Support multiple bound types for contributed bindings and multibindings

Let’s say I have two interfaces A and B and class implements both:

class C @Inject constructor(
   ......
): A, B { 
}

to bind it I can add to dagger module:

@Binds
@IntoSet
abstract fun bindA(c: C): A

@Binds
@IntoSet
abstract fun bindB(c: C): B

As I understand, currently I can bind it only to one of the interfaces via @ContributesMultibinding e.g. @ContributesMultibinding(AppScope::class, A::class) (and binding to B is still in the dagger module). Is there a better solution?

Thanks in advance!

[vRallev]

I thought about deprecating the boundType = KClass attribute in favor of boundTypes = [KClass], but I don’t like this approach for two reasons. First, the new attribute would need to be added to the end of the attribute list for source compatibility. The new ordering of the attributes would be strange (right now boundType is the 2nd attribute, boundTypes would be at the 6th position). Second, the implicit behavior of ContributesBinding and ContributesMultibinding is that if there’s one single super type, then it serves as the default bound type. It’s awkward to allow declaring multiple bound types manually, but not respecting multiple super types implicitly. I won’t move forward with this option.

The other solution is to allow repeated annotations. However, Kotlin only supports source retention for repeated annotations, but Anvil requires runtime retention to read all annotations from binaries (actually binary retention would be enough, but we decided for source retention to leave the door open for interesting options). So we can’t use repeated annotations today.

Kotlin 1.6 will support repeated annotation with a binary and source retention. The ticket is KT-12794 and the Keep is available here. For the timeline they mention:

The prototype is being worked on, and the feature is planned to be included under the -language-version 1.6 flag to Kotlin 1.5.30, and enabled by default since Kotlin 1.6.

That means we could start prototyping this solution relatively soon. My proposal for Anvil would be to make ContributesTo, ContributesBinding and ContributesMultibinding repeatable annotations.

[vRallev]

I’d like to give an update for this ticket. I’ve implemented repeatable annotations for @ContributesTo already. It’ll be a big change for the Anvil internals, but it’s definitely very convenient on the consumers side.

The problem is that Kotlin 1.6.0 shipped with significant bugs, e.g. incremental compilation is broken. We’ll ship Anvil versions for 1.5 and 1.6 for a longer period of time. But this also means that annotations can’t be repeatable, yet.