Defers creation and materialization of a Flow until there is a first element.

Simple operators




When the first element comes from upstream the actual Future[Flow] is created and when that completes it is materialized and inserted in the stream. The internal Flow will not be created if there are no elements on completion or failure of up or downstream.

The materialized value of the Flow 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.

See also lazyFlow.

Can be combined with prefixAndTail(1) to base the flow construction on the initial element triggering creation. See lazyFlow for sample.

Reactive Streams semantics

emits when the internal flow is successfully created and it emits

backpressures when the internal flow is successfully created and it backpressures

completes when upstream completes and all elements have been emitted from the internal flow

completes when upstream completes and all futures have been completed and all elements have been emitted

cancels when downstream cancels (keep reading) The operator’s default behaviour in case of downstream cancellation before nested flow materialization (future completion) is to cancel immediately. This behaviour can be controlled by setting the akka.stream.Attributes.NestedMaterializationCancellationPolicy.PropagateToNested attribute, this will delay downstream cancellation until nested flow’s materialization which is then immediately cancelled (with the original cancellation cause).

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