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Main annotations in Dagger 2
The main annotations in Dagger 2 are:
@Provides@Inject@Module@Component@Singleton
Provides
This annotation used for method that describe how to create instance of the class
@Provides
fun provideSomeClass(): SomeClass = SomeClass()
Singleton
Annotate an @Provides method or injectable class with @Singleton. The graph will use a single instance of the value for all of its clients.
@Provides
@Singleton
fun provideSomeClass(): SomeClass = SomeClass()
Inject
the @Inject annotation will tell the Dagger what all the dependencies needed to be transferred to the dependent. Can be used in constructor, field or method.
Constructor injection:
class SomeClass @Inject constructor(
private val interactor: Interactor,
private val useCase: UseCase
) {...}
Field injection:
class SomeClass() {
@Inject lateinit var viewModel: SomeViewModel
} {...}
Method injection:
class SomeClass() {
lateinit var user: User
@Inject
fun getUserName(userFromPreference: UserFromPreference) {
user = userFromPreference.getUser()
}
}
Dagger 2 manages the dependency graph of the classes that have the @Inject and @Provides annotation. Dagger cannot instantiate or inject classes that do not have neither @Inject nor @Provides annotations.
Module
All @Provides methods must belong to a module. These are just classes that have an @Module annotation. Module provides the instance of dependencies.
@Module
interface SomeModule {
@Provides
@Singleton
fun provideSomeClass(): SomeClass = SomeClass()
}
Component
The @Component is used on an interface. Such an interface is used by Dagger 2 to generate code. A @Component interface defines the connection between provider of objects (modules) and the objects which expresses a dependency.
@Component(modules = SomeModule.class)
interface ModuleComponent {
SomeClass someClassInstance();
}
Links
Using Dagger 2 for dependency injection in Android - Tutorial
Dependency injection with Dagger 2: @Inject and @Provides
Next questions
What the difference between @Bind and @Provides in Dagger 2?