How does the component hierarchy and lifecycle scopes work in Hilt?

Short answer

Scopes tie dependency objects to the lifecycle of specific Android components: @Singleton for the whole app, @ActivityScoped for the activity, and @ViewModelScoped for the ViewModel's lifecycle.

Explanation

If you do not specify a scope (unscoped), Hilt creates a new instance of the object every time it is requested. Specifying @Singleton guarantees only one instance exists in the whole app inside the SingletonComponent. Specifying @ActivityScoped guarantees the same instance is shared for as long as the Activity lives, while @ViewModelScoped keeps the instance alive across configuration changes with the ViewModel.

Code example

@Module
@InstallIn(ViewModelComponent::class)
object FeatureModule {
    @Provides
    @ViewModelScoped
    fun provideFeatureState(): FeatureState = FeatureState()
}

Common mistakes

• Using @Singleton on everything, which consumes memory with no reason and keeps unnecessary objects alive all the time.
• Trying to inject an object with a narrow scope (such as @ActivityScoped) into an object with a broader scope (such as @Singleton), causing a compile-time error.

Follow-up questions

• How do you resolve the conflict of providing two different instances of the same type using qualifiers in Hilt?

Sources

• Android Developers — Component scopes in Hilt