Defers creation and materialization of a
Sink until there is a first element.
When the first element comes from upstream the
Future[Sink] is created. When that completes successfully with a sink that is materialized and inserted in the stream. The internal
Sink will not be created if the stream completes of fails before any element got through.
The materialized value of the
Sink will be the materialized value of the created internal flow if it is materialized and failed with a
akka.stream.NeverMaterializedException if the stream fails or completes without the flow being materialized.
Can be combined with prefixAndTail to base the sink on the first element.
See also lazySink.
Reactive Streams semantics
cancels if the future fails or if the created sink cancels
backpressures when initialized and when created sink backpressures