In late 2019, Johannes released a health & fitness application called Thirstic.
Thirstic is the first smart water tracker & reminder that adapts to the users activity and the weather conditions. This means in practice that, every day, the app adapts to the current user conditions. If the user is more active or if the environment is very hot, the water intake goal will automatically increase. It even learns patterns about user workouts and weather conditions and estimates water needs for future days. You can read more about all the features in this blog post.
Johannes worked on this app, as a side-project, for almost 4 years. It is entirely written in Swift, features a simple and intuitive user interface and it supports many modern iOS system features (like Dark Mode and adaptive text). You can download Thirstic from the App Store.