Swift extensions are the smart way to add functionality to existing types, and you're going to meet them time and time again – and hopefully write quite a few of your own too. They aren't all-encompassing, because they don't let you add properties to a class whereas a full subclass would, but they are easy to use and easy to share so I'm sure you'll use them frequently.
In this project, you've also learned a little of how useful Swift playgrounds can be for prototyping code, because the immediate feedback you get makes it extremely easy to try things out and make quick adjustments.
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One of the most effective motivators of success is sharing your progress with other people – when you tell folks what you're doing and what you've learned, it encourages you to come back for more, which in turn will help you reach your app development goals faster.
So, now that you've done all the hard work it's time to share your success: tell folks that you've completed this project, either by clicking the button below to start composing a tweet, or by writing your own message from scratch. This will definitely encourage you to keep learning, but it will also help other folks discover my work – thank you!
Paul Hudson is the creator of Hacking with Swift, the most comprehensive series of Swift books in the world. He's also the editor of Swift Developer News, the maintainer of the Swift Knowledge Base, and Mario Kart world champion. OK, so that last part isn't true. If you're curious you can learn more here.