Binds

Using the @Binds annotation, it's easy to provide a type as supertypes.

@Binds
@Provides
class RepositoryImpl: Repository

interface Repository

This is a shortcut for the following implementation.

class RepositoryImpl: Repository

interface Repository

@Provides
fun provideRepository(): Repository {
    return RepositoryImpl()
}

If a type has multiple supertypes, use the to parameter to specify the target type.

@Binds(to = Type2::class)
@Provides
class Type: Type1, Type2

interface Type1
interface Type2

Provide as multiple types

@Binds can only be used to provide one type and cannot be injected as its original type.

For example, given the following code:

@Binds
@Provides
class RepositoryImpl: Repository

interface Repository

You can only inject Repository, but not RepositoryImpl.

val repository = inject<Repository>()
val repositoryImpl = inject<RepositoryImpl>() // NotProvidedException!

To provide one type as multiple types, you can use the @Provides annotation.

Here is an example that provides both RepositoryImpl and Repository:

@Provides
class RepositoryImpl: Repository {
    companion object {
        @Provides
        fun provideAsRepository(impl: RepositoryImpl): Repository {
            return impl
        }
    }
}

interface Repository

You can now inject both Repository and RepositoryImpl respectively.

val repository = inject<Repository>() // RepositoryImpl
val repositoryImpl = inject<RepositoryImpl>() // RepositoryImpl

With Qualifier

Types may conflict as a result of providing them with supertypes using @Binds. To distinguish between them, you can use qualifiers.

@Qualifier
@Retention(AnnotationRetention.BINARY)
annotation class ID1

@Qualifier
@Retention(AnnotationRetention.BINARY)
annotation class ID2

@ID1
@Binds
@Provides
class DB1: DB

@ID1
@Binds
@Provides
class DB2: DB

interface DB

In the example above, both DB1 and DB2 are bound to DB using @Binds. To distinguish between them when injecting, qualifiers ID1 and ID2 are used. In this way, you can get the specific instance you need by providing the corresponding qualifier.

val db1 = inject<DB>(ID1()) // DB1
val db2 = inject<DB>(ID2()) // DB2