FitSync is a feature-rich cross-platform mobile application built with React Native (Expo) and serverless Firebase (Auth + Firestore + Storage). It combines real-time GPS workout tracking, multi-source food intelligence, OpenAI GPT-3.5 conversational nutrition coaching, and multi-formula body metric calculations into a 5-tab interface.
Health and fitness apps traditionally silo tracking, nutrition planning, and coaching into separate tools — forcing users to manually sync calorie logs, exercise records, and meal plans. This fragmented experience leads to poor adherence, inaccurate data, and no real-time dietary intelligence. The challenge was building a unified mobile platform across 30+ screens without Redux boilerplate bloat.
We engineered FitSync using React Native (Expo) on the frontend and Firebase as a serverless backend. We created a custom reactive state factory pattern (`createDataContext`) managing 7 isolated domains (auth, user data, diet logs, GPS tracking, location, history, AI tokens). Firestore uses a nested subcollection model ensuring per-user data isolation and zero sync lag.
Engineered from scratch with clean code, scalable architecture, and strict attention to performance.
Built a `createDataContext` factory pattern generating Context + Provider pairs with bound dispatch actions across 7 state domains without Redux boilerplate.
Integrated Edamam Recipes, Edamam Food DB, and Nutritionix APIs with dual-mode search (common vs branded) normalizing schemas into unified macro breakdowns.
Built location-aware tracking via expo-location with Haversine distance formula, calorie burn computation, and coordinate polyline persistence for track replay.
Integrated OpenAI GPT-3.5-turbo with react-native-gifted-chat for conversational nutrition coaching gated behind an onboarding token system.
Implemented 4 clinical formulas (Mifflin-St Jeor, Harris-Benedict, Hamwi, Devine) for BMR, BMI, ideal weight, and personalized macro splits.
Organized Firestore documents into nested subcollections ensuring user data privacy, instant real-time sync listeners, and cost-efficient reads.
FitSync eliminated app-switching friction by consolidating workout recording, food logging, AI coaching, and body metrics into a single 5-tab mobile interface.
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