scala observable使用没有中间数据结构更新的序列统一可观察

Question

我有一个代码调用couchbase获取一些行如下:

val gotValues: Observable[JsonDocument] = Observable.from(rowKeys).flatMap(id =>
      couchbaseBucket.async().get(id))

如果我有1,2,3,4,5,6作为输入行键,并且DB中只存在1,2,3行,那么observable只会收到1,2,3的通知.

然而,我的要求是我返回一个1,2,3 true(存在于db中)和4,5,6 with false(在DB中不存在)的映射.我设法用scala observable做到了,但是我使用中间地图数据结构来返回包含所有id的总地图.下面是一个模拟我的问题的示例代码..

object Main extends App {
  import rx.lang.scala.Observable

  val idsToFetch = Seq(1,2,3,4,5,6)

  println(isInDBOrNot()) // {1=true, 2=true, 3=true, 4=false, 5=false, 6=false}

  private def isInDBOrNot(): ConcurrentHashMap[Int, Boolean] = {
    val inAndNotInDB = new java.util.concurrent.ConcurrentHashMap[Int, Boolean]
    // - How can I avoid the additional data structure?
    // - In this case a map, so that the function will return
    //   a map with all numbers and for each if exist in DB?
    // - I mean I want the function to return a map I don't 
    //   want to populate that map inside the observer,
    //   it's like a mini side effect I would rather simply 
    //   manipulate the stream.

    Observable.from(idsToFetch)
      .filterNot(x => x == 4 || x == 5 || x == 6) // Simulate fetch from DB, 4,5,6 do not exist in DB, so not returned.
      .subscribe(
      x => inAndNotInDB.put(x, true),
      e => println(e),
      () => idsToFetch.filterNot(inAndNotInDB.containsKey)
        .foreach(inAndNotInDB.put(_, false)) // mark all non-found as false.
    )

    inAndNotInDB
  }

}

无论如何没有中间映射(没有填充中间数据结构,但只能通过操纵流)？看起来不干净!! .谢谢.

Answer 1

您的问题似乎是因为您使用了flatMap这样的事实,如果DB中没有给定给定的数据id并且您得到一个空的Observable,那么flatMap就不会产生任何输出id.所以看起来你需要的是defaultIfEmpty,它被翻译成Scala的orElse.您可以使用orElse内部返回一些默认值flatMap.所以要修改你的例子:

def fetchFromDb(id: Int): Observable[String] = {
  if (id <= 3)
    Observable.just(s"Document #$id")
  else
    Observable.empty
}

def gotValue(idsToFetch: Seq[Int]): Observable[(Int, Boolean)] = {
  Observable.from(idsToFetch).flatMap((id: Int) => fetchFromDb(id).map(_ => (id, true)).orElse((id, false)))
}

println(gotValue(Seq(1, 2, 3, 4, 5, 6)).toBlocking.toList)

打印

List((1,true),(2,true),(3,true),(4,false),(5,false),(6,false))

或者你可以Option用来返回Some(JsonDocument)或者None如

def gotValueEx(idsToFetch: Seq[Int]): Observable[(Int, Option[String])] = {
  Observable.from(idsToFetch).flatMap((id: Int) => fetchFromDb(id).map(doc => (id, Option(doc))).orElse((id, None)))
}

println(gotValueEx(Seq(1, 2, 3, 4, 5, 6)).toBlocking.toList)

打印

列表((1,Some(Document#1)),(2,Some(Document#2)),(3,Some(Document#3)),(4,None),(5,None),(6,没有))