jetpack-compose

Jetpack Compose - Modern Android UI

Purpose: Build native Android UIs with declarative Jetpack Compose framework

Agent: Google Android Lead Use When: Building Android UI, migrating from XML layouts, or creating custom composables

---

Compose Basics

@Composable
fun Greeting(name: String) {
    Text(text = "Hello, $name!")
}

// Preview
@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    Greeting("Android")
}

// Column/Row/Box (like flexbox)
@Composable
fun UserCard(user: User) {
    Card(
        modifier = Modifier
            .fillMaxWidth()
            .padding(16.dp)
    ) {
        Row(
            modifier = Modifier.padding(16.dp),
            verticalAlignment = Alignment.CenterVertically
        ) {
            Image(
                painter = painterResource(R.drawable.avatar),
                contentDescription = null,
                modifier = Modifier.size(48.dp)
            )
            Spacer(modifier = Modifier.width(16.dp))
            Column {
                Text(text = user.name, style = MaterialTheme.typography.titleMedium)
                Text(text = user.email, style = MaterialTheme.typography.bodySmall)
            }
        }
    }
}

---

State Management

// remember state
@Composable
fun Counter() {
    var count by remember { mutableStateOf(0) }

    Column {
        Text("Count: $count")
        Button(onClick = { count++ }) {
            Text("Increment")
        }
    }
}

// ViewModel state
@Composable
fun UserListScreen(viewModel: UserViewModel = viewModel()) {
    val users by viewModel.users.collectAsState()
    val isLoading by viewModel.isLoading.collectAsState()

    if (isLoading) {
        CircularProgressIndicator()
    } else {
        LazyColumn {
            items(users) { user ->
                UserItem(user = user)
            }
        }
    }
}

// State hoisting
@Composable
fun SearchBar(
    query: String,
    onQueryChange: (String) -> Unit
) {
    TextField(
        value = query,
        onValueChange = onQueryChange,
        label = { Text("Search") }
    )
}

@Composable
fun SearchScreen() {
    var query by remember { mutableStateOf("") }

    Column {
        SearchBar(query = query, onQueryChange = { query = it })
        SearchResults(query = query)
    }
}

---

Lists

@Composable
fun UserList(users: List<User>) {
    LazyColumn {
        items(users) { user ->
            UserItem(user = user)
        }

        // Or with key
        items(users, key = { it.id }) { user ->
            UserItem(user = user)
        }
    }
}

// Grid
@Composable
fun PhotoGrid(photos: List<Photo>) {
    LazyVerticalGrid(
        columns = GridCells.Fixed(3),
        contentPadding = PaddingValues(8.dp)
    ) {
        items(photos) { photo ->
            PhotoItem(photo = photo)
        }
    }
}

---

Navigation

@Composable
fun AppNavigation() {
    val navController = rememberNavController()

    NavHost(navController = navController, startDestination = "home") {
        composable("home") {
            HomeScreen(
                onNavigateToDetail = { userId ->
                    navController.navigate("detail/$userId")
                }
            )
        }

        composable(
            route = "detail/{userId}",
            arguments = listOf(navArgument("userId") { type = NavType.IntType })
        ) { backStackEntry ->
            val userId = backStackEntry.arguments?.getInt("userId")
            DetailScreen(userId = userId)
        }
    }
}

---

Side Effects

@Composable
fun UserScreen(userId: Int, viewModel: UserViewModel = viewModel()) {
    // LaunchedEffect - run on composition
    LaunchedEffect(userId) {
        viewModel.loadUser(userId)
    }

    // DisposableEffect - cleanup
    DisposableEffect(Unit) {
        val listener = viewModel.addListener()
        onDispose {
            listener.remove()
        }
    }

    // rememberCoroutineScope - launch coroutines
    val scope = rememberCoroutineScope()

    Button(onClick = {
        scope.launch {
            viewModel.refreshData()
        }
    }) {
        Text("Refresh")
    }
}

---

Theming

@Composable
fun MyAppTheme(
    darkTheme: Boolean = isSystemInDarkTheme(),
    content: @Composable () -> Unit
) {
    val colors = if (darkTheme) {
        darkColorScheme()
    } else {
        lightColorScheme()
    }

    MaterialTheme(
        colorScheme = colors,
        typography = Typography,
        content = content
    )
}

// Usage
@Composable
fun App() {
    MyAppTheme {
        Surface(
            modifier = Modifier.fillMaxSize(),
            color = MaterialTheme.colorScheme.background
        ) {
            MainScreen()
        }
    }
}

---

Best Practices

• Composables should be pure functions
• Hoist state when shared between composables
• Use keys in lists for recomposition optimization
• Avoid side effects in composition (use LaunchedEffect)
• Remember expensive computations with remember
• Use derivedStateOf for computed state
• Keep composables small and focused
• Preview composables for fast iteration

---

Remember: Compose is declarative. UI = f(state). No XML, no findViewById.

Created: 2026-02-04 Maintained By: Google Android Lead