ProductsApi.kt

interface ProductsApi {

    @GET("products")
    fun searchProducts(@Query("searchTerm") searchTerm: String, @Query("page") page: Int) : Call<ProductsResponse>

    @GET
    fun getPageOfResults(@Url nextPageUrl: String) : Call<ProductsResponse>
}

ProductsDataSource.kt

class ProductsDataSource constructor(private val productsApi: ProductsApi,
                                     private val searchTerm: String) : PageKeyedDataSource<String, Product>() {

    override fun loadInitial(params: LoadInitialParams<String>, callback: LoadInitialCallback<String, Product>) {

        val productsResponse = productsApi.searchProducts(searchTerm, 1).execute()
        val nextPageUrl = productsResponse.body()?.meta?.nextPageUrl ?: null
        val products = productsResponse.body()?.products ?: listOf()

        callback.onResult(products, null, nextPageUrl)
    }

    override fun loadAfter(params: LoadParams<String>, callback: LoadCallback<String, Product>) {
        val productsResponse = productsApi.getPageOfResults(params.key).execute()

        val nextPageUrl = productsResponse.body()?.meta?.nextPageUrl ?: null
        val products = productsResponse.body()?.products ?: listOf()

        callback.onResult(products, nextPageUrl)
    }

    override fun loadBefore(params: LoadParams<String>, callback: LoadCallback<String, Product>) {
        val productsResponse = productsApi.getPageOfResults(params.key).execute()

        val nextPageUrl = productsResponse.body()?.meta?.nextPageUrl ?: null
        val products = productsResponse.body()?.products ?: listOf()

        callback.onResult(products, nextPageUrl)
    }
}

ProductsDataSourceFactory.kt

class ProductsDataSourceFactory @Inject constructor(private val productsApi: ProductsApi,
                                                    private val searchTerm: String) : DataSource.Factory<String, Product>() {
    override fun create(): DataSource<String, Product> {
        return ProductsDataSource(productsApi, searchTerm)
    }

}