Field Notes

Why double Will Break Your Fintech App (and How We Handle Money in Dart)

Floating-point money bugs aren't exotic edge cases — they're a predictable consequence of how double works, and they show up in almost every fintech codebase at least once. Here's the pattern that prevents them permanently.

A Queue That Tags Every Offline Action With a Client-Generated Idempotency Key

An offline action queue that retries blindly will eventually double-submit a transfer. The fix isn't smarter retry logic — it's making every queued action safe to replay.

Event-Driven Architecture in Flutter: Real-Time Balance Updates Without Polling

Why polling a balance endpoint every few seconds is the wrong default for a financial app, and the event-driven pattern that replaces it — including the sequence-gap recovery logic most tutorials skip.

A One-File Wrapper Around flutter_secure_storage That Fails Closed, Not Open

When secure storage throws — a Keystore invalidated by a fingerprint change, a Keychain error after a restore — most apps quietly fall back to a default. That default should never be 'logged in.'

RBI Names FACE as Fintech's First Self-Regulatory Body: What It Means for App Teams

In August 2024, the RBI granted self-regulatory-organisation status to FACE. It's an industry-body announcement, not a direct app requirement — but it usually ends up in your onboarding and grievance flows anyway.

A Money Extension Type That Makes 0.1 + 0.2 == 0.3 Fail Loudly, Not Silently

Dart's extension types turn a money precision bug from a runtime surprise into a compile error, at zero runtime cost. Here's the pattern.

Flutter 3.24: What's Actually Useful for a Fintech Team

Flutter 3.24 made Impeller the default renderer on Android. Here's what that actually changes for an app full of live charts and balance animations, and what to skip.

A Five-Line StreamController Wrapper That Prevents 'Add After Close' Crashes

The guard we add to every StreamController in a Flutter fintech app, and why checking isClosed once at the top of a function isn't enough.

Nov 2023 · 5 min read

Future vs Stream: which and why?

A practical breakdown of Dart's two async primitives: when to use each, and what happens when you mix them up.