A delicate product built by one engineer
Callie is a self-care companion app for people recovering from eating disorders. The product needed to feel warm and psychologically safe: no abrupt transitions, no jarring visuals, nothing that could read as demanding or intrusive to someone in a vulnerable emotional state.
Daehyeon Mun joined Nabi, the company behind Callie, as founding engineer and took full responsibility for the mobile app: interaction design, animation, shader-based visual effects, and the overall feel of the product. There was no separate mobile team to divide the work across.
Designing for emotional sensitivity
People recovering from eating disorders can experience heightened emotional vulnerability, where even small visual or interaction changes shift whether an app feels supportive or overwhelming. That constraint shaped Callie's design language: calm color palettes, gentle transitions, and interactions that never feel demanding.
Static screens weren't expressive enough to carry that nuance, so Mun turned to React Native Skia to build custom shaders. He evaluated Lottie and Rive first, but ruled both out: they're built for pre-designed motion graphics, not for interactions that need to respond to user input in real time or track psychological nuance frame by frame.
A page-curl shader for the AI journal
One of Callie's core features, "Callie's journal about you," analyzes a user's activity with AI and delivers a daily summary as an encouraging letter. Mun wanted the screen to read like a letter written specifically for the user, not a block of AI-generated text, so he built a page-curl shader that makes the entry unfold like a physical sheet of paper.
For simpler interactions, like the app's drag-and-drop to-do list, Mun used react-native-sortables, a library built on react-native-reanimated and react-native-gesture-handler that runs on the UI thread. It let him ship a stable drag-and-drop experience without writing custom gesture logic.
Moving off React Native CLI in the first week
Callie didn't start on Expo. In his first week on the team, Mun migrated the existing project from React Native CLI to Expo. He'd previously maintained RN CLI projects and knew how much engineering time native configuration conflicts and build instability could consume without moving the product forward.
For releases, EAS Build let him produce iOS and Android builds and publish to both stores from a single command, which he describes as a more consistent and time-efficient deployment step than what he'd dealt with on prior RN CLI projects.
Shipping fast without waiting on App Store review
That let the team fold in user feedback quickly while the user base was still small. Mun says the product changed visibly on a weekly basis during that period, and that the short feedback loop was central to shaping Callie into what users needed early on.
What this means for other small teams
Mun's takeaway: adopting Expo early cut a meaningful amount of engineering overhead, and that saved time went directly into product refinement rather than infrastructure. He says delivering Callie's level of polish in the timeframe they had would have been harder with a traditional React Native setup.
Callie went on to be named Most Creative App at the 2025 Expo App Awards and has more than 3.2k App Store reviews with a 4.9+ average rating. For a team of one mobile engineer, the constraint was never ambition. It was time, and where that time went.
“By adopting Expo’s infrastructure early, we eliminated a considerable amount of engineering overhead and reinvested that time directly into improvements users can feel. As a result, we were able to deliver a thoughtful, high quality product in a short timeframe. This would have been much more difficult with a traditional React Native setup.” (Daehyeon Mun)
Sources
Making AI feel human in a mobile app with Expo, Reanimated, and Skia (2026-01-15)
