Compose Multiplatform Patterns

data class ItemListState(    val items: List<Item> = emptyList(),    val isLoading: Boolean = false,    val error: String? = null,    val searchQuery: String = "")
class ItemListViewModel(    private val getItems: GetItemsUseCase) : ViewModel() {    private val _state = MutableStateFlow(ItemListState())    val state: StateFlow<ItemListState> = _state.asStateFlow()
    fun onSearch(query: String) {        _state.update { it.copy(searchQuery = query) }        loadItems(query)    }
    private fun loadItems(query: String) {        viewModelScope.launch {            _state.update { it.copy(isLoading = true) }            getItems(query).fold(                onSuccess = { items -> _state.update { it.copy(items = items, isLoading = false) } },                onFailure = { e -> _state.update { it.copy(error = e.message, isLoading = false) } }            )        }    }}

@Composablefun ItemListScreen(viewModel: ItemListViewModel = koinViewModel()) {    val state by viewModel.state.collectAsStateWithLifecycle()
    ItemListContent(        state = state,        onSearch = viewModel::onSearch    )}
@Composableprivate fun ItemListContent(    state: ItemListState,    onSearch: (String) -> Unit) {    // ステートレスなコンポーザブル — プレビューとテストが容易}

sealed interface ItemListEvent {    data class Search(val query: String) : ItemListEvent    data class Delete(val itemId: String) : ItemListEvent    data object Refresh : ItemListEvent}
// ViewModelの中fun onEvent(event: ItemListEvent) {    when (event) {        is ItemListEvent.Search -> onSearch(event.query)        is ItemListEvent.Delete -> deleteItem(event.itemId)        is ItemListEvent.Refresh -> loadItems(_state.value.searchQuery)    }}
// コンポーザブルの中 — 多数ではなく単一ラムダItemListContent(    state = state,    onEvent = viewModel::onEvent)

@Serializable data object HomeRoute@Serializable data class DetailRoute(val id: String)@Serializable data object SettingsRoute
@Composablefun AppNavHost(navController: NavHostController = rememberNavController()) {    NavHost(navController, startDestination = HomeRoute) {        composable<HomeRoute> {            HomeScreen(onNavigateToDetail = { id -> navController.navigate(DetailRoute(id)) })        }        composable<DetailRoute> { backStackEntry ->            val route = backStackEntry.toRoute<DetailRoute>()            DetailScreen(id = route.id)        }        composable<SettingsRoute> { SettingsScreen() }    }}

NavHost(navController, startDestination = HomeRoute) {    composable<HomeRoute> { /* ... */ }    dialog<ConfirmDeleteRoute> { backStackEntry ->        val route = backStackEntry.toRoute<ConfirmDeleteRoute>()        ConfirmDeleteDialog(            itemId = route.itemId,            onConfirm = { navController.popBackStack() },            onDismiss = { navController.popBackStack() }        )    }}

@Composablefun AppCard(    modifier: Modifier = Modifier,    header: @Composable () -> Unit = {},    content: @Composable ColumnScope.() -> Unit,    actions: @Composable RowScope.() -> Unit = {}) {    Card(modifier = modifier) {        Column {            header()            Column(content = content)            Row(horizontalArrangement = Arrangement.End, content = actions)        }    }}

Text(    text = "Hello",    modifier = Modifier        .padding(16.dp)          // 1. レイアウト（パディング、サイズ）        .clip(RoundedCornerShape(8.dp))  // 2. 形状        .background(Color.White) // 3. 描画（背景、ボーダー）        .clickable { }           // 4. インタラクション)

// commonMain@Composableexpect fun PlatformStatusBar(darkIcons: Boolean)
// androidMain@Composableactual fun PlatformStatusBar(darkIcons: Boolean) {    val systemUiController = rememberSystemUiController()    SideEffect { systemUiController.setStatusBarColor(Color.Transparent, darkIcons) }}
// iosMain@Composableactual fun PlatformStatusBar(darkIcons: Boolean) {    // iOSはUIKitインターロップまたはInfo.plistで処理}

@Immutabledata class ItemUiModel(    val id: String,    val title: String,    val description: String,    val progress: Float)

LazyColumn {    items(        items = items,        key = { it.id }  // 安定したキーによりアイテムの再利用とアニメーションが可能    ) { item ->        ItemRow(item = item)    }}

val listState = rememberLazyListState()val showScrollToTop by remember {    derivedStateOf { listState.firstVisibleItemIndex > 5 }}

// 悪い例 — リコンポジションのたびに新しいラムダとリストが作られるitems.filter { it.isActive }.forEach { ActiveItem(it, onClick = { handle(it) }) }
// 良い例 — 各アイテムにキーを付けてコールバックが正しい行に紐づくようにするval activeItems = remember(items) { items.filter { it.isActive } }activeItems.forEach { item ->    key(item.id) {        ActiveItem(item, onClick = { handle(item) })    }}

@Composablefun AppTheme(    darkTheme: Boolean = isSystemInDarkTheme(),    dynamicColor: Boolean = true,    content: @Composable () -> Unit) {    val colorScheme = when {        dynamicColor && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S -> {            if (darkTheme) dynamicDarkColorScheme(LocalContext.current)            else dynamicLightColorScheme(LocalContext.current)        }        darkTheme -> darkColorScheme()        else -> lightColorScheme()    }
    MaterialTheme(colorScheme = colorScheme, content = content)}