Pure Reducers in Kotlin: Why Your Android Unit Tests Should Run in 5 Milliseconds

Reducer unit tests clear in about five milliseconds each, with no dispatcher setup, no MockK, no Turbine, and no ViewModel. That speed is not a micro-optimization — it reshapes the test pyramid, moves 80% of Android presentation tests to pure JUnit, and keeps the feedback loop under a second on a laptop. This article walks through the Kotlin contract that makes the five-millisecond figure possible, and the disciplines that keep it there. What a reducer is object FactsReducer : MviReducer FactsSt