Android Kotlin Skill
Project Structure
project/
├── app/
│ ├── src/
│ │ ├── main/
│ │ │ ├── kotlin/com/example/app/
│ │ │ │ ├── data/ # Data layer
│ │ │ │ │ ├── local/ # Room database
│ │ │ │ │ ├── remote/ # Retrofit/Ktor services
│ │ │ │ │ └── repository/ # Repository implementations
│ │ │ │ ├── di/ # Hilt modules
│ │ │ │ ├── domain/ # Business logic
│ │ │ │ │ ├── model/ # Domain models
│ │ │ │ │ ├── repository/ # Repository interfaces
│ │ │ │ │ └── usecase/ # Use cases
│ │ │ │ ├── ui/ # Presentation layer
│ │ │ │ │ ├── feature/ # Feature screens
│ │ │ │ │ │ ├── FeatureScreen.kt # Compose UI
│ │ │ │ │ │ └── FeatureViewModel.kt
│ │ │ │ │ ├── components/ # Reusable Compose components
│ │ │ │ │ └── theme/ # Material theme
│ │ │ │ └── App.kt # Application class
│ │ │ ├── res/
│ │ │ └── AndroidManifest.xml
│ │ ├── test/ # Unit tests
│ │ └── androidTest/ # Instrumentation tests
│ └── build.gradle.kts
├── build.gradle.kts # Project-level build file
├── gradle.properties
├── settings.gradle.kts
└── CLAUDE.md
Gradle Configuration (Kotlin DSL)
App-level build.gradle.kts
plugins {
id("com.android.application")
id("org.jetbrains.kotlin.android")
id("com.google.dagger.hilt.android")
id("com.google.devtools.ksp")
}
android {
namespace = "com.example.app"
compileSdk = 34
defaultConfig {
applicationId = "com.example.app"
minSdk = 24
targetSdk = 34
versionCode = 1
versionName = "1.0"
testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
}
buildTypes {
release {
isMinifyEnabled = true
proguardFiles(
getDefaultProguardFile("proguard-android-optimize.txt"),
"proguard-rules.pro"
)
}
}
compileOptions {
sourceCompatibility = JavaVersion.VERSION_17
targetCompatibility = JavaVersion.VERSION_17
}
kotlinOptions {
jvmTarget = "17"
}
buildFeatures {
compose = true
}
composeOptions {
kotlinCompilerExtensionVersion = "1.5.8"
}
}
dependencies {
// Compose BOM
val composeBom = platform("androidx.compose:compose-bom:2024.01.00")
implementation(composeBom)
implementation("androidx.compose.ui:ui")
implementation("androidx.compose.ui:ui-tooling-preview")
implementation("androidx.compose.material3:material3")
implementation("androidx.activity:activity-compose:1.8.2")
implementation("androidx.lifecycle:lifecycle-viewmodel-compose:2.7.0")
// Coroutines
implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.7.3")
// Hilt
implementation("com.google.dagger:hilt-android:2.50")
ksp("com.google.dagger:hilt-compiler:2.50")
implementation("androidx.hilt:hilt-navigation-compose:1.1.0")
// Room
implementation("androidx.room:room-runtime:2.6.1")
implementation("androidx.room:room-ktx:2.6.1")
ksp("androidx.room:room-compiler:2.6.1")
// Testing
testImplementation("junit:junit:4.13.2")
testImplementation("io.mockk:mockk:1.13.9")
testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.7.3")
testImplementation("app.cash.turbine:turbine:1.0.0")
androidTestImplementation("androidx.test.ext:junit:1.1.5")
androidTestImplementation("androidx.compose.ui:ui-test-junit4")
debugImplementation("androidx.compose.ui:ui-tooling")
debugImplementation("androidx.compose.ui:ui-test-manifest")
}
Kotlin Coroutines & Flow
ViewModel with StateFlow
@HiltViewModel
class UserViewModel @Inject constructor(
private val getUserUseCase: GetUserUseCase,
private val savedStateHandle: SavedStateHandle
) : ViewModel() {
private val _uiState = MutableStateFlow(UserUiState())
val uiState: StateFlow<UserUiState> = _uiState.asStateFlow()
private val userId: String = checkNotNull(savedStateHandle["userId"])
init {
loadUser()
}
fun loadUser() {
viewModelScope.launch {
_uiState.update { it.copy(isLoading = true) }
getUserUseCase(userId)
.catch { e ->
_uiState.update {
it.copy(isLoading = false, error = e.message)
}
}
.collect { user ->
_uiState.update {
it.copy(isLoading = false, user = user, error = null)
}
}
}
}
fun clearError() {
_uiState.update { it.copy(error = null) }
}
}
data class UserUiState(
val user: User? = null,
val isLoading: Boolean = false,
val error: String? = null
)
Repository with Flow
interface UserRepository {
fun getUser(userId: String): Flow<User>
fun observeUsers(): Flow<List<User>>
suspend fun saveUser(user: User)
}
class UserRepositoryImpl @Inject constructor(
private val api: UserApi,
private val dao: UserDao,
private val dispatcher: CoroutineDispatcher = Dispatchers.IO
) : UserRepository {
override fun getUser(userId: String): Flow<User> = flow {
// Emit cached data first
dao.getUserById(userId)?.let { emit(it) }
// Fetch from network and update cache
