SubwayMon is a Mac OS X screensaver patterned after the in-station countdown clocks in some New York City Subway stations.
If you're on High Sierra, you have to update to 10.13.4 to use SubwayMon 2.0. Swift screensavers were broken in earlier version of High Sierra.
There are two targets: SubwayMon (the screensaver itself) and SubwayApp (a
standalone Mac OS app). The app is there so you can test the thing without
installing and previewing a screensaver in System Preferences.
SubwayMonScreenSaverView and AppDelegate are the main files of the
screensaver and app, respectively; they are pretty minimal, and as much logic
as possible should be shared between the two targets.
The display is refreshed every 5 seconds (updating the time deltas given the most recent data fetched) and the train time data is re-downloaded every minute. The static GTFS file "stops.txt" is bundled with the screensaver; I'll have to manually update it if it ever changes.
The feed is fetched from an endpoint on my server, which caches the feed fetched from the MTA for up to 30 seconds. This is to comply with the MTA's terms of use.