Android Feature Development

This skill is a bundle. The SKILL.md you are reading is a router — it points to the deep-dive references, copyable templates, evaluation rubrics, and scripts that together form the full guidance. Do not try to memorise everything at once. When you are working on a feature, open the file relevant to the layer you are touching.

When to use this skill

Activate this skill when you are:

• Creating a new feature module (any screen / flow new to the app).
• Restructuring an existing screen to follow the canonical layered MVI shape.
• Implementing the core layer scaffold, business logic, and presentation for a feature.
• Verifying code quality before opening a PR.

This skill includes:

• Software design principles — references/software-design.md (SOLID, KISS, DRY, YAGNI engineering foundation).
• Quality gates & verification — references/quality-gates.md (Detekt, Lint, coverage, scoring rubric).
• Feature architecture — 19 references covering domain, data, presentation, UI, navigation, DI, performance, and security.
• Testing patterns — references/testing.md (unit, integration, screenshot tests).

Out of scope (handle with your own process): git/PR workflow, peer code-review process, cross-feature microfrontend contracts, and KMP/multiplatform targets. This skill stays focused on building one Android feature well, from generation through quality verification.

Standards alignment (validated against Now in Android)

This skill is validated against the official Now in Android reference app and the Android architecture guide.

Matches NiA / official guidance exactly:

• Reactive Unidirectional Data Flow (events down, data up) via Kotlin Flows.
• Offline-first data layer as the single source of truth; data exposed as streams, never snapshots.
• Repository pattern as the public data API; each repository owns its models.
• Hilt for dependency injection throughout.
• Type-safe Jetpack Navigation for Compose.
• Room for local persistence; sealed UiState modelling (Loading/Success/Error).
• ViewModels as state holders exposing StateFlow.

Deliberate, documented divergences (all defensible — pick what fits your project):

The official Android architecture note is explicit: "The official Android architecture is different from other architectures, such as Clean Architecture. Concepts from other architectures may not apply here, or be applied in different ways." This skill blends NiA's layering with a Clean-Architecture-style domain layer — a common, well-supported hybrid.

For every new feature <Feature> (e.g. Transactions, CardManagement, Profile):

1. Scaffold — :feature:<feature> (single module) or :feature:<feature>:api + :feature:<feature>:impl if other features must navigate to it. See references/modularization.md.
2. Domain — model → repository interface → use case → mapper. Pure Kotlin. See references/domain-layer.md.
3. Data — DTO + Entity + RemoteDataSource + LocalDataSource + RepositoryImpl. Offline-first. See references/data-layer.md.
4. Presentation MVI — <Feature>UiState, <Feature>Intent, <Feature>Event, <Feature>ViewModel, <Feature>ViewMapper. See references/mvi-contract.md and references/presentation-layer.md.
5. UI — <Feature>Screen (stateful) + <Feature>View (stateless, previewable). See references/presentation-layer.md.
6. DI + Navigation — Hilt module binding interfaces; type-safe @Serializable routes. See references/dependency-injection.md and references/navigation.md.

File map of this skill bundle

android-feature/
├── SKILL.md                         ← You are here (complete feature-building router)
│
├── references/                      ← Deep-dive guidance (optimized for feature development)
│   ├── software-design.md           ← SOLID, KISS, DRY, YAGNI, naming conventions (CORE)
│   ├── quality-gates.md             ← Detekt, Lint, coverage, scoring, verification (CORE)
│   ├── architecture.md              ← Layered architecture, UDF, offline-first overview
│   ├── modularization.md            ← Single module vs api/impl, convention plugins
│   ├── package-layout.md            ← Authoritative package + folder structure
│   ├── domain-layer.md              ← Models, repository interfaces, use cases, mappers
│   ├── data-layer.md                ← DTOs, Entities (Room), DataSources, RepositoryImpl
│   ├── presentation-layer.md        ← ViewModel, StateFlow, stateIn(WhileSubscribed)
│   ├── mvi-contract.md              ← UiState / Intent / Event / ViewMapper rules
│   ├── ui-compose.md                ← Screen vs View split, previews, accessibility
│   ├── navigation.md                ← Type-safe Compose Navigation, routes, deep links
│   ├── dependency-injection.md      ← Hilt modules, scopes, @Binds vs @Provides
│   ├── security.md                  ← Mobile security baseline (storage, network, logging)
│   ├── performance.md               ← Cold start, baseline profiles, Compose recompositions
│   ├── testing.md                   ← Use case / VM / repo test patterns
│   ├── screenshot-testing.md        ← Roborazzi setup, snapshot conventions
│   ├── benchmarking.md              ← Macrobenchmark + Microbenchmark + baseline profiles
│   └── anti-patterns.md             ← Rejection table for code review
│
├── templates/                       ← Copy-pasteable file skeletons
│   ├── domain/
│   │   ├── Model.kt.template
│   │   ├── Repository.kt.template
│   │   └── UseCase.kt.template
│   ├── data/
│   │   ├── Dto.kt.template
│   │   ├── DtoMapper.kt.template
│   │   ├── Entity.kt.template            ← Room @Entity
│   │   ├── EntityMapper.kt.template
│   │   ├── Dao.kt.template               ← Room @Dao
│   │   ├── RemoteDataSource.kt.template  ← Ktor
│   │   ├── LocalDataSource.kt.template   ← Wraps Room Dao
│   │   ├── OfflineFirstRepository.kt.template
│   │   └── HttpClientFactory.kt.template ← Ktor + OkHttp engine
│   ├── presentation/
│   │   ├── UiState.kt.template
│   │   ├── Intent.kt.template
│   │   ├── Event.kt.template
│   │   ├── ViewMapper.kt.template
│   │   └── ViewModel.kt.template
│   ├── ui/
│   │   ├── Screen.kt.template
│   │   └── View.kt.template
│   ├── test/
│   │   └── ScreenshotTest.kt.template    ← Roborazzi (androidUnitTest)
│   ├── benchmark/
│   │   ├── Microbenchmark.kt.template
│   │   ├── Macrobenchmark.kt.template
│   │   └── BaselineProfile.kt.template
│   ├── navigation/
│   │   └── Navigation.kt.template
│   └── di/
│       └── Module.kt.template            ← Hilt + Ktor + Room + Kermit + dispatchers
│
├── eval/                            ← How to evaluate a feature against this skill
│   ├── checklist.md                 ← Pre-PR boolean checklist
│   ├── architecture-fitness.md      ← Qualitative architecture review questions
│   └── quality-rubric.md            ← Scored rubric (0–4) across 12 axes (max 48)
│
├── config/                          ← Plug-and-play starters (zero authoring needed)
│   ├── detekt/detekt.yml            ← Detekt config matching the quality-gates rules
│   └── libs.versions.toml           ← Exact dependency set for the templates' stack
│
├── scripts/
│   ├── run-quality-gates.sh         ← Runs build + detekt + lint + tests + coverage
│   └── scaffold-feature.sh          ← Generates the empty file skeleton
│
├── README.md                        ← Adoption guide + measured token usage + LICENSE (Apache-2.0)

