implementing-android-code

name	implementing-android-code
version	0.1.4
description	This skill should be used when implementing Android code in Bitwarden. Covers critical patterns, gotchas, and anti-patterns unique to this codebase. Triggered by "How do I implement a ViewModel?", "Create a new screen", "Add navigation", "Write a repository", "BaseViewModel pattern", "State-Action-Event", "type-safe navigation", "@Serializable route", "SavedStateHandle persistence", "process death recovery", "handleAction", "sendAction", "Hilt module", "Repository pattern", "implementing a screen", "adding a data source", "handling navigation", "encrypted storage", "security patterns", "Clock injection", "DataState", or any questions about implementing features, screens, ViewModels, data sources, or navigation in the Bitwarden Android app.

Implementing Android Code - Bitwarden Quick Reference

This skill provides tactical guidance for Bitwarden-specific patterns. For comprehensive architecture decisions and complete code style rules, consult docs/ARCHITECTURE.md and docs/STYLE_AND_BEST_PRACTICES.md.

---

Critical Patterns Reference

A. ViewModel Implementation (State-Action-Event Pattern)

All ViewModels follow the State-Action-Event (SAE) pattern via BaseViewModel<State, Event, Action>.

Key Requirements:

• Annotate with @HiltViewModel
• State class MUST be @Parcelize data class : Parcelable
• Implement handleAction(action: A) - MUST be synchronous
• Post internal actions from coroutines using sendAction()
• Save/restore state via SavedStateHandle[KEY_STATE]
• Private action handlers: private fun handle* naming convention

Template: See ViewModel template

Pattern Summary:

@HiltViewModel
class ExampleViewModel @Inject constructor(
    savedStateHandle: SavedStateHandle,
    private val repository: ExampleRepository,
) : BaseViewModel<ExampleState, ExampleEvent, ExampleAction>(
    initialState = savedStateHandle[KEY_STATE] ?: ExampleState(),
) {
    init {
        stateFlow.onEach { savedStateHandle[KEY_STATE] = it }.launchIn(viewModelScope)
    }

    override fun handleAction(action: ExampleAction) {
        // Synchronous dispatch only
        when (action) {
            is Action.Click -> handleClick()
            is Action.Internal.DataReceived -> handleDataReceived(action)
        }
    }

    private fun handleClick() {
        viewModelScope.launch {
            val result = repository.fetchData()
            sendAction(Action.Internal.DataReceived(result))  // Post internal action
        }
    }

    private fun handleDataReceived(action: Action.Internal.DataReceived) {
        mutableStateFlow.update { it.copy(data = action.result) }
    }
}

Reference:

• ui/src/main/kotlin/com/bitwarden/ui/platform/base/BaseViewModel.kt (see handleAction method)
• app/src/main/kotlin/com/x8bit/bitwarden/ui/auth/feature/login/LoginViewModel.kt (see class declaration)

Critical Gotchas:

• ❌ NEVER update mutableStateFlow directly inside coroutines
• ✅ ALWAYS post internal actions from coroutines, update state in handleAction()
• ❌ NEVER forget @IgnoredOnParcel for sensitive data (causes security leak)
• ✅ ALWAYS use @Parcelize on state classes for process death recovery
• ✅ State restoration happens automatically if properly saved to SavedStateHandle

---

B. Navigation Implementation (Type-Safe)

All navigation uses type-safe routes with kotlinx.serialization.

Pattern Structure:

1. @Serializable route data class with parameters
2. ...Args helper class for extracting from SavedStateHandle
3. NavGraphBuilder.{screen}Destination() extension for adding screen to graph
4. NavController.navigateTo{Screen}() extension for navigation calls

Template: See Navigation template

Pattern Summary:

