Over on some of the other Swift mailing lists, people have been asking about Swift logging engines; since this list seems to be the right place to discuss them, I hope you'll indulge me in letting you know about CleanroomLogger, a real logging engine written in pure Swift:
Why do I say it's a real logging engine? Because the other pure Swift loggers I've run across are all just wrappers around print().
Loggers that just call print() aren't actually sending messages to the system console; if you use these loggers and expect to read their output through Console.app, you'll be out of luck.
CleanroomLogger writes to the Apple System Log (ASL) facility, just like NSLog() does. But unlike NSLog(), which can bog down your app if not used judiciously, CleanroomLogger is designed to be performant, so you don't have to worry that your desire for diagnostics is going to kill your app's performance.
CleanroomLogger is mature enough to be shipping in significantly-revenue-generating apps, and the project now has over 500 stars on GitHub. I hope you'll find it useful.
In the meantime, if you're interested in furthering the state of Swift logging, please reach out to me. Alex Kolov has set up a swift-logging org on GitHub to help drive Swift logging standards; community involvement desired:
Thanks for your time,