How to unit-test Mercari Hallo Flutter app

Heejoon from Mercari's Work Mobile team reveals how they built a comprehensive unit testing strategy for their Flutter app. Their approach catches bugs early and actually speeds up development.

The Mercari Hallo team shares their production-tested approach to unit testing in Flutter, covering everything from simple logic tests to complex time-dependent scenarios. They've automated code coverage analysis in their CI/CD pipeline and built custom tooling to streamline widget testing across multiple themes and screen sizes.

Key Takeaways

• Uses clock package to freeze time in tests, making time-dependent logic deterministic
• Custom testThemedWidgets() runs tests across light/dark themes and multiple screen sizes
• Automated CI/CD pipeline generates differential coverage reports for every pull request
• Custom linter prevents DateTime.now() usage, enforcing clock.now() for testability
• Golden image testing with dynamic placeholders catches visual regressions automatically