@Serializable
data class ExampleRoute(val userId: String, val isEditMode: Boolean = false)

data class ExampleArgs(val userId: String, val isEditMode: Boolean)

fun SavedStateHandle.toExampleArgs(): ExampleArgs {
    val route = this.toRoute<ExampleRoute>()
    return ExampleArgs(userId = route.userId, isEditMode = route.isEditMode)
}

fun NavController.navigateToExample(
    userId: String,
    isEditMode: Boolean = false,
    navOptions: NavOptions? = null,
) {
    this.navigate(route = ExampleRoute(userId, isEditMode), navOptions = navOptions)
}

fun NavGraphBuilder.exampleDestination(onNavigateBack: () -> Unit) {
    composableWithSlideTransitions<ExampleRoute> {
        ExampleScreen(onNavigateBack = onNavigateBack)
    }
}

Reference: app/src/main/kotlin/com/x8bit/bitwarden/ui/auth/feature/login/LoginNavigation.kt (see LoginRoute and extensions)

Key Benefits:

• ✅ Type safety: Compile-time errors for missing parameters
• ✅ No string literals in navigation code
• ✅ Automatic serialization/deserialization
• ✅ Clear contract for screen dependencies

---

C. Screen/Compose Implementation

All screens follow consistent Compose patterns.

Template: See Screen/Compose template

Key Patterns:

@Composable
fun ExampleScreen(
    onNavigateBack: () -> Unit,
    viewModel: ExampleViewModel = hiltViewModel(),
) {
    val state by viewModel.stateFlow.collectAsStateWithLifecycle()

    EventsEffect(viewModel = viewModel) { event ->
        when (event) {
            ExampleEvent.NavigateBack -> onNavigateBack()
        }
    }

    BitwardenScaffold(
        topBar = {
            BitwardenTopAppBar(
                title = stringResource(R.string.title),
                navigationIcon = rememberVectorPainter(BitwardenDrawable.ic_back),
                onNavigationIconClick = { viewModel.trySendAction(ExampleAction.BackClick) },
            )
        },
    ) {
        // UI content
    }
}

Reference: app/src/main/kotlin/com/x8bit/bitwarden/ui/auth/feature/login/LoginScreen.kt (see LoginScreen composable)

Essential Requirements:

• ✅ Use hiltViewModel() for dependency injection
• ✅ Use collectAsStateWithLifecycle() for state (not collectAsState())
• ✅ Use EventsEffect(viewModel) for one-shot events
• ✅ Use Bitwarden* prefixed components from :ui module

State Hoisting Rules:

• ViewModel state: Data that needs to survive process death or affects business logic
• UI-only state: Temporary UI state (scroll position, text field focus) using remember or rememberSaveable

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D. Data Layer Implementation

The data layer follows strict patterns for repositories, managers, and data sources.

Interface + Implementation Separation (ALWAYS)

Template: See Data Layer template

Pattern Summary:

// Interface (injected via Hilt)
interface ExampleRepository {
    suspend fun fetchData(id: String): ExampleResult
    val dataFlow: StateFlow<DataState<ExampleData>>
}

// Implementation (NOT directly injected)
class ExampleRepositoryImpl(
    private val exampleDiskSource: ExampleDiskSource,
    private val exampleService: ExampleService,
) : ExampleRepository {
    override suspend fun fetchData(id: String): ExampleResult {
        // NO exceptions thrown - return Result or sealed class
        return exampleService.getData(id).fold(
            onSuccess = { ExampleResult.Success(it.toModel()) },
            onFailure = { ExampleResult.Error(it.message) },
        )
    }
}

// Sealed result class (domain-specific)
sealed class ExampleResult {
    data class Success(val data: ExampleData) : ExampleResult()
    data class Error(val message: String?) : ExampleResult()
}

// Hilt Module
@Module
@InstallIn(SingletonComponent::class)
object ExampleRepositoryModule {
    @Provides
    @Singleton
    fun provideExampleRepository(
        exampleDiskSource: ExampleDiskSource,
        exampleService: ExampleService,
    ): ExampleRepository = ExampleRepositoryImpl(exampleDiskSource, exampleService)
}

Reference:

• app/src/main/kotlin/com/x8bit/bitwarden/data/auth/repository/AuthRepository.kt
• app/src/main/kotlin/com/x8bit/bitwarden/data/tools/generator/repository/di/GeneratorRepositoryModule.kt

Three-Layer Data Architecture:

1. Data Sources - Raw data access (network, disk, SDK). Return Result<T>, never throw.
2. Managers - Single responsibility business logic. Wrap OS/external services.
3. Repositories - Aggregate sources/managers. Return domain-specific sealed classes.

