How It Works

Dejavu hooks into the Compose runtime's CompositionTracer API (available since compose-runtime 1.2.0):

1. Intercepts trace calls — Composer.setTracer() receives callbacks for every composable enter/exit
2. Maps testTag to composable — walks the CompositionData group tree to find which composable encloses each Modifier.testTag()
3. Counts recompositions — maintains a thread-safe counter per composable, incrementing on recomposition (not initial composition)
4. Tracks causality — Snapshot.registerApplyObserver detects state changes; dirty bits detect parameter-driven recompositions
5. Reports on failure — assembles source location, timeline, tracked composables, and causality into a structured error

All tracking runs in the app process on the main thread, directly accessible to instrumented tests.

Compatibility

Minimum: compose-runtime 1.2.0 (CompositionTracer API). Requires Kotlin 2.0+ with the Compose compiler plugin.

Compose BOM	Compose	Kotlin	Status
2024.06.00	1.6.x	2.0.x	Tested
2024.09.00	1.7.x	2.0.x	Tested
2025.01.01	1.8.x	2.0.x	Tested
2026.01.01	1.10.x	2.0.x	Baseline

Known Limitations

• Off-screen lazy items — LazyColumn/LazyRow only compose items that are visible. Items that haven't been composed don't exist in the composition tree, so Dejavu has nothing to track. Scroll them into view before asserting.
• Activity-owned Recomposer clock — createAndroidComposeRule uses the Activity's real Recomposer, not a test-controlled one. This means mainClock.advanceTimeBy() can't drive infinite animations forward. Use createComposeRule (without an Activity) if you need a controllable clock.
• Parameter change tracking precision — parameter diffs use Group.parameters from the Compose tooling data API, which was designed for Layout Inspector rather than programmatic diffing. Parameter names may be unavailable, and values are compared via hashCode/toString, so custom types without meaningful toString show opaque values.