Jetpack Compose & Compose Multiplatform

This skill covers the full Compose app development lifecycle — from architecture and state management through UI, networking, persistence, performance, accessibility, cross-platform sharing, build configuration, and distribution. Jetpack Compose and Compose Multiplatform share the same core APIs and mental model. Not all Jetpack libraries work in commonMain — many remain Android-only. A subset of AndroidX libraries now publish multiplatform artifacts (e.g., lifecycle-viewmodel, lifecycle-runtime-compose, datastore-preferences), but availability and API surface vary by version. Before adding any Jetpack/AndroidX dependency to commonMain, verify the artifact is published for all required targets by checking Maven Central or the library's official documentation. CMP uses expect/actual or interfaces for platform-specific code. MVI (Model-View-Intent) is the recommended architecture, but the skill adapts to existing project conventions.

Existing Project Policy

Do not force migration. If a project already follows MVI with its own conventions (different base class, different naming, different file layout), respect that. Adapt to the project's existing patterns. The architecture pattern — unidirectional data flow with Event, State, and Effect — is what matters, not a specific base class or framework. Only suggest structural changes when the user asks for them or when the existing code has clear architectural violations (business logic in composables, scattered state mutations, etc.).

Workflow

When helping with Jetpack Compose or Compose Multiplatform code, follow this process:

1. Read the existing code first — understand the project's current conventions, base classes, naming, and file layout before writing anything.
2. Identify the concern — is this architecture, state modeling, performance, navigation, DI, animation, cross-platform, or testing?
3. Apply the core rules below — the decision heuristics and defaults in this file cover most cases.
4. Consult the right reference — load the relevant file from references/ only when deeper guidance is needed. Each reference is listed in the Detailed References section with its scope.
5. Verify dependencies before recommending — before adding or upgrading any dependency, verify coordinates, target support, and API shape via a documentation MCP tool or official docs (see Dependency Verification Rule).
6. Flag anti-patterns — if the user's code violates architectural best practices, call it out and suggest the correct pattern.
7. Write the minimal correct solution — do not over-engineer. Prefer feature-specific code over generic frameworks.

Dependency Verification Rule

Before recommending any new dependency or version upgrade, verify:

1. Coordinates — Confirm the exact Maven coordinates (group:artifact:version) exist and are current.
2. Target support — Confirm the artifact supports the project's targets (Android, iOS, Desktop, commonMain). Do not assume a Jetpack library works in commonMain unless verified.
3. API shape — Confirm the API you plan to use actually exists in that version. Function signatures, parameter names, and return types change between major versions.

How to verify:

• Documentation MCP tool (preferred) — If a documentation MCP server is available (e.g., Context7), verify exact tool names and schemas first, then use it to fetch current official documentation for the library.
• Official docs — Search the library's official documentation or release notes.
• Maven Central / Google Maven — Check artifact availability and supported platforms.

If verification is not possible (no documentation tool, no network access, docs unavailable), state this explicitly and note that coordinates or APIs may need adjustment.

Fetching Up-to-Date Documentation

When adding a new dependency, upgrading major versions, or verifying latest API patterns, use a documentation MCP tool (e.g., Context7) if available. Before invoking, verify the tool's exact name and parameter schema — tool names vary across environments.

1. Resolve library ID — if the tool requires a resolution step, call it first.
2. Query docs — call with the resolved ID and a specific question.

Alternative: Users can add use context7 (or equivalent) to their prompt. Bundled references remain the primary source for architectural patterns and MVI guidance; use documentation tools for API-specific and version-specific queries.

Core Architecture: MVI with Event, State, Effect

This is the recommended architecture for all Compose work. If the project already uses a different pattern, suggest MVI as the preferred approach but do not force-migrate working code — follow Existing Project Policy.

MVI (Model-View-Intent) enforces unidirectional data flow: UI renders state → user acts → event dispatched → new state computed → UI re-renders. Every feature defines 3 types:

• Event — user actions and lifecycle signals (sealed interface). The only input from the UI to the ViewModel.
• State — immutable data class that fully describes the screen. Owned by the ViewModel via StateFlow.
• Effect — one-shot commands (navigate, snackbar, share) delivered via Channel. Not state — fire and forget.

The ViewModel owns StateFlow<State>, Channel<Effect>, and a single onEvent(event: Event) entry point. All event handling, state transitions, effect emissions, and async launches happen inside onEvent().

For detailed rationale (why 3 types not 4, data flow diagrams, ViewModel internals, file structure) see Architecture & State Management.

