android-jetpack-compose-expert

## Overview

A comprehensive guide for building production-quality Android applications using Jetpack Compose. This skill covers architectural patterns, state management with ViewModels, navigation type-safety, and performance optimization techniques.

## When to Use This Skill

- Use when starting a new Android project with Jetpack Compose.
- Use when migrating legacy XML layouts to Compose.
- Use when implementing complex UI state management and side effects.
- Use when optimizing Compose performance (recomposition counts, stability).
- Use when setting up Navigation with type safety.

## Step-by-Step Guide

### 1. Project Setup & Dependencies

Ensure your `libs.versions.toml` includes the necessary Compose BOM and libraries.

```kotlin
[versions]
composeBom = "2024.02.01"
activityCompose = "1.8.2"

[libraries]
androidx-compose-bom = { group = "androidx.compose", name = "compose-bom", version.ref = "composeBom" }
androidx-ui = { group = "androidx.compose.ui", name = "ui" }
androidx-ui-graphics = { group = "androidx.compose.ui", name = "ui-graphics" }
androidx-ui-tooling-preview = { group = "androidx.compose.ui", name = "ui-tooling-preview" }
androidx-material3 = { group = "androidx.compose.material3", name = "material3" }
androidx-activity-compose = { group = "androidx.activity", name = "activity-compose", version.ref = "activityCompose" }
```

### 2. State Management Pattern (MVI/MVVM)

Use `ViewModel` with `StateFlow` to expose UI state. Avoid exposing `MutableStateFlow`.

```kotlin
// UI State Definition
data class UserUiState(
    val isLoading: Boolean = false,
    val user: User? = null,
    val error: String? = null
)

// ViewModel
class UserViewModel @Inject constructor(
    private val userRepository: UserRepository
) : ViewModel() {

    private val _uiState = MutableStateFlow(UserUiState())
    val uiState: StateFlow<UserUiState> = _uiState.asStateFlow()

    fun loadUser() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true) }
            try {
                val user = userRepository.getUser()
                _uiState.update { it.copy(user = user, isLoading = false) }
            } catch (e: Exception) {
                _uiState.update { it.copy(error = e.message, isLoading = false) }
            }
        }
    }
}
```

### 3. Creating the Screen Composable

Consume the state in a "Screen" composable and pass data down to stateless components.

```kotlin
@Composable
fun UserScreen(
    viewModel: UserViewModel = hiltViewModel()
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()

    UserContent(
        uiState = uiState,
        onRetry = viewModel::loadUser
    )
}

@Composable
fun UserContent(
    uiState: UserUiState,
    onRetry: () -> Unit
) {
    Scaffold { padding ->
        Box(modifier = Modifier.padding(padding)) {
            when {
                uiState.isLoading -> CircularProgressIndicator()
                uiState.error != null -> ErrorView(uiState.error, onRetry)
                uiState.user != null -> UserProfile(uiState.user)
            }
        }
    }
}
```

## Examples

### Example 1: Type-Safe Navigation

Using the new Navigation Compose Type Safety (available in recent versions).

```kotlin
// Define Destinations
@Serializable
object Home

@Serializable
data class Profile(val userId: String)

// Setup NavHost
@Composable
fun AppNavHost(navController: NavHostController) {
    NavHost(navController, startDestination = Home) {
        composable<Home> {
            HomeScreen(onNavigateToProfile = { id ->
                navController.navigate(Profile(userId = id))
            })
        }
        composable<Profile> { backStackEntry ->
            val profile: Profile = backStackEntry.toRoute()
            ProfileScreen(userId = profile.userId)
        }
    }
}
```

## Best Practices

- ✅ **Do:** Use `remember` and `derivedStateOf` to minimize unnecessary calculations during recomposition.
- ✅ **Do:** Mark data classes used in UI state as `@Immutable` or `@Stable` if they contain `List` or other unstable types to enable smart recomposition skipping.
- ✅ **Do:** Use `LaunchedEffect` for one-off side effects (like showing a Snackbar) triggered by state changes.
- ❌ **Don't:** Perform expensive operations (like sorting a list) directly inside the Composable function body without `remember`.
- ❌ **Don't:** Pass `ViewModel` instances down to child components. Pass only the data (state) and lambda callbacks (events).

## Troubleshooting

**Problem:** Infinite Recomposition loop.
**Solution:** Check if you are creating new object instances (like `List` or `Modifier`) inside the composition without `remember`, or if you are updating state inside the composition phase instead of a side-effect or callback. Use Layout Inspector to debug recomposition counts.