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.

Note
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.

Note

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 not always what you want, if not, use @ref(prependLazy)[prependLazy.md]

Example

Scala
source
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
sourceimport akka.stream.javadsl.Keep;
import akka.stream.javadsl.Source;
import akka.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).runWith(Sink.foreach(System.out::print), 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).runWith(Sink.foreach(System.out::print), system);
// this will print "Emma", "Emily", "Liam", "William"

Reactive Streams semantics

emits when the given stream has an element available; if the given input completes, it tries the current one

backpressures when downstream backpressures

completes when all upstreams complete

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