AlertSpot: An open-source, battery-friendly transit alarm for Android

Building AlertSpot: A Privacy-First Location Alarm for Android

Commuting on public transit comes with an everyday annoyance: falling asleep or getting absorbed in a book and missing your stop.

While location-based reminder apps exist, many require cloud accounts, track location telemetry, or aggressively drain battery by pinging continuous high-accuracy GPS in the background.

To solve this, I built AlertSpot — a lightweight, completely free, and open-source Android app that fires high-priority audio and full-screen alerts when you enter a destination geofence.

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🏗️ Architecture & Core Tech Stack

• Language & UI: Kotlin, Jetpack Compose, Material Design 3
• Architecture: MVVM with ViewModel and StateFlow
• Maps & Tiles: OSMDroid (OpenStreetMap)
• Location & Geofencing: Google Play Services FusedLocationProvider + GeofencingClient
• Navigation & Serialization: Jetpack Navigation Compose, Gson
• Target SDK: Android 8.0 (API 26) up to Android 15 (API 35)

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⚡ Technical Challenges & Solutions

1. Zero Google Maps API Costs (OSMDroid)

Instead of relying on commercial Google Maps billing/keys, AlertSpot uses OSMDroid for map rendering.

• Tile Caching: Caches up to 600 MB of OpenStreetMap tiles locally so maps load reliably even with unstable transit mobile data.
• Interactive Geofencing: Users can drop pins or search locations to define custom radius zones (in meters or kilometers) rendered directly on the map layer.

2. Battery-Smart Adaptive GPS Polling

Continuous location updates quickly drain battery on long trips. AlertSpot uses a dynamic polling mechanism:

• Far: When far from all active geofences, polling frequency drops to save power.
• Approaching: As distance shrinks, polling frequency scales up dynamically to guarantee detection precision.
• Dual Detection: Combines the Android Geofencing API with background timer distance checks to prevent dropped triggers.

3. Reliable Waking (Surviving Doze Mode & Reboots)

A transit alarm is useless if the system kills it or delays the notification due to Android Doze mode:

• Foreground Service: Runs a continuous foreground service with persistent notification status.
• Full-Screen Intent & Wake Lock: When triggered, it fires a full-screen alarm overlay with looping audio and vibration, waking the screen even if the device is locked.
• Reboot Resilience: Listens for BOOT_COMPLETED intents to automatically re-register active geofences on device restart.

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🔒 Privacy First

• No Accounts / Logins: No backend servers, analytics tracking, or telemetry.
• Local Storage: All locations and history logs remain strictly on the device.

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📦 Try AlertSpot / Source Code

The code and pre-compiled APKs are publicly available on GitHub:

• 💻 GitHub Repository: vasanth-kandasamy/alert-spot-android
• 📦 Download APK Release: GitHub Releases

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🚀 Supporting the Store Launch

I am an independent developer working to raise the one-time $25 Google Play Developer fee to release AlertSpot on the Play Store for non-technical users.

If you'd like to support the project, feel free to check out my Ko-fi page.

Pull requests, feedback, and architecture suggestions are always welcome!