Defers creation and materialization of a
Flow until there is a first element.
When the first element comes from upstream the actual
Flow is created and materialized. 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.
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