Android App Development Skill

Overview

This skill guides production-grade Android and cross-platform (non-iOS) app development following practices used at big tech companies. It covers the entire development lifecycle — architecture, UI, code quality, testing, error handling, release, and maintenance.

When to Use This Skill

Use when deciding on a tech stack (see §1 Stack Selection)
Use when setting up project architecture (see §2 Architecture)
Use when designing UI, screens, or a design system (see §3 UI & Design)
Use when ensuring code quality, patterns, or APIs (see Best Practices)
Use when implementing error handling or debugging crashes (see §5 Error Handling)
Use when planning testing strategy (see §6 Testing)
Use when configuring build, CI/CD, or release pipelines (see §7 Build & Release)
Use when optimizing performance or memory (see §8 Performance)
Use when debugging or fixing bugs (see §9 Debugging)
Use when following the full development roadmap (see §10 Development Roadmap)
Use when needing deep reference for a stack (see references/ directory)

§1 Stack Selection

Choose based on team, requirements, and platform targets. Do not recommend iOS-specific paths.

Native Android — Kotlin + Jetpack Compose

Best for: Android-only apps, hardware-intensive features, best-in-class UX, new projects.

Language: Kotlin
UI: Jetpack Compose (modern declarative UI)
Key libs: Room, Retrofit/Ktor, Hilt, WorkManager, DataStore, Navigation Compose
Reference: references/native-android.md

Native Android — Java + XML Views

Best for: Existing Java codebases, teams without Kotlin experience, legacy app maintenance, incremental Kotlin migration.

Language: Java (fully supported by Google, not deprecated)
UI: XML Layouts (ConstraintLayout, RecyclerView, ViewBinding)
Key libs: Room, Retrofit, Hilt, WorkManager, LiveData, ViewModel
Java and Kotlin coexist seamlessly in the same project — migrate incrementally
Reference: references/java-android.md

Flutter (Dart)

Best for: Android + Web (+ desktop) from one codebase, fast iteration, pixel-perfect custom UI.

Language: Dart
UI: Flutter Widget tree (Material 3 / Cupertino widgets available but target Material for Android)
Key libs: Provider/Riverpod/Bloc, Dio, Drift/Isar, go_router, flutter_local_notifications
Reference: references/flutter.md

React Native (JavaScript/TypeScript)

Best for: Web + Android code sharing, JS/TS teams, rich ecosystem.

Language: TypeScript (preferred)
UI: React Native core components + NativeWind / React Native Paper
Key libs: React Navigation, Zustand/Redux Toolkit, React Query, MMKV
Reference: references/react-native.md

Kotlin Multiplatform (KMM / Compose Multiplatform)

Best for: Sharing business logic across Android + Desktop + Web while keeping native Android UI.

Language: Kotlin everywhere
UI: Native Compose on Android; Compose Multiplatform for shared UI
Key libs: Ktor, SQLDelight, Koin, kotlinx.serialization, Napier
Reference: references/kmm.md

Hybrid (Capacitor / Ionic)

Best for: Web-first teams, simple apps, PWA-like content apps.

Language: TypeScript + HTML/CSS
UI: Ionic components or custom web UI
Avoid for: Heavy animations, native sensor access, high-performance games
Reference: references/hybrid.md

Decision Matrix

Requirement	Native Kotlin	Native Java	Flutter	RN	KMM	Hybrid
Android-only (new)	✅ Best	✅	✅	✅	✅	✅
Android-only (existing Java)	⚠️ migrate	✅ Best	❌	❌	⚠️	❌
Android + Web	❌	❌	✅	✅	✅	✅ Best
Android + Desktop	❌	❌	✅	⚠️	✅	⚠️
Shared business logic only	N/A	N/A	N/A	N/A	✅ Best	N/A
Native performance	✅	✅	✅	⚠️	✅	❌
JS/TS team	❌	❌	❌	✅ Best	❌	✅
Custom pixel-perfect UI	✅	⚠️	✅ Best	⚠️	✅	❌

§2 Architecture

Core Principle: Separation of Concerns

Every production Android project must separate UI, business logic, and data into distinct, independently testable layers.

