How to build a resilient activity tracker with Expo

Daniel Vinojčić describes how Calda built resilient Expo-based walking sessions that continue tracking through backgrounding, crashes, and process termination by treating persisted state rather than in-memory state as authoritative. The system uses Expo TaskManager and background location services, platform-specific permissions and service settings, and recovery logic that reseeds tracking after restarts while acknowledging limits such as user force-quitting an app and aggressive Android battery managers. To improve distance accuracy, it filters poor GPS fixes, rejects vehicle-like speeds and outliers, applies Kalman smoothing, and primarily integrates Doppler-derived speed over time instead of summing noisy position changes. It protects session data through layered persistence, serialized monotonic merges between foreground and background writers, server-side monotonic updates, and offline-aware relaunch recovery. Step tracking uses iOS Core Motion and a custom Kotlin Expo Module for Android sensors, with activity recognition and cumulative counters helping preserve counts across process loss. The project argues that modern Expo tooling can support sophisticated cross-platform activity tracking, native modules, and iOS Live Activities without abandoning the managed workflow.

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