I've been building a mobile app for a smartwatch, and the hardest part wasn't the UI. It was getting the phone to find the watch, connect over Bluetooth, and keep that connection alive. The watch runs custom firmware. The app is React Native with Expo. There's no SDK, no nice wrapper library from the hardware team. Just a protocol spec and a lot of trial and error. This post is about that connection layer only: scanning, pairing, subscribing to the BLE channel, and handling drops...