I just released my first ever playground book for Apple's Swift Playgrounds app on iPad: Learn Core Graphics. This gives you the ability to work through examples of drawing shapes, text, images, and more, all while seeing the results of your code played back right next to it.
The free book is structured as a series of challenges of increasing difficulties, and by the end you'll have learned a fair chunk of Core Graphics that you can then go on to apply elsewhere. Because it uses the real Core Graphics API rather than a simplified approach, you can literally take the code you wrote and apply it directly in your own iOS apps.
If this first book benefits enough people it's likely I'll do more in the future. So, give it a try and let me know what you think!
To learn more, and subscribe to the playground, go to this page on an iPad that has Swift Playgrounds installed: Hacking with Swift in Swift Playgrounds. You'll see a "Click to Subscribe" button there that only works on iPads with Swift Playgrounds installed, and it will subscribe you to my playgrounds feed.
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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.