I build the software that moves money.

Senior Flutter Engineer · Better-Pro Limited · Jan 2026 — Present

HoyaPay — Crypto & Fiat Wallet

App StoreLive · US & TW Google PlayLive · US & TW

A cross-platform crypto & fiat wallet for a US fintech — deposits, withdrawals, transfers, exchange, buy/sell, and virtual & physical cards. I own the mobile side end to end: authentication, transaction safety, real-time, release engineering.

AUTH.PASSKEYS

Killed the password

Complete passwordless system: passkey (WebAuthn/FIDO2) registration & login, OTP fallback via email / Google Authenticator, step-up re-auth for sensitive actions, per-device credential management with multi-OEM & iOS hardening.

TX.IDEMPOTENCY

15+ money flows, zero double-spends

Idempotency across every money-moving flow, plus payload-freeze snapshots, double-submit guards, and on-device transaction lifecycle tracking with persistence and crash recovery for in-flight transactions.

TX.INTENT_HASH

Tamper-proof by construction

Intent hashing binds each payment to a canonical fingerprint that's rejected on any mismatch, tamper-protecting it from prompt to execution. (See the live demo above for how the pattern works.)

NET.REALTIME

Live transaction status

WebSocket layer with connectivity-aware reconnection; hardened Dio API stack with typed errors, encrypted requests, request signing, and strict logging hygiene — no tokens, balances, or PII.

REL.HARDENING

CI/CD built from scratch

Signed builds, secure keystore management, obfuscation with regression guards; co-owned iOS & Android store deployments and release sign-off with QA.

UI.SYSTEM

One design system, 250+ files

Spacing, color and typography tokens with full light/dark theming; centralized typed navigation; 7-locale i18n; bilingual (EN/中文) docs for an international team.

“Server-authoritative by default — the client displays and forwards; it never decides.”

Flutter / DartGetXDio WebSocketsWebAuthn / FIDO2 Keychain / KeystoreGitLab CI/CD

Not an app — a distributed system I designed and built solo across four codebases: a 114k-LOC Flutter client, a Laravel API that owns the truth, a Node.js real-time layer, and a Next.js moderation console. The hard part wasn't the screens — it was making location, presence and money-grade trust behave correctly across all four. Live on TestFlight, iOS & Android.

GEO.TERMINATED

Geofencing that fires with the app closed

OS-level geofence regions wake the app on boundary crossings even when fully terminated — handling the iOS 20-region cap and asymmetric enter/exit registration to kill false "you left" events. The genuinely hard part of a location app, done right.

PRESENCE.AUTH

Server-authoritative, anti-spoof presence

GPS only decides when to ask; the Laravel server decides inside/outside via Haversine and is the single source of truth. Every affirmative ping doubles as a presence refresh, and transient network errors never eject a user. Location integrity + anti-bot guards on top.

RT.THREE_ENGINE

Three-engine real-time architecture

Clients emit over Socket.IO; the Node layer verifies identity against Laravel (no client-supplied identity is trusted) and fans out; Laravel persists and authorizes every event over a shared-secret internal bridge. Room membership is replayed on reconnect, preserving nested join order.

CHAT.RICH

Chat with the details that are hard

4-state WhatsApp-style delivery/read ticks driven by acks, typing & voice-recording presence, live polls, reactions, mentions, pinning, nested "flows", and timed "sprint" matchmaking — plus a custom-built voice-note UI with waveform scrubbing.

MEDIA.S3

Presigned media, zero bytes over chat

Two-path upload contract — presign → PUT → complete to S3 with a multipart fallback. Messages carry only a media_id; private objects are served via short-TTL signed URLs. Client-side compression before upload.

ADMIN.LIVE

A real-time moderation console

Next.js 16 admin: a live firehose room surgically patches React Query caches for live sprint/DM monitoring, a Leaflet map moderates bubbles in place, plus approval queues, RBAC tiers, warnings/bans, and a versioned legal-document CMS.

Flutter / DartCubitGetItSocket.IO Laravel 12SanctumNode.jsNext.js 16 React QueryAWS S3FCM (HTTP v1)Leaflet