val remoteUser = api.getUser(userId)
dao.insert(remoteUser)
emit(remoteUser)
}.flowOn(dispatcher)
override fun observeUsers(): Flow<List<User>> =
dao.observeAllUsers().flowOn(dispatcher)
override suspend fun saveUser(user: User) = withContext(dispatcher) {
api.saveUser(user)
dao.insert(user)
}
}
Jetpack Compose
Screen with ViewModel
@Composable
fun UserScreen(
viewModel: UserViewModel = hiltViewModel(),
onNavigateBack: () -> Unit
) {
val uiState by viewModel.uiState.collectAsStateWithLifecycle()
UserScreenContent(
uiState = uiState,
)
}
@Composable
private fun UserScreenContent(
uiState: UserUiState,
onRefresh: () -> Unit,
onErrorDismiss: () -> Unit,
onNavigateBack: () -> Unit
) {
Scaffold(
topBar = {
TopAppBar(
title = { Text("User Profile") },
navigationIcon = {
IconButton(onClick = onNavigateBack) {
Icon(Icons.AutoMirrored.Filled.ArrowBack, "Back")
}
}
)
}
) { padding ->
Box(
modifier = Modifier
.fillMaxSize()
.padding(padding)
) {
when {
uiState.isLoading -> {
CircularProgressIndicator(
modifier = Modifier.align(Alignment.Center)
)
}
uiState.user != null -> {
UserContent(user = uiState.user)
}
}
uiState.error?.let { error ->
Snackbar(
modifier = Modifier.align(Alignment.BottomCenter),
action = {
TextButton(onClick = onErrorDismiss) {
Text("Dismiss")
}
}
) {
Text(error)
}
}
}
}
}
Sealed Classes for State
Result Wrapper
sealed interface Result<out T> {
data class Success<T>(val data: T) : Result<T>
data class Error(val exception: Throwable) : Result<Nothing>
data object Loading : Result<Nothing>
}
fun <T> Result<T>.getOrNull(): T? = (this as? Result.Success)?.data
inline fun <T, R> Result<T>.map(transform: (T) -> R): Result<R> = when (this) {
is Result.Success -> Result.Success(transform(data))
is Result.Error -> this
is Result.Loading -> this
}
Testing with MockK & Turbine
ViewModel Tests
@OptIn(ExperimentalCoroutinesApi::class)
class UserViewModelTest {
@get:Rule
val mainDispatcherRule = MainDispatcherRule()
private val getUserUseCase: GetUserUseCase = mockk()
private val savedStateHandle = SavedStateHandle(mapOf("userId" to "123"))
private lateinit var viewModel: UserViewModel
@Before
fun setup() {
viewModel = UserViewModel(getUserUseCase, savedStateHandle)
}
@Test
fun `loadUser success updates state with user`() = runTest {
val user = User("123", "John Doe", "john@example.com")
coEvery { getUserUseCase("123") } returns flowOf(user)
viewModel.uiState.test {
val initial = awaitItem()
assertFalse(initial.isLoading)
viewModel.loadUser()
val loading = awaitItem()
assertTrue(loading.isLoading)
val success = awaitItem()
assertFalse(success.isLoading)
assertEquals(user, success.user)
}
}
}
class MainDispatcherRule(
private val dispatcher: TestDispatcher = UnconfinedTestDispatcher()
) : TestWatcher() {
override fun starting(description: Description) {
Dispatchers.setMain(dispatcher)
}
override fun finished(description: Description) {
Dispatchers.resetMain()
}
}
GitHub Actions
name: Android Kotlin CI
on:
push:
branches: [main]
pull_request:
branches: [main]
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Set up JDK 17
uses: actions/setup-java@v4
with:
java-version: '17'
distribution: 'temurin'
- name: Setup Gradle
uses: gradle/actions/setup-gradle@v3
- name: Run Detekt
run: ./gradlew detekt
- name: Run Ktlint
run: ./gradlew ktlintCheck
- name: Run Unit Tests
run: ./gradlew testDebugUnitTest
- name: Build Debug APK
run: ./gradlew assembleDebug
Lint Configuration
detekt.yml
build:
maxIssues: 0
complexity:
LongMethod:
threshold: 20
LongParameterList:
functionThreshold: 4
TooManyFunctions:
thresholdInFiles: 10
style:
MaxLineLength:
maxLineLength: 120
WildcardImport:
active: true
coroutines:
GlobalCoroutineUsage:
active: true
Kotlin Anti-Patterns
- ❌ Blocking coroutines on Main - Never use
runBlockingon main thread - ❌ GlobalScope usage - Use structured concurrency with viewModelScope/lifecycleScope
- ❌ Collecting flows in init - Use
repeatOnLifecycleorcollectAsStateWithLifecycle - ❌ Mutable state exposure - Expose
StateFlownotMutableStateFlow - ❌ Not handling exceptions in flows - Always use
catchoperator - ❌ Lateinit for nullable - Use
lazyor nullable with? - ❌ Hardcoded dispatchers - Inject dispatchers for testability
- ❌ Not using sealed classes - Prefer sealed for finite state sets
- ❌ Side effects in Composables - Use
LaunchedEffect/SideEffect - ❌ Unstable Compose parameters - Use stable/immutable types or
@Stable
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