Kotlin+Dagger best practices/documentation/pain points

Opening this as a tracking bug for all kotlin related documentation that we should be add/best practices that we should call out to make using Dagger w/ Kotlin easier.

One example: How to achieve the effect of static @Provides in Kotlin.

Feel free to comment new ideas, but don’t make “me too” or “i agree with XYZ” comments.

[ZacSweers]

Edit: This resolved in Dagger 2.25+ by https://github.com/google/dagger/commit/646e0336cdbe454baa5fe73c0af11f211a92deeb

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If you have injected properties (as “fields”), qualifiers must have field: designation.

Good

@Inject @field:ForApi lateinit var gson: Gson

Bad

@Inject @ForApi lateinit var gson: Gson // @ForApi is ignored!

Forgetting this can lead to subtle bugs if an unqualified instance of that type also exists on the graph at that scope, as that’s the one that will end up being injected. This would make a great lint as well

[ZacSweers]

Edit: Objects are handled in Dagger 2.25+. Companion objects are handled in Dagger 2.26+.

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Static provides can be achieved via @JvmStatic. There are two scenarios I see this come up:

top-level objects

@Module
object DataModule {
  @JvmStatic @Provides fun provideDiskCache() = DiskCache()
}

If you have an existing class module, things get a bit weirder

@Module
abstract class DataModule {
  @Binds abstract fun provideCache(diskCache: DiskCache): Cache

  @Module
  companion object {
    @JvmStatic @Provides fun provideDiskCache() = DiskCache()
  }
}

The way this works is as follows:

• the companion object must also be annotated as @Module
• under the hood, the kotlin compiler will duplicate those static provides methods into the DataModule class. Dagger will see those and treat them like regular static fields. Dagger will also see them in the companion object, but that “module” will get code gen from dagger but be marked as “unused”. The IDE will mark this as such, as the provideDiskCache method will be marked as unused. You can tell IntelliJ to ignore this for annotations annotated with @Provides via quickfix

I sent Ron a braindump once of how dagger could better leverage this, i.e. no requirement for JvmStatic and just call through to the generated Companion class, but I think that’s out of the scope of this issue 😃. I’ve been meaning to write up a more concrete proposal for how that would work.