Hi! I just launched an app to help developers learn Kotlin through a series of 40+ short coding challenges (credits for the content go to Jetbrains). If you’re in the process of learning Kotlin, take a look!
Now I know not many people want to code on their phones, but the motivation is that since each koan is short (a few minutes), you can do these as you go about your day. Also, I’m betting some devs may not get enough time in a day to dedicate to learning Kotlin, hence this app.
Feedback is welcome! If you run into any bugs, feel free to open a Github issue 
Play store: https://play.google.com/store/apps/details?id=me.vickychijwani.kotlinkoans
Demo: Imgur
Code: GitHub - vickychijwani/kotlin-koans-android: Learn Kotlin programming right on your phone 📱 through a series of 40+ short coding challenges.