Dependency Injection in Android With Dagger2 (5)

Subcomponent

Guowei Lv

2 minute read

In previous post, we introduced the component dependencies. But there is one interesting subtlety: If Component B depends on Component A, B has access to all A’s exposed objects. Note that there is a difference between what a component exposes and what it provides.

Let’s look at PresentationComponent and ActivityComponent.

@PresentationScope
@Component(dependencies = [ActivityComponent::class], modules = [PresentationModule::class])
interface PresentationComponent {
    fun inject(fragment: QuestionsListFragment)
    fun inject(questionDetailsActivity: QuestionDetailsActivity)
}

Here in PresentationComponent, even though it provides all the objects that QuestionsListFragment and QuestionDetailsActivity need, it exposes nothing.

But in ActivityComponent:

@ActivityScope
@Component(dependencies = [AppComponent::class], modules = [ActivityModule::class])
interface ActivityComponent {
    fun layoutInflater(): LayoutInflater
    fun fragmentManager(): FragmentManager
    fun screensNavigator(): ScreensNavigator
    fun stackOverflowApi(): StackoverflowApi
}

It exposes LayoutInflater, FragmentManager, ScreensNavigator, StackoverflowApi. If we don’t write these functions to expose these objects, the PresentationComponent will not be able to access them.

Sometimes this can feel a bit inconvenient. E.g. in order for the PresentationComponent to have access to StackoverflowApi, ActivityComponent has to expose it, and also AppComponent has to expose it as well. Is there a way to get rid of this expose chain?

The answer is subcomponent.

Here is how it works:

  1. Subcomponents specified by @Subcomponent annotation.
  2. Parent Component exposes factory method which returns Subcomponent.
  3. The arguments of the factory method are Subcomponent’s Modules.
  4. Subcomponent get access to all services provided by parent(provided, not just exposed).

Now the Components are dead simple:

@AppScope
@Component(modules = [AppModule::class])
interface AppComponent {
    fun newActivityComponent(activityModule: ActivityModule): ActivityComponent
}
@ActivityScope
@Subcomponent(modules = [ActivityModule::class])
interface ActivityComponent {
    fun newPresentationComponent(presentationModule: PresentationModule): PresentationComponent
}
@PresentationScope
@Subcomponent(modules = [PresentationModule::class])
interface PresentationComponent {
    fun inject(fragment: QuestionsListFragment)
    fun inject(questionDetailsActivity: QuestionDetailsActivity)
}
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