As technologies change, we see how basic patterns recur over time. In many ways, there aren’t that many new things to learn — just new variations and combinations of existing technologies and concepts. (See my book “Learn Computer Science with Swift” for more on the patterns that recur). As always, there are many people to thank for helping on this book. Most important are the people who have contributed to the technologies. When it comes to the many open source technologies (including ReactiveX and its projects), there are more and more people working on the technologies, and that makes it easier for everyone. Closer to home, Aaron Crabtree has provided very helpful and watchful comments on the manuscript. And, as always, Carole Jelen at Waterside Productions has helped make this book possible.