Mobiai Android Architecture

Android Architecture

Common architecture patterns and project structures in Android apps.

When to Use

• Navigating an unfamiliar Android codebase
• Understanding where to make changes for a bug fix
• Writing code that follows the project's existing patterns

Detecting the Architecture

Check project structure

# Find the main source directory
find app/src/main -type d | head -30

# Look for common architecture indicators
grep -r "ViewModel\|UseCase\|Repository\|Interactor\|Presenter" app/src/main --include="*.kt" -l | head -20

Common Patterns

Clean Architecture (Most Common)

Layer Structure

feature/
  presentation/        # UI Layer
    MyFragment.kt         # Fragment (XML) or Screen (Compose)
    MyViewModel.kt        # ViewModel — UI state + user actions
    MyUiState.kt          # State model for the UI
  domain/              # Business Logic Layer
    usecase/
      GetItemsUseCase.kt  # Single-purpose business operation
    model/
      Item.kt              # Domain model (plain Kotlin)
    repository/
      ItemRepository.kt   # Interface — defined here, implemented in data layer
  data/                # Data Layer
    repository/
      ItemRepositoryImpl.kt  # Implementation of domain interface
    local/
      ItemDao.kt           # Room DAO
      ItemEntity.kt        # Database entity
    remote/
      ItemApi.kt           # Retrofit API interface
      ItemDto.kt           # Network response model
    mapper/
      ItemMapper.kt        # DTO ↔ Entity ↔ Domain model mapping

Tracing a Bug Through Layers

1. Start from the UI: Find the Fragment/Activity/Composable for the screen
2. Follow to ViewModel: The UI observes ViewModel state. Find what triggers the bug.
3. Follow to UseCase: The ViewModel calls UseCases. Which one is involved?
4. Follow to Repository: The UseCase calls Repository methods. Where does data come from?
5. Check Data Sources: Repository uses DAOs (local) and APIs (remote). Is the data correct?

Quick Navigation

# Find ViewModel for a feature
grep -r "class.*ViewModel" app/src/main --include="*.kt" | grep -i "<feature>"

# Find UseCase
grep -r "class.*UseCase\|class.*Interactor" app/src/main --include="*.kt" | grep -i "<feature>"

# Find Repository interface
grep -r "interface.*Repository" app/src/main --include="*.kt" | grep -i "<feature>"

# Find Repository implementation
grep -r "class.*RepositoryImpl\|class.*Repository(" app/src/main --include="*.kt" | grep -i "<feature>"

Jetpack Compose

Project Structure (Compose)

feature/
  presentation/
    MyScreen.kt           # @Composable function
    MyViewModel.kt        # ViewModel with StateFlow
    components/           # Reusable composables for this feature
      ItemCard.kt
      LoadingIndicator.kt

Common Compose Patterns

// ViewModel exposes state
class MyViewModel : ViewModel() {
    private val _state = MutableStateFlow(MyState())
    val state: StateFlow<MyState> = _state.asStateFlow()

    fun onAction(action: MyAction) { ... }
}

// Screen collects state
@Composable
fun MyScreen(viewModel: MyViewModel = hiltViewModel()) {
    val state by viewModel.state.collectAsStateWithLifecycle()
    MyScreenContent(state = state,
}

Finding Compose Screens

# Find all @Composable screens
grep -r "@Composable" app/src/main --include="*.kt" -l | head -20

# Find navigation graph
grep -r "NavHost\|composable(" app/src/main --include="*.kt" -l

XML Views (Legacy/Hybrid)

Project Structure (XML)

feature/
  presentation/
    MyFragment.kt        # Fragment with ViewBinding
    MyViewModel.kt       # ViewModel
    adapter/             # RecyclerView adapters
      MyAdapter.kt
res/
  layout/
    fragment_my.xml      # Layout file
    item_my.xml          # RecyclerView item layout

Finding XML Layouts

# Find layout for a fragment
grep -r "R.layout\.\|inflate(" app/src/main --include="*.kt" | grep -i "<feature>"

# Find string resources
grep -r "<feature-keyword>" app/src/main/res/values/strings.xml

Dependency Injection

Hilt (Most Common)

# Find modules
grep -r "@Module\|@Provides\|@Binds" app/src/main --include="*.kt" -l

# Find injected classes
grep -r "@Inject\|@HiltViewModel" app/src/main --include="*.kt" | grep -i "<feature>"

Koin

grep -r "module\s*{" app/src/main --include="*.kt" -l

Build Flavors and Resources

Country/Environment-specific code

# Check for flavor-specific source sets
ls app/src/*/java/ 2>/dev/null

# Check for resource overrides
ls app/src/main/res/values*/strings.xml 2>/dev/null

String Resources

# Find a string by its visible text
grep -r "visible text here" app/src/main/res/values/strings.xml

# Find a string by its ID
grep -r "@string/my_string_id" app/src/main --include="*.xml" --include="*.kt"

Navigation

Jetpack Navigation

# Find nav graphs
find app/src/main/res -name "*nav*" -o -name "*navigation*"

# Find navigation actions
grep -r "navigate\|findNavController\|NavController" app/src/main --include="*.kt" | grep -i "<feature>"

Activity-based Navigation

grep -r "startActivity\|Intent(" app/src/main --include="*.kt" | grep -i "<feature>"