Hilt - Inject a list of Interface implementations - dagger-hilt

Question

Let me show what I mean by code.
Interface:
interface MyInterface {
}
I have 2 implementation classes:
#Singleton
class Implementation1 #Inject constructor(
private val gson: Gson,
) : MyInterface {
}
#Singleton
class Implementation2 #Inject constructor(
#ApplicationContext private val context: Context,
) : MyInterface {
}
I need a repository class with list of implementations of MyInterface:
#Singleton
class MyRepository #Inject constructor(
private val implementations: List<MyInterface>,
) {
}
Problem part: I am trying to inject as below:
#InstallIn(SingletonComponent::class)
#Module
object MyDiModule {
#Provides
fun providesImplementations(
imp1: Implementation1,
imp2: Implementation2,
): List<MyInterface> {
return listOf(imp1, imp2)
}
}
But I get compilation error:
/home/sayantan/AndroidStudioProjects/example/app/build/generated/hilt/component_sources/debug/com/example/ExampleApplication_HiltComponents.java:188: error: [Dagger/MissingBinding] java.util.List<? extends com.example.MyInterface> cannot be provided without an #Provides-annotated method.
public abstract static class SingletonC implements FragmentGetContextFix.FragmentGetContextFixEntryPoint,
^
Any way to achieve this?

Answer 1

This might be late but would like to drop my thoughts.
Firstly, you need to annotate your #Provides methods in the module class because dagger would certainly throw an error of multiple bindings of MyInterface.
For example:
enum class ImplType {
Impl1, Impl2
}
#Qualifier
#Retention(AnnotationRetention.RUNTIME)
annotation class ImplTypeAnnotation(val type: ImplType)
#Singleton
#Provides
#ImplTypeAnnotation(ImplType.Impl1)
fun provideImplementation1(impl1: Implementation1): MyInterface
// Do the same for Implementation2
Secondly, once these are done, you need to individually inject them into the Repository class. e.g.
class MyRepository #Inject constructor(
#ImplTypeAnnotation(ImplType.Impl1) private val implementation1: MyInterface,
#ImplTypeAnnotation(ImplType.Impl2) private val implementation2: MyInterface
) {
// get the implementations as list here using Kotlin listOf()
}
NB: I'm not sure if providing List<MyInterface> would work.
Hope this helps!

Answer 2

I do not know how correct this answer is, but it works for me, so posting as answer.
This can be done using the annotation #JvmSuppressWildcards.
In the above example, everything is fine, I just need to add this annotation to the class where the dependencies are being injected, i.e. the MyRepository class.
#JvmSuppressWildcards
#Singleton
class MyRepository #Inject constructor(
private val implementations: List<MyInterface>,
) {
}

Answer 3

I was seeing the same problem in the project I've been working on, but moving from List<> to Array<> seemed to fix it for me.
So in your case the hilt injection would look like:
#Provides
fun providesImplementations(
imp1: Implementation1,
imp2: Implementation2,
): Array<MyInterface> {
return arrayOf(imp1, imp2)
}
Then used with:
#Singleton
class MyRepository #Inject constructor(
private val implementations: Array<MyInterface>,
) {
}