docs: add mobile app design document (local-first, Expo/React Native)
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Davide Scaini
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# Bincio Mobile App — Design Document
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## Philosophy
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The Bincio mobile app follows a **local-first** model:
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- All activity data lives on the device. The app works fully offline — no account or internet connection required.
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- An online instance (bincio.org or a self-hosted server) is an optional upgrade, not a prerequisite.
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- Activities are stored in the open **BAS format** (the same JSON schema the server uses), so data is always portable and readable without the app.
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- Sync is explicit and user-initiated — the app never silently overwrites local data.
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The goal is a personal activity log that you own. The cloud is a backup and a sharing mechanism, not the source of truth.
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---
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## What already exists
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Several pieces of the mobile app are already implemented or proven:
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| Piece | Where | Notes |
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|---|---|---|
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| BAS schema | `docs/schema.md` | The on-device data format — identical to the server format |
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| In-browser FIT/GPX/TCX parsing | `site/src/pages/convert/` | Pyodide + the Python extractor running in a browser tab. Proves the extraction works without a server. Not suitable for mobile (Pyodide is 30 MB, browser-only). |
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| Local activity storage | `site/src/pages/convert/` | IndexedDB + service worker serving local activities in the feed. Proves the concept; needs a native equivalent. |
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| Content-addressed dedup | `bincio/extract/dedup.py` | `source_hash` (SHA-256 of raw file) already prevents duplicates on upload |
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| Sync-ready REST API | `bincio/serve/server.py` | Login, upload, activity detail, index.json — the sync primitives are already there |
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| Settings persistence | `bincio/serve/db.py` | `settings` table (key/value) in the multi-user SQLite DB; the mobile_app branch added this |
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| Elevation algorithms | `bincio/extract/metrics.py`, `bincio/extract/dem.py` | Hysteresis accumulation and DEM correction — need a TypeScript port |
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---
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## Technology choice
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### Cross-platform framework: Expo (React Native)
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**Expo** (React Native + the Expo SDK) is the recommended platform:
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- TypeScript-first, large ecosystem
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- `expo-sqlite` (v2+) provides a fast on-device SQLite database with WAL mode — good fit for BAS-style JSON blobs
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- File picking and sharing: `expo-document-picker`, `expo-sharing`
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- Maps: MapLibre React Native (`@maplibre/maplibre-react-native`) — same tile standard as the web app, self-hostable
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- Background tasks: `expo-background-fetch` / `expo-task-manager`
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- OTA updates via Expo's update service
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- Expo Go app for rapid development without a build step
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- Builds for iOS and Android from a single codebase via EAS Build
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**Why not alternatives:**
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| Option | Reason for skipping |
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|---|---|
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| Capacitor + Svelte | Reuses web components but WebView performance is poor for map-heavy activities; Pyodide can't run on mobile |
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| Flutter | Dart is unfamiliar, requires rewriting all logic; no advantage over RN for this use case |
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| PWA | iOS severely limits background sync, local storage quotas, and file access — not viable for an activity logger |
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### Extraction engine: TypeScript port
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The Python extractor (`bincio/extract/`) cannot run on mobile. It must be re-implemented in TypeScript. The math is straightforward:
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- **FIT parsing**: `@garmin/fitsdk` or `fit-file-parser` (existing JS libraries)
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- **GPX/TCX parsing**: standard XML parsing (`DOMParser` or `fast-xml-parser`)
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- **Metrics**: distance (Haversine), speed, HR/power averages, lap splits — all simple loops
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- **Elevation**: direct port of the hysteresis algorithm from `metrics.py` and `dem.py`
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- **DEM correction**: same HTTP API call to Open-Elevation, works from mobile
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The TypeScript extraction library (`bincio-extract-ts` or similar) should produce BAS-compatible JSON — the same schema that the server writes. This makes the output directly uploadable.
