There are still a couple of more places in the code that need Akka Streams based refactoring work, which will be done by you as homework. The first place is really a continuation of the work that we did in the previous section. As part of that refactoring, we built a system where a ViewBuilder class can define a subflow to handle the DeferredCreate situation. We have one such situation with the SalesOrderViewBuilder class where we need to define that subflow. This is one of the pieces of work that you will need to do for this chapter. Go into the -incomplete code folder for this chapter and fill in the subflow implementation for the OrderCreated event handling. Currently, I have stubbed that flow out, as follows:

val flow = Flow[EnvelopeAndAction] 

As it stands, this Flow will do nothing, just passing the input EnvelopeAndAction back out as it is. You will need to add processing stages to this Flow so that it loads the books for each line item and then uses that...