foldAsync

Just like fold but receives a function that results in a Future CompletionStage to the next value.

Simple operators

Signature

Source.foldAsyncSource.foldAsync Flow.foldAsyncFlow.foldAsync

Description

Just like fold but receives a function that results in a Future CompletionStage to the next value.

Example

foldAsync is typically used to ‘fold up’ the incoming values into an aggregate asynchronously. For example, you might want to summarize the incoming values into a histogram:

Scala

copysourceimport akka.actor.ActorSystem
import akka.stream.scaladsl.Source

import scala.concurrent.{ ExecutionContext, Future }
case class Histogram(low: Long = 0, high: Long = 0) {
  def add(i: Int): Future[Histogram] =
    if (i < 100) Future { copy(low = low + 1) } else Future { copy(high = high + 1) }
}

Source(1 to 150).foldAsync(Histogram())((acc, n) => acc.add(n)).runForeach(println)

// Prints: Histogram(99,51)

Java

copysourceclass Histogram {
  final long low;
  final long high;

  private Histogram(long low, long high) {
    this.low = low;
    this.high = high;
  }

  // Immutable start value
  public static Histogram INSTANCE = new Histogram(0L, 0L);

  public CompletionStage<Histogram> addAsync(Integer n) {
    if (n < 100) {
      return CompletableFuture.supplyAsync(() -> new Histogram(low + 1L, high));
    } else {
      return CompletableFuture.supplyAsync(() -> new Histogram(low, high + 1L));
    }
  }
}

  // Folding over the numbers from 1 to 150:
  Source.range(1, 150)
      .foldAsync(Histogram.INSTANCE, Histogram::addAsync)
      .runForeach(h -> System.out.println("Histogram(" + h.low + ", " + h.high + ")"), system);

  // Prints: Histogram(99, 51)

Reactive Streams semantics