UI Rendering Boundary

• Route composable: obtains ViewModel, collects state via collectAsStateWithLifecycle(), collects effects via CollectEffect (see compose-essentials.md), binds navigation/snackbar/platform APIs
• Screen composable: stateless renderer — receives state and onEvent callback, renders the screen, adapts callbacks for leaf composables
• Leaf composables: render sub-state, emit specific callbacks, keep only tiny visual-local state (focus, scroll, animation)

Decision Heuristics

• Composable functions render state and emit events, never decide business rules
• If a value can be derived from state, do not store it redundantly unless async/persistence/performance justifies it
• Event handling in the ViewModel owns state transitions; composables do not mutate state
• UI-local state is acceptable only for ephemeral visual concerns: focus, scroll, animation progress, expansion toggles
• Do not push animation-only flags into global screen state unless business logic depends on them
• Pass the narrowest possible state to leaf composables
• Implement onEvent() in the ViewModel — the single entry point from the UI for all user actions
• Do not introduce a use case for every repository call
• Cross-platform sharing prioritizes business logic and presentation state before platform behavior
• Least recomposition is achieved by state shape and read boundaries first, Compose APIs second
• When a project has an existing MVI base class or pattern, use it — don't introduce a competing abstraction

State Modeling

For calculator/form screens, split state into four buckets:

1. Editable input — raw text and choice values as the user edits them
2. Derived display/business — parsed, validated, calculated values
3. Persisted domain snapshot — saved entity for dirty tracking or reset
4. Transient UI-only — purely visual, not business-significant

Recommended Defaults

Apply these unless the project already follows a different coherent pattern.

Do / Don't Quick Reference

Do

• Model raw editable text separately from parsed values
• Keep state immutable and equality-friendly
• Reuse unchanged nested objects when possible
• Emit semantic effects instead of making platform calls from event handling
• Preserve old content during refresh
• Map domain data to UI state close to the presentation boundary
• Use feature-specific ViewModel names
• Key list items by stable domain ID
• Import all types and functions at the top of the file; use import ... as ... aliases to resolve name clashes
• Guard no-op state emissions (don't update state if nothing changed)
• Respect the project's existing MVI conventions

Don't

• Parse numbers in composable bodies
• Run network requests from composables
• Store MutableState, controllers, lambdas, or platform objects in screen state
• Encode snackbar/navigation as "consume once" booleans in state — use effects
• Keep every minor visual toggle in the ViewModel state
• Pass entire state to every child composable
• Wrap every repository call in a use case class
• Wipe the screen with a full-screen spinner during refresh
• Force-migrate a working codebase to a different architecture or base class
• Use fully qualified package paths inline (e.g., com.example.pkg.SomeClass.method()) — always import at file top

Detailed References

Load these only when the task requires deeper guidance:

Kotlin Foundations

• Coroutines & Flow — StateFlow/SharedFlow/Channel decisions, Flow operators, structured concurrency, Turbine testing

Architecture

• Architecture & State Management — ViewModel pipeline, state modeling, domain layer, inter-feature communication
• Clean Code & Organization — file organization, naming, disciplined vs bloated MVI
• Anti-Patterns — cross-cutting anti-pattern table with replacements

Compose APIs

• Material 3 Theming & Components — M3 theme, dynamic color, components, adaptive layouts
• Image Loading (Coil 3) — AsyncImage, cache policy, SVG, CMP resources
• Compose Essentials — three phases, state primitives, side effects, modifiers
• Lists & Grids — LazyColumn/Row, keys, grids, pager, scroll state
• Paging 3 — PagingSource, Pager, RemoteMediator, MVI integration
• Navigation 3 — Nav 3 routes, NavDisplay, tabs, scenes, ViewModel scoping, modularization

Performance & Quality

• Performance & Recomposition — stability, Compiler Metrics, baseline profiles, recomposition rules
• Animations — animation API decision tree, shared elements, gesture-driven, Canvas
• UI/UX Patterns — loading states, skeleton/shimmer, inline validation
• Accessibility — semantics, touch targets, WCAG contrast, custom actions
• Testing Strategy — Turbine, ViewModel tests, Macrobenchmark, lean test matrix

Data & Persistence

• DataStore — Preferences & Typed DataStore, KMP setup, MVI integration
• Room Database — entities, DAOs, migrations, relationships, MVI integration

Networking, DI & Cross-Platform

• Networking with Ktor — HttpClient, ApiResponse wrapper, auth, WebSockets
• Dependency Injection — Hilt vs Koin decision guide
• Koin — CMP setup, Nav 3 integration, scoped navigation
• Hilt — Android-only setup, @HiltViewModel, scopes, testing
• Cross-Platform (KMP) — commonMain sharing, expect/actual, platform bridges
• iOS Swift Interop — SKIE, Flow→AsyncSequence, SwiftUI/UIKit interop
• Multiplatform Resources — CMP Res class, qualifiers, localization, Android interop

Build, Distribution & CI/CD

• Gradle & Build Configuration — AGP 9+, version catalog, composite builds, convention plugins
• CI/CD & Distribution — GitHub Actions, desktop packaging, signing, notarization