jetpack-compose

Jetpack Compose Expert Skill

Non-opinionated, practical guidance for writing correct, performant Jetpack Compose code. Focuses on real pitfalls developers encounter daily, backed by analysis of the actual androidx/androidx source code (branch: androidx-main).

Workflow

When helping with Compose code, follow this checklist:

1. Understand the request

• What Compose layer is involved? (Runtime, UI, Foundation, Material3, Navigation)
• Is this a state problem, layout problem, performance problem, or architecture question?

2. Consult the right reference

Read the relevant reference file(s) from references/ before answering:

Topic	Reference File
@State, remember, mutableStateOf, state hoisting, derivedStateOf, snapshotFlow	references/state-management.md
Structuring composables, slots, extraction, preview	references/view-composition.md
Modifier ordering, custom modifiers, Modifier.Node	references/modifiers.md
LaunchedEffect, DisposableEffect, SideEffect, rememberCoroutineScope	references/side-effects.md
CompositionLocal, LocalContext, LocalDensity, custom locals	references/composition-locals.md
LazyColumn, LazyRow, LazyGrid, Pager, keys, content types	references/lists-scrolling.md
NavHost, type-safe routes, deep links, shared element transitions	references/navigation.md
animate*AsState, AnimatedVisibility, Crossfade, transitions	references/animation.md
MaterialTheme, ColorScheme, dynamic color, Typography, shapes	references/theming-material3.md
Recomposition skipping, stability, baseline profiles, benchmarking	references/performance.md
Semantics, content descriptions, traversal order, testing	references/accessibility.md
Removed/replaced APIs, migration paths from older Compose versions	references/deprecated-patterns.md
Styles API (experimental): Style {}, MutableStyleState, Modifier.styleable()	references/styles-experimental.md

3. Apply and verify

• Write code that follows the patterns in the reference
• Flag any anti-patterns you see in the user's existing code
• Suggest the minimal correct solution — don't over-engineer

4. Cite the source

When referencing Compose internals, point to the exact source file:

// See: compose/runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/Composer.kt

Key Principles

1. Compose thinks in three phases: Composition → Layout → Drawing. State reads in each phase only trigger work for that phase and later ones.

2. Recomposition is frequent and cheap — but only if you help the compiler skip unchanged scopes. Use stable types, avoid allocations in composable bodies.

3. Modifier order matters. Modifier.padding(16.dp).background(Color.Red) is visually different from Modifier.background(Color.Red).padding(16.dp).

4. State should live as low as possible and be hoisted only as high as needed. Don't put everything in a ViewModel just because you can.

5. Side effects exist to bridge Compose's declarative world with imperative APIs. Use the right one for the job — misusing them causes bugs that are hard to trace.

Source Code Receipts

Beyond the guidance docs, this skill bundles the actual source code from androidx/androidx (branch: androidx-main). When you need to verify how something works internally, or the user asks "show me the actual implementation", read the raw source from references/source-code/:

Module	Source Reference	Key Files Inside
Runtime	references/source-code/runtime-source.md	Composer.kt, Recomposer.kt, State.kt, Effects.kt, CompositionLocal.kt, Remember.kt, SlotTable.kt, Snapshot.kt
UI	references/source-code/ui-source.md	AndroidCompositionLocals.android.kt, Modifier.kt, Layout.kt, LayoutNode.kt, ModifierNodeElement.kt, DrawModifier.kt
Foundation	references/source-code/foundation-source.md	LazyList.kt, LazyGrid.kt, BasicTextField.kt, Clickable.kt, Scrollable.kt, Pager.kt
Material3	references/source-code/material3-source.md	MaterialTheme.kt, ColorScheme.kt, Button.kt, Scaffold.kt, TextField.kt, NavigationBar.kt
Navigation	references/source-code/navigation-source.md	NavHost.kt, ComposeNavigator.kt, NavGraphBuilder.kt, DialogNavigator.kt

Two-layer approach

1. Start with guidance — read the topic-specific reference (e.g., references/state-management.md)
2. Go deeper with source — if the user wants receipts or you need to verify, read from references/source-code/

Source tree map

compose/
├── runtime/runtime/src/commonMain/kotlin/androidx/compose/runtime/
├── ui/ui/src/androidMain/kotlin/androidx/compose/ui/platform/
├── ui/ui/src/commonMain/kotlin/androidx/compose/ui/
├── foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/
├── material3/material3/src/commonMain/kotlin/androidx/compose/material3/
└── navigation/navigation-compose/src/commonMain/kotlin/androidx/navigation/compose/