Recommended Architecture: Clean Architecture + MVI/MVVM

app/
├── ui/              # Composables / Activities / Fragments / Screen states
├── presentation/    # ViewModels, UI State, UI Events
├── domain/          # Use cases, domain models, repository interfaces
├── data/            # Repository impl, remote (API), local (DB), mappers
└── di/              # Dependency injection modules

Data flow (unidirectional):

User Action → ViewModel/Store → Use Case → Repository → Data Source
                    ↓
             UI State (sealed class / StateFlow)
                    ↓
             Composable / View renders state

Key Architecture Patterns by Stack

Native (MVVM + MVI):

StateFlow / SharedFlow for reactive state
sealed class UiState + sealed class UiEvent
Hilt for DI, coroutines + Flow for async
Repository pattern wrapping Room + Retrofit

Flutter (BLoC or Riverpod):

Bloc or Cubit for business logic isolation
AsyncNotifierProvider (Riverpod) for data + state
Repositories as abstract classes with impl injected

React Native (Redux Toolkit or Zustand):

RTK Query or React Query for server state
Zustand slices for client state
Custom hooks to encapsulate business logic per feature

KMM:

Shared commonMain holds domain + data layers
expect/actual for platform-specific implementations
Kotlin coroutines + Flow bridged to platform (StateFlow on Android)

Module Structure (Multi-module for large apps)

:app            # Entry point, DI wiring
:core:ui        # Design system, shared composables
:core:network   # API client, interceptors
:core:database  # Room / SQLDelight setup
:feature:home
:feature:profile
:feature:settings

§3 UI & Design

Design System First

Before writing screens, define:

Color tokens — Primary, secondary, surface, on-surface, error; light + dark variants
Typography scale — Display, headline, title, body, label (Material 3 type system)
Spacing scale — 4dp grid system (4, 8, 12, 16, 24, 32, 48dp)
Shape tokens — Corner radii per component family
Component library — Button, TextField, Card, BottomSheet, TopAppBar, etc.

Jetpack Compose UI Rules

Use MaterialTheme tokens; never hardcode colors/dimensions
CompositionLocal for theme, locale, haptics
remember / rememberSaveable correctly (saveable for UI state surviving rotation)
Extract large composables into sub-composables; each function ≤ 80 lines
Use LazyColumn/LazyVerticalGrid for lists; never Column with forEach for large data
Side effects only in LaunchedEffect, DisposableEffect, SideEffect
Avoid state hoisting anti-patterns: hoist state to the lowest common ancestor

Accessibility (Non-Negotiable)

All interactive elements: contentDescription or semantics { }
Min touch target: 48×48dp
TalkBack compatibility tested before every release
Dynamic text size support (sp not dp for text)
Color contrast ratio ≥ 4.5:1 (WCAG AA)

Navigation

Native: Navigation Compose with typed NavHost and SafeArgs equivalent
Flutter: go_router with named routes and guards
RN: React Navigation v7 with typed NavigationProp
Deep link handling registered for every screen that can be externally opened
Back stack managed deliberately — don't push duplicates, use popUpTo / launchSingleTop

Responsive & Adaptive UI

Support all screen sizes: phones, foldables, tablets (WindowSizeClass)
Test at 320dp, 360dp, 411dp, 600dp+, 840dp+ widths
Foldable hinge awareness via WindowInfoTracker
Edge-to-edge display + WindowInsets handling required for Android 15+

Best Practices

Language Standards

Kotlin:

Prefer data class, sealed class, object, enum class appropriately
No !! null assertions — use ?.let, ?: return, requireNotNull with message
Coroutines: always specify CoroutineScope + Dispatcher explicitly; never GlobalScope
Use @Stable / @Immutable on Compose state classes for smart recomposition

Java:

@NonNull / @Nullable annotations on every method param and return type
Never call methods on unchecked objects — null-check explicitly or use Objects.requireNonNull
Always null binding reference in Fragment's onDestroyView() to prevent memory leaks
Use ExecutorService (not AsyncTask — deprecated) for background work; or LiveData + Room's built-in threading
Prefer ListAdapter + DiffUtil over manual notifyDataSetChanged() in RecyclerView
Use ViewBinding — never findViewById

Dart (Flutter):

Null safety required — no ! without explicit null check above
Immutable state objects with copyWith
const constructors on all stateless widgets

TypeScript (RN):

strict: true in tsconfig always
Zod or io-ts for runtime type validation of API responses
No any — use unknown and narrow

Dependency Management

Pin all dependency versions in build.gradle.kts / pubspec.yaml / package.json
Audit dependencies monthly for security vulnerabilities
Avoid transitive dependency conflicts — use dependency resolution strategies
Keep dependency count minimal — every added lib is a maintenance burden

Code Review Checklist (PR gate)

New public APIs have KDoc / DartDoc / JSDoc
No hardcoded strings — use string resources / l10n
No hardcoded dimensions or colors outside design tokens
No blocking I/O on main thread
No memory leaks (no Activity context stored in singletons)
Coroutine scopes / streams properly cancelled / disposed
Feature flag guarding any non-trivial feature

§5 Error Handling

The Golden Rule

Never let exceptions propagate to the user silently or crash the app.

