A group shares a fenced area, and the moment anyone leaves it, everyone else is told, without GPS drift crying wolf.
For the people using it
On a trek, at a festival or on a work site, someone drifts away and nobody notices until it matters. I kept seeing companies and families wanting a simple way to know when someone has left an area. GeoFence does that: create a fence, share a code, and the whole group is told when anyone steps outside.
Why I built it
I wanted to try OpenStreetMap and the free alternatives to Google Maps, and to solve the part that's actually hard. Checking whether someone crossed a circle is one line of maths. Deciding they really did is the whole problem: near the edge, GPS wanders tens of metres either way, and a naive check tells your friends you left and came back nine times while you queued for coffee.
An app that cries wolf trains people to ignore it, and then the real alert is missed too.
Treks, festivals and car parks are exactly where the network is weakest.
No Maps API key and no paid Firebase plan, so the maps and the logic had to be free.
Clone it and it builds against a demo project, with nothing to sign up for.
GPS drift inside the band can't flip anyone from inside to outside. That's the demo setting; a release build uses 30 m.
A step out and back, or a fix that jumps and returns, never reaches the group. A release build waits 30 s.
A fix less accurate than this proves nothing, so it's dropped, not guessed at.
The engine is a pure function of its inputs, so a whole walk replays in a unit test.
Join
Anonymous Firebase sign-in: nothing to remember.
Type the name the group will see and you're in. Sign-in is anonymous, and only your name is shared.
Place a fence
The fence is drawn to scale, so it stays true as you zoom.
Name the group, place the fence and slide the radius. If location permission is denied, the screen falls back to a default centre instead of a dead end.
Share the code
0/O and 1/I are left out, because people read codes aloud.
Each group gets a six-character join code. Start sharing and you appear on everyone's map with your distance from the centre.
Someone left
Past the band, for long enough, with an accurate fix: 25 m and 5 s in the demo, 30 m and 30 s in release.
Alice walks past the band and stays out. Only then does her dot turn red on every phone and the crossing land in the activity feed, with how far out she is.
The alert
One alert per real crossing, never a storm.
Once the crossing is confirmed, the other phones raise a notification. You're never told about yourself, and opening a group doesn't replay its old history as a burst of alerts.
Layer 1
Jetpack Compose and Material 3 screens, and a real vector map with no API key. The screens on this page show the earlier, simpler map.
Layer 2
The geofence engine, the domain and the whole presentation layer live in shared Kotlin with no Android types, wired with Koin.
Layer 3
Firebase for sign-in, live positions and crossings, and Play services for location. Alerts travel over live Firestore listeners.
A built-in simulator feeds made-up positions into the same pipeline as real GPS, so anyone can watch a crossing happen, and watch drift do nothing, from a desk.
Drift inside the band, or a step back before the wait is over, ends the journey early: nobody is told. Demo settings shown; a release build uses 30 m and 30 s.
A server could check every position, but losing signal is the normal case on a trek or at a festival. On the device, a crossing is still detected offline, queued and synced later, and each check is one cheap distance calculation.
Works with no signalAndroid's built-in geofencing hides its debounce rules, can't be driven by a simulator and leaves nothing to test. Doing it in shared Kotlin makes every rule explicit, tuned and tested.
MapLibre with OpenFreeMap vector tiles in light and dark. The fence is drawn as a geodesic shape, not a pixel circle, so it keeps its true size at every zoom.
Joining while already outside never alerts. Your own writes don't echo back as news. A wrong device clock can't hide a real crossing. A broken document is skipped instead of taking the listener down.
17 edge cases written downA working portfolio project: two phones on real Firebase, a fence, and alerts that only fire for real crossings. The screens here are from an earlier version of the design.
Geofences, tracking and alerts that people trust because they don't cry wolf.
Real maps with no API key or billing, and logic that runs on the phone instead of a server.
Shared domain and presentation, with platform code only where it has to be.
Tests that read like the spec, with the trade-offs and the limits written down.