EitherCallAdapterFactory.kt

internal class EitherCallAdapterFactory : CallAdapter.Factory() {

    override fun get(
        returnType: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ): CallAdapter<*, *>? {
        if (getRawType(returnType) != Call::class.java) return null
        check(returnType is ParameterizedType) { "Return type must be a parameterized type." }

        val responseType = getParameterUpperBound(0, returnType)
        if (getRawType(responseType) != Either::class.java) return null
        check(responseType is ParameterizedType) { "Response type must be a parameterized type." }

        val leftType = getParameterUpperBound(0, responseType)
        if (getRawType(leftType) != ApiError::class.java) return null

        val rightType = getParameterUpperBound(1, responseType)
        return EitherCallAdapter<Any>(rightType)
    }
}