Mobile App Builder

// Modern SwiftUI component with performance optimization
import SwiftUI
import Combine

struct ProductListView: View {
    @StateObject private var viewModel = ProductListViewModel()
    @State private var searchText = ""
    
    var body: some View {
        NavigationView {
            List(viewModel.filteredProducts) { product in
                ProductRowView(product: product)
                    .onAppear {
                        // Pagination trigger
                        if product == viewModel.filteredProducts.last {
                            viewModel.loadMoreProducts()
                        }
                    }
            }
            .searchable(text: $searchText)
            .onChange(of: searchText) { _ in
                viewModel.filterProducts(searchText)
            }
            .refreshable {
                await viewModel.refreshProducts()
            }
            .navigationTitle("Products")
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    Button("Filter") {
                        viewModel.showFilterSheet = true
                    }
                }
            }
            .sheet(isPresented: $viewModel.showFilterSheet) {
                FilterView(filters: $viewModel.filters)
            }
        }
        .task {
            await viewModel.loadInitialProducts()
        }
    }
}

// MVVM Pattern Implementation
@MainActor
class ProductListViewModel: ObservableObject {
    @Published var products: [Product] = []
    @Published var filteredProducts: [Product] = []
    @Published var isLoading = false
    @Published var showFilterSheet = false
    @Published var filters = ProductFilters()
    
    private let productService = ProductService()
    private var cancellables = Set<AnyCancellable>()
    
    func loadInitialProducts() async {
        isLoading = true
        defer { isLoading = false }
        
        do {
            products = try await productService.fetchProducts()
            filteredProducts = products
        } catch {
            // Handle error with user feedback
            print("Error loading products: \(error)")
        }
    }
    
    func filterProducts(_ searchText: String) {
        if searchText.isEmpty {
            filteredProducts = products
        } else {
            filteredProducts = products.filter { product in
                product.name.localizedCaseInsensitiveContains(searchText)
            }
        }
    }
}

// Modern Jetpack Compose component with state management
@Composable
fun ProductListScreen(
    viewModel: ProductListViewModel = hiltViewModel()
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()
    val searchQuery by viewModel.searchQuery.collectAsStateWithLifecycle()
    
    Column {
        SearchBar(
            query = searchQuery,
            onQueryChange = viewModel::updateSearchQuery,
            onSearch = viewModel::search,
            modifier = Modifier.fillMaxWidth()
        )
        
        LazyColumn(
            modifier = Modifier.fillMaxSize(),
            contentPadding = PaddingValues(16.dp),
            verticalArrangement = Arrangement.spacedBy(8.dp)
        ) {
            items(
                items = uiState.products,
                key = { it.id }
            ) { product ->
                ProductCard(
                    product = product,
                    onClick = { viewModel.selectProduct(product) },
                    modifier = Modifier
                        .fillMaxWidth()
                        .animateItemPlacement()
                )
            }
            
            if (uiState.isLoading) {
                item {
                    Box(
                        modifier = Modifier.fillMaxWidth(),
                        contentAlignment = Alignment.Center
                    ) {
                        CircularProgressIndicator()
                    }
                }
            }
        }
    }
}

// ViewModel with proper lifecycle management
@HiltViewModel
class ProductListViewModel @Inject constructor(
    private val productRepository: ProductRepository
) : ViewModel() {
    
    private val _uiState = MutableStateFlow(ProductListUiState())
    val uiState: StateFlow<ProductListUiState> = _uiState.asStateFlow()
    
    private val _searchQuery = MutableStateFlow("")
    val searchQuery: StateFlow<String> = _searchQuery.asStateFlow()
    
    init {
        loadProducts()
        observeSearchQuery()
    }
    
    private fun loadProducts() {
        viewModelScope.launch {
            _uiState.update { it.copy(isLoading = true) }
            
            try {
                val products = productRepository.getProducts()
                _uiState.update { 
                    it.copy(
                        products = products,
                        isLoading = false
                    ) 
                }
            } catch (exception: Exception) {
                _uiState.update { 
                    it.copy(
                        isLoading = false,
                        errorMessage = exception.message
                    ) 
                }
            }
        }
    }
    
    fun updateSearchQuery(query: String) {
        _searchQuery.value = query
    }
    
    private fun observeSearchQuery() {
        searchQuery
            .debounce(300)
            .onEach { query ->
                filterProducts(query)
            }
            .launchIn(viewModelScope)
    }
}

// React Native component with platform-specific optimizations
import React, { useMemo, useCallback } from 'react';
import {
  FlatList,
  StyleSheet,
  Platform,
  RefreshControl,
} from 'react-native';
import { useSafeAreaInsets } from 'react-native-safe-area-context';
import { useInfiniteQuery } from '@tanstack/react-query';

interface ProductListProps {
  onProductSelect: (product: Product) => void;
}

export const ProductList: React.FC<ProductListProps> = ({ onProductSelect }) => {
  const insets = useSafeAreaInsets();
  
  const {
    data,
    fetchNextPage,
    hasNextPage,
    isLoading,
    isFetchingNextPage,
    refetch,
    isRefetching,
  } = useInfiniteQuery({
    queryKey: ['products'],
    queryFn: ({ pageParam = 0 }) => fetchProducts(pageParam),
    getNextPageParam: (lastPage, pages) => lastPage.nextPage,
  });

  const products = useMemo(
    () => data?.pages.flatMap(page => page.products) ?? [],
    [data]
  );

  const renderItem = useCallback(({ item }: { item: Product }) => (
    <ProductCard
      product={item}
      onPress={() => onProductSelect(item)}
      style={styles.productCard}
    />
  ), [onProductSelect]);

  const handleEndReached = useCallback(() => {
    if (hasNextPage && !isFetchingNextPage) {
      fetchNextPage();
    }
  }, [hasNextPage, isFetchingNextPage, fetchNextPage]);

  const keyExtractor = useCallback((item: Product) => item.id, []);

  return (
    <FlatList
      data={products}
      renderItem={renderItem}
      keyExtractor={keyExtractor}
      onEndReached={handleEndReached}
      onEndReachedThreshold={0.5}
      refreshControl={
        <RefreshControl
          refreshing={isRefetching}
          onRefresh={refetch}
          colors={['#007AFF']} // iOS-style color
          tintColor="#007AFF"
        />
      }
      contentContainerStyle={[
        styles.container,
        { paddingBottom: insets.bottom }
      ]}
      showsVerticalScrollIndicator={false}
      removeClippedSubviews={Platform.OS === 'android'}
      maxToRenderPerBatch={10}
      updateCellsBatchingPeriod={50}
      windowSize={21}
    />
  );
};

const styles = StyleSheet.create({
  container: {
    padding: 16,
  },
  productCard: {
    marginBottom: 12,
    ...Platform.select({
      ios: {
        shadowColor: '#000',
        shadowOffset: { width: 0, height: 2 },
        shadowOpacity: 0.1,
        shadowRadius: 4,
      },
      android: {
        elevation: 3,
      },
    }),
  },
});