Multibinding with @ContributesBinding

I’d like to propose a(couple) multibinding version(s) of @ContributesBinding to reduce the need for obvious, hand-written modules.

I often use multibindings in tandem with factories to enable constructor injection for types the Android framework is ultimately responsible for instantiating.

Map Multibinding

@ContributesBindingIntoMap(AppScope::class, mapKey = PlaylistViewModel::class)
class PlaylistViewModel @Inject constructor(streamer: AudioStreamer): ViewModel

// resulting in

[...]
interface Module {
    @Binds
    @IntoMap
    @Named("com.example.PlaylistViewModel")
    fun providesComExamplePlaylistViewModel(viewModel: PlaylistViewModel): ViewModel

Set Multibinding

@ContributesBindingIntoSet(AppScope::class)
class IndexMigration @Inject constructor(currentUser: User): Migration

// resulting in

interface Module {
    @Binds
    @IntoSet
    fun providesComExampleIndexMigration(migration: IndexMigration): Migration

Alternative: Modify the existing ContributesBinding annotation

@Target(CLASS)
@Retention(RUNTIME)
annotation class ContributesBinding(
  val scope: KClass<*>,
  val boundType: KClass<*> = Unit::class,
  val replaces: Array<KClass<*>> = [],
  val mapKey: KClass<*> = Unit::class,
  val intoSet: Boolean = false
)

This would minimize the Anvil API surface, but it doesn’t provide a strong API. For example, there’s nothing preventing a person from suggesting they would like map and set multibinding.

I have a rough prototype running as well, with a few issues in the map multibinding scenario.

[vRallev]

PRs are up #216 and #219.

[vRallev]

That is pretty much what we agreed on internally. At the moment we’re leaning towards option 1. The first step will be to make qualifiers work for @ContributesBinding without multibindings. There’s no reason to not support this qualifiers for that use case.

We want to do quite some testing internally first (also benchmarks). I’m currently on parental leave, but want to take a look at this in Q2.