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📚 Software Engineering Articles
🗞️ Tech and AI Trends
👨🏻💻 Coding Tip
Fixture setups in TypeScript
Time-to-digest: 5 minutes
Context
Shopify is rebuilding its mobile apps as native Swift and Kotlin applications while exploring how coding agents can work on them. The team designed Reflex around a short feedback loop: an agent needs to exercise app behavior without waiting for a simulator and a rendered screen.
Challenge
Two native implementations can drift, while UI tests need devices or emulators, and duplicate unit suites may not establish that both apps support the same journeys. Shopify wanted fast behavior checks that could run against both platforms.
Implementation Highlights
Separate logic from presentation. A UI-free core holds ViewModels, services, navigation, and other app logic; the mobile host provides platform integrations.
Expose a small contract. Each ViewModel accepts actions and exposes state through JSON. A CLI can inspect a screen and invoke actions against the headless core or a real build.
Share tests, not implementation. The same behavior test runs against the Swift and Kotlin versions. Fixtures stand in at network and device boundaries while the tests exercise the real logic underneath.
Results/Learnings
Shopify reports that more than 1,700 behavior tests run on both platforms in about four minutes in CI. This is a gray-box layer: it checks state and actions, not pixels, so it complements rather than replaces tests of rendered UI.
Closing Insight
The interesting design move is making behavior observable before asking an agent to improve it. A small, stable interface can shorten the feedback loop for humans and agents alike.
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ARTICLE (fixtures without the fuss)
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ARTICLE (the cost of a newline)
We can do better than line-buffered output
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This week’s tip:
Make fixture setup fail loudly. A small generic helper can unwrap a valid test value or stop immediately when setup fails. Keep real validation tests separate; this helper is for fixtures the test expects to be valid.
When?
When test setup needs a value that should always be valid.
When several fixture constructors return
undefinedfor invalid input.When you want setup to fail immediately instead of passing an invalid fixture into a test.
Perplexity is the beginning of knowledge.
Kahlil Gibran
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