Kotlin Android

Building modern Android apps with Kotlin and Jetpack Compose.

Quick Start

# Android Studio -> New Project -> Empty Compose Activity
./gradlew assembleDebug

When to Use

• Native Android apps
• Modern UI with Jetpack Compose
• Teams using Kotlin coroutines
• Apps targeting Play Store

Best Practices

UI with Compose

• Build declarative UI with composables
• Use state hoisting; keep composables stateless
• Prefer `remember`/`derivedStateOf` for state
• Use Material 3 components

Architecture

• Use MVVM with ViewModel + StateFlow
• Repository pattern for data sources
• Keep UI free of business logic
• Use `collectAsStateWithLifecycle`

Concurrency

• Use coroutines for async work
• Scope with viewModelScope/lifecycleScope
• Use Flow for streams
• Avoid blocking main thread

Navigation & DI

• Use Compose Navigation
• Use Hilt for dependency injection
• Define nav graphs per feature
• Scope dependencies correctly

Dependencies

# build.gradle.kts
implementation("androidx.compose.ui:ui")
implementation("androidx.lifecycle:lifecycle-viewmodel-compose")
implementation("com.google.dagger:hilt-android")

Examples

// Composable UI
@Composable
fun CounterScreen(viewModel: CounterViewModel = viewModel()) {
    val count by viewModel.count.collectAsStateWithLifecycle()
    Column(
        horizontalAlignment = Alignment.CenterHorizontally,
        modifier = Modifier.padding(24.dp)
    ) {
        Text(text = "Count: $count", style = MaterialTheme.typography.headlineMedium)
        Button(onClick = { viewModel.increment() }) { Text("Add") }
    }
}

// ViewModel with StateFlow
class CounterViewModel : ViewModel() {
    private val _count = MutableStateFlow(0)
    val count: StateFlow<Int> = _count.asStateFlow()
    fun increment() { _count.update { it + 1 } }
}

// Coroutine + repository
class UserRepository(private val api: Api) {
    suspend fun loadUsers(): List<User> = withContext(Dispatchers.IO) {
        api.fetchUsers()
    }
}

// Navigation
@Composable
fun App() {
    val nav = rememberNavController()
    NavHost(navController = nav, startDestination = "home") {
        composable("home") { HomeScreen(onOpen = { nav.navigate("detail") }) }
        composable("detail") { DetailScreen() }
    }
}

Step-by-Step

1. Create a Compose project.

2. Set up Hilt and navigation.

3. Build the UI with composables and Material 3.

4. Add ViewModels with StateFlow.

5. Wire repositories with coroutines.

6. Handle loading/error states.

7. Test with Compose UI tests.

8. Build and publish.

Validation

1. `./gradlew build` passes

2. UI renders and reacts to state

3. Coroutines are scoped and cancelled

4. Navigation works across screens

5. Unit/UI tests pass

Troubleshooting

• Main thread block: move work to Dispatchers.IO.
• State not updating: use StateFlow + collectAsStateWithLifecycle.
• Memory leaks: scope coroutines to viewModelScope.