prepend

Prepends the given source to the flow, consuming it until completion before the original source is consumed.

Fan-in operators

Signature

Source.prependSource.prepend Flow.prependFlow.prepend

Description

Prepends the given source to the flow, consuming it until completion before the original source is consumed.

The `prepend` operator is for backwards compatibility reasons "detached" and will eagerly
demand an element from both upstreams when the stream is materialized and will then have a
one element buffer for each of the upstreams, this is most often not what you want, instead
use @ref(prependLazy)[prependLazy.md]

If materialized values needs to be collected prependMat is available.

Example

Scala

copysource
val ladies = Source(List("Emma", "Emily"))
val gentlemen = Source(List("Liam", "William"))

gentlemen.prepend(ladies).runWith(Sink.foreach(println))
// this will print "Emma", "Emily", "Liam", "William"

Java

copysourceimport org.apache.pekko.stream.javadsl.Keep;
import org.apache.pekko.stream.javadsl.Source;
import org.apache.pekko.stream.javadsl.Sink;

import java.util.*;

Source<String, NotUsed> ladies = Source.from(Arrays.asList("Emma", "Emily"));
Source<String, NotUsed> gentlemen = Source.from(Arrays.asList("Liam", "William"));
gentlemen.prepend(ladies).runForeach(System.out::println, system);
// this will print "Emma", "Emily", "Liam", "William"

Source<String, NotUsed> ladies = Source.from(Arrays.asList("Emma", "Emily"));
Source<String, NotUsed> gentlemen = Source.from(Arrays.asList("Liam", "William"));
gentlemen.prependLazy(ladies).runForeach(System.out::println, system);
// this will print "Emma", "Emily", "Liam", "William"

Reactive Streams semantics