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---
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## Architecture
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```
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Bincio Mobile
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├── UI Layer (React Native / Expo)
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│ ├── Feed screen — list of local activities
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│ ├── Activity detail — map + chart + stats
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│ ├── Import screen — pick FIT/GPX/TCX from device or cloud storage
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│ ├── Sync screen — configure instance, push/pull
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│ └── Settings screen — instance URL, account, preferences
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│
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├── Extraction Engine (TypeScript)
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│ ├── FIT parser — wraps @garmin/fitsdk
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│ ├── GPX parser — XML → BAS points
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│ ├── TCX parser — XML → BAS points
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│ ├── Metrics — port of metrics.py (distance, elevation, HR, power)
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│ └── Hysteresis — port of dem.py _hysteresis_gain_loss + _moving_average
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│
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├── Local Store (expo-sqlite)
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│ ├── activities — BAS detail JSON + summary fields as columns
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│ ├── timeseries — 1 Hz arrays stored as JSON blobs per activity
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│ ├── geojson — simplified GPS track per activity
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│ └── settings — instance_url, auth_token, sync preferences
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│
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└── Sync Layer
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├── Auth — POST /api/auth/login → session cookie or token
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├── Manifest fetch — GET /{handle}/index.json → list of remote IDs
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├── Push — POST /api/upload (multipart, original file)
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└── Pull — GET /api/activity/{id} + timeseries + geojson
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```
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---
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## Data model on device
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Each activity is stored in SQLite with:
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- `id` — BAS activity ID (`2026-04-17T074238Z`)
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- `source_hash` — SHA-256 of the original file (deduplication key)
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- `detail_json` — full BAS detail JSON blob
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- `timeseries_json` — 1 Hz arrays blob (nullable — loaded lazily)
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- `geojson` — simplified GPS track (nullable)
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- `synced_at` — timestamp of last successful push to remote instance (nullable = not yet synced)
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- `origin` — `"local"` (parsed on device) | `"remote"` (pulled from instance)
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The `settings` table stores:
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| Key | Value |
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|---|---|
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| `instance_url` | `https://bincio.org` |
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| `handle` | user's handle on the remote instance |
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| `session_token` | auth cookie/token value |
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| `last_sync_at` | ISO timestamp of last sync |
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---
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## Sync protocol
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Sync is a two-way, hash-based diff — no custom server protocol needed:
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### Push (local → server)
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1. Fetch `{instance_url}/{handle}/index.json` to get remote activity IDs.
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2. Find activities where `synced_at IS NULL` or `origin = "local"` and `id NOT IN remote_ids`.
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3. For each unsynced activity, `POST /api/upload` with the original file (stored separately from the extracted JSON) or reconstruct a minimal file from the BAS JSON.
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4. On 200, set `synced_at = now()`.
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### Pull (server → local)
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1. Fetch `{instance_url}/{handle}/index.json` (and yearly shards if present).
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2. Find IDs in the remote list that are not in the local DB.
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3. For each missing activity:
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- `GET {instance_url}/activities/{id}.json` → store as `detail_json`
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- `GET {instance_url}/activities/{id}.timeseries.json` → store as `timeseries_json` (lazy, on demand)
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- `GET {instance_url}/activities/{id}.geojson` → store as `geojson`
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4. Insert with `origin = "remote"`, `synced_at = now()`.
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### Conflict handling
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Activities are immutable once created (same philosophy as the server). Conflicts only arise if the same source file is imported on two devices before sync. The `source_hash` prevents double-counting — whichever copy arrives at the server first wins; the duplicate is rejected silently.
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---
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## What's NOT in scope (v1)
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- Live activity recording (GPS track + sensor recording during a ride) — this is a separate, harder problem
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- Offline map tiles — initial version requires network for map rendering
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- Photo sync — photos are not included in the BAS timeseries, sync deferred
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- Watch / ANT+ / Bluetooth sensors — out of scope for v1
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- Editing activities on mobile — read-only feed in v1; edits happen on the web
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---
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## Open questions
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1. **Original file storage**: To push to the server, we need the original FIT/GPX/TCX file. Do we always keep it, or reconstruct a minimal file from extracted BAS JSON? The `POST /api/import-bas` endpoint already exists on the edit server for this use case (see `convert/` page).
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2. **Auth model**: The multi-user server uses HTTP session cookies. React Native handles cookies but a token-based auth (`Authorization: Bearer …`) would be cleaner for a mobile client. A `POST /api/auth/token` endpoint may be worth adding.
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3. **iOS file access**: FIT/GPX files from Garmin Connect, Komoot, etc. arrive via the share sheet or Files app. `expo-document-picker` handles this but needs testing with each source.
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4. **Offline DEM**: DEM correction requires a network call. On mobile, the hysteresis method (offline) should be the default; DEM correction is opt-in when connected.
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---
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## Roadmap
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| Phase | Scope |
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| **0 — Foundation** | Expo project scaffold, local SQLite store, settings screen, BAS reader (load an existing `.json` file into the feed) |
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| **1 — Import** | TypeScript FIT/GPX/TCX parser + metrics engine, local feed, activity detail with map and chart |
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| **2 — Sync** | Auth, push/pull sync with bincio.org or a self-hosted instance |
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| **3 — Polish** | Offline map tiles, share sheet integration, widgets, performance |
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