Create akka-stream Source from a pagination, using Source.unfoldAsync

UnfoldPages.scala

  // Inspired by a tweet from @trautonen 1/13/2016
  // Use Source.unfoldAsync to turn paginated database results into an akka-streams Source
  // unfold is the inverse of fold

  case class Page[T](pageNumber:Long, totalPages:Long, contents:List[T])
  case class Thing(id: Long, name: String = "foo")
  
  val totalPages = 5 // 
  val pageSize = 3

  // Imagine instead a Slick 3 Database call. The total pages would come from the base query, rather than being supplied
  def fetchPage(pageNumber: Long, pageSize: Long, totalPages: Long): Future[Page[Thing]] = Future {
    val start = pageNumber * pageSize
    Page(pageNumber, totalPages, List(Thing(start), Thing(start + 1), Thing(start + 2)))
  }

  val sink: Sink[Any, Future[Unit]] = Sink.foreach(println)

  // Create a Source using Source.unfoldAsync
  val startPage = 1
  val source = Source.unfoldAsync(startPage) { pageNum => 
    val futurePage: Future[Page[Thing]] = fetchPage(pageNum, pageSize, totalPages)
    
    val next = futurePage.map(page => if (page.pageNumber > page.totalPages) None else Some((pageNum + 1, page)))
    next
  }

  val result: Future[Unit] = source.runWith(sink)
  result.onComplete { _ => system.shutdown() }
/*
Page(1,5,List(Thing(3,foo), Thing(4,foo), Thing(5,foo)))
Page(2,5,List(Thing(6,foo), Thing(7,foo), Thing(8,foo)))
Page(3,5,List(Thing(9,foo), Thing(10,foo), Thing(11,foo)))
Page(4,5,List(Thing(12,foo), Thing(13,foo), Thing(14,foo)))
Page(5,5,List(Thing(15,foo), Thing(16,foo), Thing(17,foo)))
 */