Android + Kotlin (Jetpack Compose & Media3)

This playbook defines the architecture, UI design system, performance guidelines, and testing strategy for native Android applications.

android-ui-design owns specialized Jetpack Compose phone UI/UX design, implementation, review, and audit guidance. This skill remains the general guide for architecture, Hilt dependency injection, Media3, performance, and testing.

1. Lead Architect & Modularization

Architecture (MVI / MVVM with Clean Architecture)

• Prefer MVI (Model-View-Intent) for state management. Ensure a strict unidirectional data flow:
  • UiState: Single source of truth for the UI state.
  • UiIntent: Actions initiated by the user or system.
  • UiEffect: One-time events (navigation, toasts, snackbars).
• Separate the project into clean logical modules:
  • app: Application entry point, launcher activity, global dependency injection configuration.
  • core: Shared resources, network layer, media playback logic (ExoPlayer/Media3), storage.
  • feature: Individual business features (isolated and independent of other features).

Dependency Injection (Hilt)

• Annotate the Application class with @HiltAndroidApp.
• Annotate Activities and Fragments with @AndroidEntryPoint.
• Use constructor injection for ViewModels with @HiltViewModel.

2. Jetpack Compose UI/UX

Recomposition & State Management

• Consume StateFlow from ViewModels safely using collectAsStateWithLifecycle() to respect the activity lifecycle.
• Keep data models immutable. Mark configuration classes with @Immutable or @Stable to prevent redundant recompositions.
• Use contentType and key in LazyColumn and LazyRow to optimize item recycling and rendering performance.
• Avoid fixed hardcoded margins. Apply WindowInsets to support modern edge-to-edge screens.

3. Performance & Multimedia (Media3 / ExoPlayer)

Playback & Services

• Run ExoPlayer / Media3 inside a robust Foreground Service (using MediaSessionService) to ensure playback continues when the app is in the background.
• Clean up resources: release player instances in onDestroy or when the service is stopped.
• Control wake locks strictly to avoid draining the user's battery when playback is paused.

Kotlin Coroutines & Threading

• Always inject CoroutineDispatcher instances instead of hardcoding them:
  • Dispatchers.IO: Database access, network operations, file reading.
  • Dispatchers.Default: CPU-intensive operations (parsing JSON, filtering large lists).
  • Dispatchers.Main: UI modifications and light interactions.

R8 & Resource Optimization

• Enable shrinking and obfuscation in production builds:
  • isMinifyEnabled = true
  • isShrinkResources = true
• Convert all static images to the modern WebP format to minimize the application binary size.

4. Quality Assurance & Automated Testing

Unit Testing

• Use JUnit 5 and MockK for mocking dependencies.
• Test ViewModels and Reducers by asserting StateFlow updates. Use Turbine to easily test Kotlin Flows.
• Mock all multimedia/ExoPlayer components to avoid loading actual audio/video resources in unit tests.

UI Testing

• Create UI tests using Compose test rules: createComposeRule() or createAndroidComposeRule<MainActivity>().
• Assert correct rendering, visibility states, and user interactions without executing real network calls.

5. Key Gradle & ADB Commands

Use these commands for building, checking code style, running tests, and profiling performance: