Gradle dependency not shared with dependent module

Question

Context

I have two modules in my project:

Java/Kotlin module common
Android/Kotlin module app

common depends on Koin, which is a Kotlin library for dependency injection:

dependencies {
  implementation 'org.koin:koin-core:1.0.2'
}

Usage example:

class MyPresenter: KoinComponent {
  ...
}

app does not depend on the Koin library because I do not need to inject anything in the Android code, all the injections are in the common code (presenters, interceptors, etc).

But app depends on common:

dependencies {
  implementation project(':common')
}

Usage example:

class MyFragment {
  private val presenter = MyPresenter()
}

Problem

I can compile common, I can run unit tests in common, but when I try to compile app I get this error:

Supertypes of the following classes cannot be resolved. Please make sure you have the required dependencies in the classpath: class xxx.common.presenter.MyPresenter, unresolved supertypes: org.koin.standalone.KoinComponent

When I run ./gradlew :app:dependencies

debugCompileClasspath
+--- project :common
debugRuntimeClasspath
+--- project :common
|    +--- org.koin:koin-core:1.0.2

The dependency is in the runtime configuration, but is missing from the compile configuration.

What I have tried so far:

Obviously I don't want to declare the Koin dependency in app so I have tried several things:

Change Koin dependency for api:

dependencies {
  api 'org.koin:koin-core:1.0.2'
}

Not working - I get the exact same dependency tree as with implementation.

Change project dependency configuration:

dependencies {
  implementation project(path: ':common', configuration: `compile`)
}

Not working - I wasn't sure of this one but I was hoping it would get the dependencies of common in compile configuration.

Change Koin dependency for compile:

dependencies {
  compile 'org.koin:koin-core:1.0.2'
}

Working! The dependency appears in the debugCompileClasspath and I am able to run the app.

Questions

Now I am confused:

As app does not use Koin directly, I though it would not need the dependency. Why does it? Is it because the static type of MyPresenter is KoinComponent?
I thought api was the same than the deprecated compile. It seems not.
Is there another way other than using the deprecated compile?

Louis Jacomet Louis Jacomet · Accepted Answer · 2019-01-30T17:31:34

Because you make Koin types appear in the APIs of common, then consumers of common need to know about the Koin types. They effectively become API.
The api configuration is what you should use and should work
The most likely explanation is that between the Android/Kotlin project on one side and the Java/Kotlin project on the other side have a different definition of what api is, how the consumable configuration apiElements is built or accessed or ...

To debug that, I would recommend creating a simple project that reproduces the problem and that can be shared as there might be a bug behind that in the android or kotlin plugin.