components work in the same way as with subcomponents. Because LoginComponent uses an instance of AppComponent , they cannot use the same scope annotation.

If you wanted to scope LoginViewModel to LoginComponent , you would do it as you did previously using the custom @ActivityScope annotation.

Kotlin @ActivityScope @Component(dependencies = [AppComponent::class]) interface LoginComponent { ... } @ActivityScope class LoginViewModel @Inject constructor( private val userRepository: UserRepository ) { ... } Java @ActivityScope @Component(dependencies = AppComponent.class) public interface LoginComponent { ... } @ActivityScope public class LoginViewModel { private final UserRepository userRepository; @Inject public LoginViewModel(UserRepository userRepository) { this.userRepository = userRepository; } }

Note: Not exposing all the types that a dependent component needs from a component will result in a Dagger compile time error because it cannot provide certain types for the dependent component.

Best practices