Critical Rules:

• ❌ NEVER throw exceptions in data layer
• ✅ ALWAYS use interface + ...Impl pattern
• ✅ ALWAYS inject interfaces, never implementations
• ✅ Data sources return Result<T>, repositories return domain sealed classes
• ✅ Use StateFlow for continuously observed data

KDoc on Interfaces vs. Implementations

KDoc on an interface member describes the contract — what the caller can rely on. It must not describe how the implementation fulfills that contract. Implementation details (which data source is consulted, what is cached, what is logged, which manager is delegated to, ordering of internal calls, etc.) belong on the ...Impl member — and only when the why is non-obvious from the code itself.

This applies to every interface in the codebase: repositories, managers, data sources, validators, and UI-layer interfaces alike. The rule is the same one CLAUDE.md states for comments in general — describe the contract or the non-obvious why, never the what a well-named identifier already conveys.

❌ Wrong — interface KDoc leaks the implementation:

interface ExampleRepository {
    /**
     * Fetches data by reading from [ExampleDiskSource] first, falling back to
     * [ExampleService] on cache miss and writing the network result back to disk.
     */
    suspend fun fetchData(id: String): ExampleResult
}

✅ Right — interface KDoc describes the contract; implementation details (if needed at all) live on the override:

interface ExampleRepository {
    /** Returns the [ExampleData] for [id], or an error result on failure. */
    suspend fun fetchData(id: String): ExampleResult
}

class ExampleRepositoryImpl(...) : ExampleRepository {
    // No KDoc needed — the disk-then-network pattern is visible in the code.
    override suspend fun fetchData(id: String): ExampleResult { ... }
}

Red flags that an interface KDoc has drifted into implementation territory:

• Names a concrete collaborator (...DiskSource, ...Service, ...Manager, ...Impl)
• Describes ordering of internal calls ("first ... then ...", "falls back to ...")
• Mentions caching, retries, logging, or threading behavior that callers don't depend on
• Restates the method signature in prose ("Suspends until X returns Y")

If callers genuinely depend on a behavior (e.g., "this method is safe to call before vault unlock", "result is cached for the session"), that is part of the contract and belongs on the interface. The test: would a different valid implementation be free to change this? If yes, it's implementation detail — strip it.

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E. UI Components

Use Existing Components First:

The :ui module provides reusable Bitwarden* prefixed components. Search before creating new ones.

Common Components:

• BitwardenFilledButton - Primary action buttons
• BitwardenOutlinedButton - Secondary action buttons
• BitwardenTextField - Text input fields
• BitwardenPasswordField - Password input with show/hide
• BitwardenSwitch - Toggle switches
• BitwardenTopAppBar - Toolbar/app bar
• BitwardenScaffold - Screen container with scaffold
• BitwardenBasicDialog - Simple dialogs
• BitwardenLoadingDialog - Loading indicators

Component Discovery: Search ui/src/main/kotlin/com/bitwarden/ui/platform/components/ for existing Bitwarden* components. For build, test, and codebase discovery commands, use the build-test-verify skill.

When to Create New Reusable Components:

• Component used in 3+ places
• Component needs consistent theming across app
• Component has semantic meaning (accessibility)
• Component has complex state management

New Component Requirements:

• Prefix with Bitwarden
• Accept themed colors/styles from BitwardenTheme
• Include preview composables for testing
• Support accessibility (content descriptions, semantics)

String Resources:

New strings belong in the :ui module: ui/src/main/res/values/strings.xml

• Use typographic apostrophes and quotes to avoid escape characters: you’ll not you\'ll, “word” not \"word\"
• Reference strings via generated BitwardenString resource IDs
• Do not add strings to other modules unless explicitly instructed

