The BoundaryCallback example from the Google talk on the Android Paging Library for future reference.

BoundaryCallbackExample.kt

class ItemBoundaryCallback(private val service: MyApi,
                           private val database: MyDatabase) : PagedList.BoundaryCallback<Item> {
	val isLoading = false
	
	override fun onItemAtEndLoaded(itemAtEnd: Item) {
		if (isLoading) return
    		isLoading = true
  		networkExecutor.execute {
        		val response = service
				.getTopAfter(itemAtEnd.name, NETWORK_PAGE_SIZE)
                        	.execute()
        		if (response.isSucessful) {
        			ioExecutor.execute {
            				database.dao().insert(response.body())
            				isLoading = false
            			}
        		}
    		}
  	} 
}