What is the use case for @Binds vs @Provides annotation in Dagger2

Question

I am not certain on the purpose for Dagger2\'s @Bind annotation.

From what i have read online im still not clear but here is an example:

@Module
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Answer 1

Here a concrete case where you need Bind annotation, imagine you got a BaseActivityModule which is include in all your activity modules that provides your activity viewmodel.

@Module
object BaseActivityModule {
    @Provides
    @ActivityScope
    @ActivityContext
    @JvmStatic
    fun provideViewModelProvider(
        activity: AppCompatActivity,
        viewModelFactory: ViewModelProvider.Factory
    ): ViewModelProvider = ViewModelProviders.of(activity, viewModelFactory)
}

Here you see we need to provide an AppCompatActivity and a ViewModelProvider.Factory. You cannot provide AppCompatActivity with a Provide annotation since activities are created by android.

We're assuming your concrete ActivityModule for example MainActivityModule will provide MainActivity class either because you create a MainActivity sub component or you used ContributesAndroidInjector to automatically create your sub components (but this is another talk).

So we have our MainActivityModule providing MainActivity and our MainActivityModule includes our BaseActivityModule which need an AppCompatActivity. So here the Bind magic, let's tell Dagger that when you need an AppCompatActivity you can use our MainActivity.

@Module(includes = [BaseActivityModule::class])
abstract class MainActivityModule {
    @Binds
    @ActivityScope
    abstract fun bindActivity(activity: MainActivity): AppCompatActivity
}

You can see more from my project template here