Error Classification

Type	Strategy
Network errors	Retry with exponential backoff; show retry UI
Auth errors (401/403)	Refresh token → re-request → logout if fails
Validation errors	Show inline field errors immediately
Data parsing errors	Log + fallback to cached/default state
Unexpected crashes	Catch at top-level; show error screen + report
Background task failures	Retry via WorkManager; notify user if critical

Result / Either Pattern (Kotlin)

sealed class AppResult<out T> {
    data class Success<T>(val data: T) : AppResult<T>()
    data class Error(val exception: AppException) : AppResult<Nothing>()
}

sealed class AppException(msg: String) : Exception(msg) {
    class NetworkException(msg: String) : AppException(msg)
    class AuthException(msg: String) : AppException(msg)
    class ParseException(msg: String) : AppException(msg)
    class UnknownException(msg: String) : AppException(msg)
}

Use AppResult<T> as return type for all repository + use case functions. ViewModels map to UiState.Error.

Crash Reporting

Integrate Firebase Crashlytics or Sentry from day one
Set user identifiers and custom keys before crash occurs
Non-fatal exceptions logged for all caught errors
ANR monitoring enabled
Crash-free sessions target: ≥ 99.5%

Offline / Network Resilience

Cache-first strategy: show stale data, fetch fresh in background
Room / Drift / MMKV as single source of truth
Expose network state via ConnectivityManager and reflect in UI
All network calls wrapped with timeout + retry policy

§6 Testing

Testing Pyramid

         /\
        /E2E\        ← 10%  (UI tests: Espresso, Maestro, Appium)
       /------\
      / Integr \     ← 20%  (Repository, DB, API contract tests)
     /----------\
    /    Unit    \   ← 70%  (ViewModels, Use Cases, Utilities)
   /--------------\

Unit Tests (70%)

Every ViewModel, UseCase, Repository, Mapper tested
Native: JUnit5 + MockK + Turbine (Flow testing) + Kotest assertions
Flutter: flutter_test + mocktail
RN: Jest + @testing-library/react-native + msw for API mocking
Coverage target: ≥ 80% on domain + presentation layers

Integration Tests (20%)

Room DB tests with in-memory database
Retrofit/Ktor tests with MockWebServer (OkHttp)
Repository tests verifying cache + remote coordination
API contract tests against real staging endpoint

UI / E2E Tests (10%)

Espresso for critical user journeys (login, checkout, core action)
Maestro for cross-platform E2E flows (recommended for Flutter + RN too)
Run on real device farm (Firebase Test Lab / BrowserStack) before release
Smoke test suite runs on every PR; full E2E suite nightly

Test Data Management

Use factories / builders for test data, never copy-paste objects
Hermetic tests: never share mutable state between test cases
Fakes over mocks for complex dependencies (repositories, data sources)

§7 Build & Release

Build Variants

debug       → dev API, logging on, no minification, debuggable
staging     → staging API, logging on, minified, not debuggable
release     → prod API, logging off, minified, signed

Gradle Best Practices (Native)

build.gradle.kts only — no Groovy DSL in new projects
Version catalog (libs.versions.toml) for all dependency versions
buildConfig for environment-specific constants
Baseline profiles for startup performance
R8 full mode enabled in release; maintain proguard rules in version control

CI/CD Pipeline

PR Opened
  └─ lint + unit tests + build debug APK          [< 5 min]

Merge to main
  └─ unit + integration tests + staging build     [< 15 min]
  └─ deploy to Firebase App Distribution (QA)

Release tag
  └─ full test suite + E2E on device farm         [< 45 min]
  └─ build release AAB
  └─ upload to Play Console (internal track)
  └─ promote: internal → closed testing → open → production

Recommended CI: GitHub Actions, Bitrise, or CircleCI.

Play Store Release Strategy

Always release to internal → closed → open testing before production
Use staged rollouts: 5% → 20% → 50% → 100% with 24-48h monitoring
Monitor Crashlytics + ANR rate + rating before expanding rollout
Never skip staged rollout for significant changes

App Signing

Upload key (Play App Signing): stored in CI secrets, never committed
Use Google Play App Signing for distribution key management
Document key recovery procedure in team runbook

§8 Performance

Startup Performance

App startup time target: cold start < 1s, warm start < 500ms
Use App Startup library for initializing libraries lazily
Baseline profiles generated + committed to repo
Heavy initialization moved off main thread

UI Performance

Target: 60fps (90/120fps on supported devices); zero jank
Measure with Android Studio Profiler + FrameMetrics API
Avoid allocation in draw() / onMeasure() / composition
Use derivedStateOf in Compose to avoid unnecessary recompositions
Image loading: Coil (Compose) / Glide / Picasso — never load full-res in thumbnails

Memory

No Activity / Context references in ViewModels or singletons
WeakReferences for listeners stored beyond their owner's lifecycle
Bitmap recycling and memory cache sizing
Heap dump + leak detection via LeakCanary in debug builds (always)

Network

HTTP caching headers respected
Image CDN + WebP format
Gzip/Brotli compression verified
Request batching where applicable
Connection pooling configured

