configuring-lazy-prefetch
Configuring Lazy Prefetch — Cache Window and Pausable Composition
Lazy layouts pre-compose items just outside the viewport so they're ready when the user scrolls. Compose Foundation 1.9 added LazyLayoutCacheWindow for configurable ahead/behind extents. Compose Foundation 1.10 made prefetch composition pausable by default — work spreads across multiple frames instead of one. Most apps still rely on legacy single-frame prefetch and unknowingly pay for it during heavy scrolling. This skill teaches Claude when (and only when) to widen the window or implement nested prefetch, and how to validate the change with Macrobenchmark.
When to use this skill
- Macrobenchmark
FrameTimingMetricshows dropped frames at high scroll velocity even though item composables are skippable. - Items are content-heavy: large images, decoded videos, nested grids, or expensive measurement work.
- The developer wants composition retained for items the user scrolled past briefly (e.g. flick-back gesture).
- An outer lazy layout contains items with their own inner lazy layouts (e.g.
LazyColumnrows that each host aHorizontalPageror innerLazyRow). - The developer mentions
LazyLayoutCacheWindow,NestedPrefetchScope, "prefetch window", "ahead extent", or "pausable composition for prefetch".
When NOT to use this skill
- Items are cheap and short; default behavior is sufficient — do not pre-emptively configure a window.
- Item composables are non-skippable / unstable; widening the window only spreads the same waste over more frames. First fix
../optimizing-lazy-layouts/SKILL.mdand the underlying stability via../../stability/diagnosing-compose-stability/SKILL.md. - The bottleneck is per-item layout or draw rather than composition. Use Android Studio Profiler / Layout Inspector to confirm; if measure or draw dominates, prefetch tuning will not help — adjust item content instead.
- Wrong-phase state reads inside items (animation reads
state.valuein Composition) — use../../recomposition/deferring-state-reads/SKILL.md.
More from skydoves/compose-performance-skills
diagnosing-compose-stability
Use this skill to diagnose Jetpack Compose stability problems by enabling and reading the Compose Compiler Reports (classes.txt, composables.txt, composables.csv, module.json). Covers the Gradle DSL, the release-only build requirement, and how to interpret per-class and per-composable stability annotations including stable, unstable, runtime, restartable, skippable, readonly, @static, and @dynamic markers. Use when the developer asks "why does this recompose", reports jank, dropped frames, slow scroll, high recomposition count, suspects an unstable parameter, mentions Compose Compiler Reports, classes.txt, composables.txt, module.json, or wants to know which composables are non-skippable. The fix lives in a sibling skill — this one only diagnoses.
10deferring-state-reads
Use this skill to push frequently-changing Jetpack Compose state reads (scroll position, animation values, drag offsets) out of the Composition phase and down into Layout or Draw using lambda-based modifiers like Modifier.offset { }, Modifier.layout { }, Modifier.graphicsLayer { }, Modifier.drawBehind { }, and Modifier.drawWithCache { }. Covers the three-phase model (Composition, Layout, Draw), why a state read at phase N invalidates phase N and every phase below, the modifier-phase cheat sheet, and lambda providers (() -> T) for hoisting hot values across composables. Use when the developer mentions every-frame work, scroll jank, animation jank, dropped frames, animated alpha or offset, "the whole subtree recomposes on scroll", Modifier.alpha(state.value), Modifier.offset(x.dp), or graphicsLayer.
10collecting-flows-safely
Use this skill to migrate Compose UI from `collectAsState()` to `collectAsStateWithLifecycle()`, hoist `Flow<T>` parameters out of composables, and apply `.conflate()` / `.distinctUntilChanged()` / `snapshotFlow` so background CPU and battery stop draining and chatty flows stop invalidating the UI per emission. Covers ViewModel `StateFlow`/`SharedFlow` consumers, sensor and location streams, and the "Flow as composable parameter" antipattern. Trigger when the user mentions `collectAsState`, `collectAsStateWithLifecycle`, lifecycle-aware flow collection, `Lifecycle.State.STARTED`, background battery drain from a Compose screen, `snapshotFlow`, `Flow` parameter on a composable, conflate, or distinctUntilChanged.
10debugging-recompositions
Use this skill to find which Jetpack Compose composables are recomposing and why, using Android Studio Layout Inspector recomposition counts and skip counts, the per-parameter Argument Change Reasons (Changed / Unchanged / Uncertain / Static / Unknown) introduced in Android Studio Hedgehog and later, and runtime `@TraceRecomposition` from `compose-stability-analyzer` for production-like measurement. Walks through enabling counts, mapping each Argument Change Reason to a fix, and confirming the result in a release build. Use when the developer says "this should be skipping but isn't", "I want to see recomposition counts", asks what "Uncertain" or "Unknown" means in the inspector, or needs to confirm a stability or strong-skipping fix actually worked end-to-end.
10auditing-compose-performance
Use this skill to run an end-to-end Jetpack Compose performance audit when the symptom is broad ("the app feels sluggish", "scroll is rough everywhere", "we're starting a perf sprint", "what should we fix first?"). Orchestrates the four-phase Measure → Diagnose → Fix → Verify loop by sequencing the 25 focused skills (release-mode setup, R8, Baseline Profiles, Compose Compiler reports, stability inference, Layout Inspector, `@TraceRecomposition`, stabilization, strong skipping, phase-deferral, derivedStateOf, lazy layouts, lazy prefetch, Modifier.Node, modifier ordering, flow collection, effects, CI gates, hot-reload) and produces a written audit report with Before/After Macrobenchmark numbers. Use when the developer wants a perf sprint kickoff, a pre-release perf gate, onboarding to a perf-troubled codebase, or a written deliverable. Use when the user mentions "audit", "perf review", "perf sprint", "where do I start", or has no specific symptom yet.
10optimizing-lazy-layouts
Use this skill to fix scroll jank, lost item state, and broken animateItem() animations in LazyColumn, LazyRow, LazyVerticalGrid, and LazyHorizontalGrid. Covers stable item keys, contentType for mixed-type feeds, Modifier.animateItem() requirements, hoisting modifier chains and painters out of the items lambda, and validating item composable stability. Use when the developer mentions LazyColumn jank, dropped frames while scrolling, items losing scroll state on insert/remove/reorder, mixed feeds of cards/headers/ads feeling sluggish, animateItem() not animating, RecyclerView view-type analog, key parameter, or contentType parameter. The prefetch-window tuning lives in a sibling skill.
9