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F. Security Patterns

Encrypted vs Unencrypted Storage:

Template: See Security templates

Pattern Summary:

class ExampleDiskSourceImpl(
    @EncryptedPreferences encryptedSharedPreferences: SharedPreferences,
    @UnencryptedPreferences sharedPreferences: SharedPreferences,
) : BaseEncryptedDiskSource(
    encryptedSharedPreferences = encryptedSharedPreferences,
    sharedPreferences = sharedPreferences,
),
    ExampleDiskSource {
    fun storeAuthToken(token: String) {
        putEncryptedString(KEY_TOKEN, token)  // Sensitive — uses base class method
    }

    fun storeThemePreference(isDark: Boolean) {
        putBoolean(KEY_THEME, isDark)  // Non-sensitive — uses base class method
    }
}

Android Keystore (Biometric Keys):

• User-scoped encryption keys: BiometricsEncryptionManager
• Keys stored in Android Keystore (hardware-backed when available)
• Integrity validation on biometric state changes

Input Validation:

// Validation returns boolean, NEVER throws
interface RequestValidator {
    fun validate(request: Request): Boolean
}

// Sanitization removes dangerous content
fun String?.sanitizeTotpUri(issuer: String?, username: String?): String? {
    if (this.isNullOrBlank()) return null
    // Sanitize and return safe value
}

Security Checklist:

• ✅ Use @EncryptedPreferences for credentials, keys, tokens
• ✅ Use @UnencryptedPreferences for UI state, preferences
• ✅ Use @IgnoredOnParcel for sensitive ViewModel state
• ❌ NEVER log sensitive data (passwords, tokens, vault items)
• ✅ Validate all user input before processing
• ✅ Use Timber for non-sensitive logging only

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G. Testing Patterns

ViewModel Testing:

Template: See Testing templates

Pattern Summary:

class ExampleViewModelTest : BaseViewModelTest() {
    private val mockRepository: ExampleRepository = mockk()

    @Test
    fun `ButtonClick should fetch data and update state`() = runTest {
        val expectedResult = ExampleResult.Success(data = "test")
        coEvery { mockRepository.fetchData(any()) } returns expectedResult

        val viewModel = createViewModel()
        viewModel.trySendAction(ExampleAction.ButtonClick)

        viewModel.stateFlow.test {
            assertEquals(EXPECTED_STATE.copy(data = "test"), awaitItem())
        }
    }

    private fun createViewModel(): ExampleViewModel = ExampleViewModel(
        savedStateHandle = SavedStateHandle(mapOf(KEY_STATE to EXPECTED_STATE)),
        repository = mockRepository,
    )
}

Reference: app/src/test/kotlin/com/x8bit/bitwarden/ui/tools/feature/generator/GeneratorViewModelTest.kt

Key Testing Patterns:

• ✅ Extend BaseViewModelTest for proper dispatcher management
• ✅ Use runTest from kotlinx.coroutines.test
• ✅ Use Turbine's .test { awaitItem() } for Flow assertions
• ✅ Use MockK: coEvery for suspend functions, every for sync
• ✅ Test both state changes and event emissions
• ✅ Test both success and failure Result paths

Flow Testing with Turbine:

// Test state and events simultaneously
viewModel.stateEventFlow(backgroundScope) { stateFlow, eventFlow ->
    viewModel.trySendAction(ExampleAction.Submit)
    assertEquals(ExpectedState.Loading, stateFlow.awaitItem())
    assertEquals(ExampleEvent.ShowSuccess, eventFlow.awaitItem())
}

MockK Quick Reference:

coEvery { repository.fetchData(any()) } returns Result.success("data")  // Suspend
every { diskSource.getData() } returns "cached"  // Sync
coVerify { repository.fetchData("123") }  // Verify

---

H. Clock/Time Handling

All code needing current time must inject Clock for testability.

