Metro Timer: Kyiv
A ride countdown that lives in the Dynamic Island — with no GPS, no network, and no background execution.
The constraint that shapes everything
On the metro you read, doze off, or put headphones on — and then you look up at the wrong station. Three things are unavailable to an app that wants to help, and only one of them is a choice.
No GPS
Eight of the network's fifty-two stations are above ground. The rest are in tunnels, where satellites are not an option.
No network
Same tunnels. Nothing can be fetched at the moment it is actually needed.
No background modes
This one is a decision, not a limitation. They cost battery, invite App Review questions, and buy nothing a pre-scheduled notification does not.
So the train's position cannot be measured. It has to be computed — from a timetable, anchored to the single event a passenger can supply for free: the moment the train starts moving.
That one tap is the whole contract. Everything else follows from it.
How it works
- Route. One line, or two via a transfer. In Kyiv any pair of lines meets at exactly one node, which keeps routing to a lookup rather than a graph search.
- Anchor. The tap on Start becomes t₀, recorded synchronously before any permission dialog can appear and skew it.
- Schedule. Arrival and departure for every station on the route are computed up front, along with the alert moment and the arrival moment.
- Notifications. Scheduled immediately, so they fire even if the app is force-quit on the platform.
- Countdown. Drawn by SwiftUI itself via
Text(timerInterval:). No process has to stay alive for it to tick. - Corrections. On iOS 17+ the −1 / +1 and “train departed” buttons in the Live Activity are App Intents: one tap on the Lock Screen wakes the app's own process and reschedules everything.
Where the model is uncertain, the passenger is the sensor — and a correction never requires unlocking the phone.
Screens
Swipe to see all five.
What it can't know
Being explicit about this is part of the design, not a caveat bolted on afterwards.
Waiting for a train after a transfer. Genuinely unknowable — anywhere from zero to a full headway. The model books a quarter of the current headway rather than the “correct” half, because the two errors do not cost the same. Underestimate, and the countdown runs ahead of the train: the passenger looks up early and keeps riding. Overestimate, and the warning arrives after their station. The whole product exists to prevent the second one, so the estimate leans early on purpose — and a “train departed” button erases the uncertainty entirely.
The stop counter freezes in the background. With no background modes, ActivityKit cannot be handed a deferred update. The timer keeps running on its own, but “3 stops left” holds its last value until something wakes the app. That is the price of the third constraint above, and it was paid knowingly.
Under the hood
| Target | Contents |
|---|---|
| App | SwiftUI screens and app-only services: accelerometer, GPS corrector, air-raid polling |
| Shared | Compiled into both app and widget: models, planner, trip engine, strings |
| Widget | Live Activity presentation only — Dynamic Island and Lock Screen |
Segment times are derived from the official Kyiv Metro timetable published on the city's open data portal — not estimated, not crowdsourced. A generator turns the published departure times into per-segment running and dwell times; on device, field calibration refines them per segment and per direction.
Both languages live as pairs on one line
(tr("Наступна — ваша", "Yours is next")), so there is no key to forget
and no .strings file to fall out of sync. 28 tests cover the planner,
the schedule, data consistency and localization.
Privacy: no analytics, no advertising, no third-party SDKs, no account. Exactly one feature touches the network — optional air-raid alerts — and it is off until you turn it on. Everything written to disk is excluded from backups and protected while the device is locked.
Status
Pre-release. Not on the App Store yet.
Feature-complete: routing with transfers, service hours, Live Activity with interactive corrections, calibration, trip journal, both languages throughout, VoiceOver labels and Reduce Motion support, store metadata and privacy policy.
What remains is field validation. The trip journal records the plan against the outcome and counts every correction, precisely so accuracy can be measured rather than asserted — and that data does not exist yet. No accuracy figure is claimed here for that reason.