Battery

Background work only via WorkManager with appropriate constraints
Location updates: request only needed accuracy level; stop when backgrounded
Wakelocks used sparingly with explicit release

§9 Debugging & Bug Fixing

Debugging Process

Reproduce reliably — document exact steps, device, OS version, account state
Isolate — is it UI, business logic, network, or persistence?
Instrument — add targeted logs / breakpoints, NOT shotgun logging
Hypothesize — form 1-3 specific hypotheses before touching code
Fix the root cause — never patch symptoms; trace back to the source
Regression test — write a test that fails before fix, passes after
Document — comment explaining why the fix works, not just what it does

Common Android Bug Patterns

Bug	Likely Cause	Fix
ANR	Main thread I/O / long computation	Move to coroutine/Dispatcher.IO
Memory leak	Context stored in singleton	Use `applicationContext`; WeakRef
Crash on rotation	ViewModel not used; state not saved	`rememberSaveable` / ViewModel
UI lag	Recomposition loops	`derivedStateOf`, stable params
Blank screen after API call	Error swallowed silently	Check error state propagation
Deep link not working	Manifest intent-filter missing	Verify `adb shell am start` test
Push notification silent	Background restrictions	Test on real devices across OEMs

Logging Standards

Production: Firebase Crashlytics only (no Log.d in release builds)
Debug/Staging: Timber with debug tree
Log levels: ERROR (crashes), WARN (recoverable), INFO (key events), DEBUG (dev only)
Never log PII — mask emails, phone numbers, tokens in logs

OEM-Specific Issues

Test on Samsung, Xiaomi/MIUI, OnePlus/OxygenOS, Huawei (no GMS) for critical flows
Background restrictions vary widely by OEM — test push, alarms, background sync
Maintain a physical or cloud device farm with top market-share devices

§10 Development Roadmap

Follow this phase structure for any new Android project:

Phase 0 — Foundation (Week 1-2)

Stack decision documented with rationale
Module structure defined
Design system tokens defined (colors, type, spacing, shapes)
CI pipeline running (lint + unit tests + build)
Crash reporting integrated (Crashlytics/Sentry)
Analytics baseline integrated (Firebase/Amplitude)
API contract / mock server set up
DI framework configured
Navigation skeleton implemented
Flavor/build variant config complete

Phase 1 — Core Features (Weeks 3-8)

Auth flow (login, register, token refresh, logout)
Core screen shells with real navigation
Network layer (client, interceptors, error handling)
Local persistence layer (DB schema + DAOs)
Repository layer wiring remote + local
ViewModels + UI states for each feature
Unit tests for all ViewModels + use cases
Feature flags infrastructure

Phase 2 — Polish (Weeks 9-12)

Design QA pass against Figma/spec
Accessibility audit (TalkBack, contrast, touch targets)
Dark mode implementation + verification
Localization (strings externalized, RTL support if needed)
Loading, empty, error states on every screen
Deep link handling
Widget / notification implementation
Offline mode verification

Phase 3 — Hardening (Weeks 12-14)

Performance profiling (startup, scroll, memory)
E2E test suite on device farm (Firebase Test Lab)
Security review (certificate pinning, biometrics, secure storage)
Proguard / R8 rules verified
Crash-free rate ≥ 99.5% on staging
Play Store listing, screenshots, privacy policy

Phase 4 — Release

AAB signed and uploaded to internal track
Staged rollout plan defined
Monitoring dashboard set up (Crashlytics, Play Console vitals)
Rollback plan documented
On-call rotation assigned

Phase 5 — Post-Launch (Ongoing)

Crash-free rate monitored daily
ANR rate < 0.47% (Play Store threshold)
App rating monitored; negative reviews triaged weekly
Dependency updates reviewed monthly
OS beta testing with each new Android release

Limitations

This skill is scoped to Android and Android-adjacent delivery paths; it does not cover iOS-only architecture, App Store release operations, or Apple platform UI guidance.
Version numbers, Play Console policy thresholds, and recommended libraries can change; verify release-critical details against current Android, Google Play, and library documentation before shipping.
Code snippets are architecture patterns, not complete applications; adapt package names, dependency versions, permissions, privacy disclosures, and security controls to the actual project.
The guidance does not replace device QA, accessibility review, security review, legal/privacy review, or store compliance checks for a production release.

Additional Resources

For stack-specific deep dives, read:

references/native-android.md — Kotlin, Compose, Room, Hilt, Coroutines
references/java-android.md — Java, XML Views, ViewBinding, LiveData, Retrofit, Room, Hilt, migration path
references/flutter.md — Dart, BLoC/Riverpod, Drift, go_router
references/react-native.md — TypeScript, RN architecture, Hermes, New Architecture
references/kmm.md — KMM shared modules, SQLDelight, Ktor, Compose Multiplatform
references/hybrid.md — Capacitor, Ionic, PWA considerations