Key Requirements:

• ✅ Inject Clock via Hilt in ViewModels
• ✅ Pass Clock as parameter in extension functions
• ✅ Use clock.instant() to get current time
• ❌ Never call Instant.now() or DateTime.now() directly
• ❌ Never use mockkStatic for datetime classes in tests

Pattern Summary:

// ViewModel with Clock
class MyViewModel @Inject constructor(
    private val clock: Clock,
) {
    val timestamp = clock.instant()
}

// Extension function with Clock parameter
fun State.getTimestamp(clock: Clock): Instant =
    existingTime ?: clock.instant()

// Test with fixed clock
val FIXED_CLOCK = Clock.fixed(
    Instant.parse("2023-10-27T12:00:00Z"),
    ZoneOffset.UTC,
)

Reference:

• docs/STYLE_AND_BEST_PRACTICES.md (see Time and Clock Handling section)
• core/src/main/kotlin/com/bitwarden/core/di/CoreModule.kt (see provideClock function)

Critical Gotchas:

• ❌ Instant.now() creates hidden dependency, non-testable
• ❌ mockkStatic(Instant::class) is fragile, can leak between tests
• ✅ Clock.fixed(...) provides deterministic test behavior

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I. Kotlin Style Rules

Project-specific style conventions enforced in code review. These supplement (not replace) docs/STYLE_AND_BEST_PRACTICES.md.

when branches with wrapped right-hand side require curly braces.

When a when branch's expression is too long to fit on the same line as the arrow and is wrapped to the next line, wrap the body in { }. A bare -> followed by an indented expression on its own line is rejected in review.

❌ Wrong — wrapped body without braces:

when (type) {
    VaultItemCipherType.LOGIN -> VaultAddEditState.ViewState.Content.ItemType.Login()
    VaultItemCipherType.BANK_ACCOUNT ->
        VaultAddEditState.ViewState.Content.ItemType.BankAccount()
    VaultItemCipherType.DRIVERS_LICENSE ->
        VaultAddEditState.ViewState.Content.ItemType.DriversLicense()
}

✅ Right — wrapped body with braces:

when (type) {
    VaultItemCipherType.LOGIN -> VaultAddEditState.ViewState.Content.ItemType.Login()
    VaultItemCipherType.BANK_ACCOUNT -> {
        VaultAddEditState.ViewState.Content.ItemType.BankAccount()
    }
    VaultItemCipherType.DRIVERS_LICENSE -> {
        VaultAddEditState.ViewState.Content.ItemType.DriversLicense()
    }
}

Single-line branches (body fits on the same line as ->) do not need braces.

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Bitwarden-Specific Anti-Patterns

General anti-patterns are documented in CLAUDE.md. This section covers violations specific to Bitwarden's State-Action-Event, navigation, and data layer patterns:

❌ NEVER update ViewModel state directly in coroutines

• Post internal actions, update state synchronously in handleAction()

❌ NEVER inject ...Impl classes

• Only inject interfaces via Hilt

❌ NEVER create navigation without @Serializable routes

• No string-based navigation, always type-safe

❌ NEVER use raw Result<T> in repositories

• Use domain-specific sealed classes for better error handling

❌ NEVER make state classes without @Parcelize

• All ViewModel state must survive process death

❌ NEVER skip SavedStateHandle persistence for ViewModels

• Users lose form progress on process death

❌ NEVER forget @IgnoredOnParcel for passwords/tokens

• Causes security vulnerability (sensitive data in parcel)

❌ NEVER use generic Exception catching

• Catch specific exceptions only (RemoteException, IOException)

❌ NEVER call Instant.now() or DateTime.now() directly

• Inject Clock via Hilt, use clock.instant() for testability

❌ NEVER put implementation details in interface KDoc

• Interface KDoc describes the contract; concrete collaborators, call ordering, caching, and fallback behavior belong on the ...Impl override (and only when the why is non-obvious). See section D for examples.

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Quick Reference

For build, test, and codebase discovery commands, use the build-test-verify skill.

File Reference Format: When pointing to specific code, use: file_path:line_number

Example: ui/src/main/kotlin/com/bitwarden/ui/platform/base/BaseViewModel.kt (see handleAction method)