This is not an issue with the book per se, except in the fact that I wish the book addressed this problem, even if it can not be done. If an event that creates new data occurs in the app, I want that event to be handled first by a sink on a background thread. Then after that sink processes the event I would like to transform the data and pass the transformed data on to another sink on the main thread. The idea is to have an order of events, like a dependency. How is this done with combine?