I can finally reveal what I have been playing with recently:
It is a workout timer which plays music, aimed at training sessions with repeated high-intensity and rest phases. The app will play music when you are working and will be quiet when you are not.
A typical usage example is circuit sessions where you work hard for instance for 45 seconds and rest for 15 seconds, and repeat. In a group, you may want to use the quiet periods to tell your friends what the next exercise is.
You can freely configure the timings and your music playlist, even add songs from Apple Music.
As an universal app it supports both iPhone and iPad. There is even a playback app for Apple TV.
There are neither subscriptions nor in-app purchases, no ads, no pestering review requests, and no usage tracking.
The app is written entirely in Swift, uses MusicKit for playback, and CloudKit with Core Data to synchronise sessions between devices via iCloud. I suppose it could have been finished earlier, but I decided to transition to SwiftUI, which requires iOS 14. Fastlane is used to automate the creation of screenshots.