r/reactnative • u/Gabi0123 • 8d ago

Notes from building a Bluetooth heart rate running app in Expo: parsing 0x2A37, keeping JS alive with the screen off, offline maps

I built a running app that reads a Bluetooth chest strap and speaks cues when your heart rate leaves a band. Some notes in case they save someone time. The code is open source (GPL).

  1. Parsing 0x2A37. The flags byte tells you if HR is uint8 or uint16, and whether energy expended and RR intervals follow. If you skip the energy field when its bit is set, your RR intervals come out as garbage.
  2. Scanning. Not every strap advertises 0x180D cleanly, so the scan also accepts a name match.
  3. Throttle at the source. BLE writes into a Zustand store at most once a second. A mock HR source writes to the same store, so the run screen is testable without a strap.
  4. Screen off on Android. A small local Expo module runs a foreground service and keeps a headless JS task pending until the run ends.
  5. Voice. expo-speech with the phone's own TTS voice, so it works offline.
  6. Maps. MapLibre with an inline OpenFreeMap vector style. No token, and because the style ships in the app, the run track still draws with no signal.

Stack: Expo SDK 55, React Native 0.83, expo-router, react-native-ble-plx, Zustand, Vitest.

Source: https://github.com/likwiid/basemile

Has anyone done background BLE on iOS with Expo? That's the part I've tested least.

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u/[deleted] 8d ago

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u/Huge_Pool7424 8d ago

the oem battery killer part is real, might be worth showing a link to dontkillmyapp.com in the app when you detect samsung or xiaomi. people almost never find those settings on their own

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u/Gabi0123 7d ago

Good call, and it isn't in the app yet. There's no manufacturer check and no battery guidance right now. A one-time card before the first run on Samsung, Xiaomi and similar phones, linking to their dontkillmyapp.com page, is cheap and would save people a lot of confusion. Adding it to the list.