Legend:

• CORE = Loaded with feature generation, always available
• Other = Reference on-demand during development

Authoritative package layout (summary)

For a feature transactions in a module :feature:transactions, package com.<org>.feature.transactions:

presentation/
├── screen/      ← <Feature>Screen.kt (stateful) + <Feature>View.kt (stateless)
├── viewmodel/   ← <Feature>ViewModel.kt
├── state/       ← <Feature>UiState.kt (sealed)
├── intent/      ← <Feature>Intent.kt (sealed)
├── event/       ← <Feature>Event.kt (sealed, one-shot)
└── viewmapper/  ← <Feature>ViewMapper.kt

domain/
├── model/       ← Pure Kotlin data classes / sealed interfaces
├── repository/  ← interfaces only
├── usecase/     ← one operation per class, operator fun invoke
└── mapper/      ← cross-domain mappers (rare)

data/
├── repository/  ← OfflineFirst<Feature>Repository.kt
└── datasource/
    ├── network/  ← <Feature>RemoteDataSource + Api + dto/
    └── local/    ← <Feature>LocalDataSource + Dao + entity/

di/              ← <Feature>Module.kt (Hilt @Module)
navigation/      ← <Feature>Navigation.kt (typed routes)

Full per-folder rules in references/package-layout.md.

Feature Development Workflow

Build (Steps 1–6)

Generate the complete feature end-to-end using the TL;DR recipe above.

Verify (Built-in Guidance)

Step 7: Quality Verification — See references/quality-gates.md for:

• Automated checks (Detekt, Lint, tests, coverage)
• Manual verification checklist
• Architecture fitness evaluation
• Quality scoring (0–40 rubric)

Step 8: Design Review — See references/software-design.md for:

• SOLID principles validation
• KISS, DRY, YAGNI compliance
• Naming conventions alignment
• Pre-commit quality checklist

Step 9: Cross-Feature Concerns (if applicable):

• For PRs/release workflow → follow your team's git and PR process.
• For peer code review → use eval/checklist.md and eval/architecture-fitness.md as the review rubric.
• For multi-module / cross-feature navigation → see the api/impl split in references/modularization.md.

Quick rules — never violate

These are the non-negotiable rules. Detailed reasoning lives in references/.

• Domain has zero Android imports. Pure Kotlin only.
• DTOs and Entities are `internal`. Domain models are public.
• Mappers are extension functions, never methods inside the DTO/Entity.
• Repository is `@Singleton`, offline-first (Flow from local, sync writes to local).
• `@Dispatcher(IO)` injected — never hardcoded Dispatchers.IO.
• ViewModel exposes only `StateFlow<UiState>` built with stateIn(viewModelScope, WhileSubscribed(5_000), Loading).
• State / Intent / Event are sealed. when is exhaustive.
• Events via `Channel(Channel.BUFFERED).receiveAsFlow()` for one-shot effects.
• Screen uses `collectAsStateWithLifecycle()`. Never collectAsState().
• View is stateless with @PreviewLightDark for every meaningful state.
• Hilt binds interfaces, never concrete classes.
• Navigation routes are `@Serializable`. Features expose NavGraphBuilder.<feature>Screen(...).
• No `LiveData` anywhere. StateFlow always.
• Run quality gates before opening a PR.

Full rationale per rule in the relevant references/ file. Anti-pattern table in references/anti-patterns.md.

References

• Now in Android — Architecture Learning Journey

<https://github.com/android/nowinandroid/blob/main/docs/ArchitectureLearningJourney.md>

• Now in Android — Modularization Learning Journey

<https://github.com/android/nowinandroid/blob/main/docs/ModularizationLearningJourney.md>

• Official Android architecture guide

<https://developer.android.com/topic/architecture>

• WhileSubscribed(5000) deep-dive

<https://blog.p-y.wtf/whilesubscribed5000>