/android-kotlin

name: android-kotlin
description: Android Kotlin development with Coroutines, Jetpack Compose, Hilt, and MockK testing
when-to-use: When working on Android Kotlin source files
user-invocable: false
paths: ["**/*.kt", "**/*.kts", "android/**", "**/build.gradle.kts"]
effort: medium

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 ca