Practical Mobile App Development Interview Questions

sealed class Result<T> {} class Success<T> extends Result<T> { Success(this.data); final T data; } class Failure<T> extends Result<T> { Failure(this.message); final String message; } Widget view(Result<List<String>> r) => switch (r) { Success(:final data) => ListView(children: [for (final s in data) Text(s)]), Failure(:final message) => Center(child: Text(message)), };

class UsersPage extends StatefulWidget { const UsersPage({super.key}); @override State<UsersPage> createState() => _UsersPageState(); } class _UsersPageState extends State<UsersPage> { late final Future<List<User>> _future = api.fetchUsers(); @override Widget build(BuildContext context) => FutureBuilder<List<User>>( future: _future, builder: (context, snap) { if (snap.connectionState != ConnectionState.done) { return const Center(child: CircularProgressIndicator()); } if (snap.hasError) return const Center(child: Text('Something went wrong')); final users = snap.data!; return ListView.builder( itemCount: users.length, itemBuilder: (_, i) => ListTile(title: Text(users[i].name)), ); }, ); }

class Counter extends StatefulWidget { const Counter({super.key}); @override State<Counter> createState() => _CounterState(); } class _CounterState extends State<Counter> { int _count = 0; @override Widget build(BuildContext context) => Column( mainAxisSize: MainAxisSize.min, children: [ Text('Count: $_count'), FilledButton( onPressed: () => setState(() => _count++), child: const Text('Add'), ), ], ); }

class Product { const Product({required this.id, required this.name, required this.price}); final String id; final String name; final double price; factory Product.fromJson(Map<String, dynamic> json) => Product( id: json['id'] as String, name: json['name'] as String, price: (json['price'] as num).toDouble(), ); Map<String, dynamic> toJson() => {'id': id, 'name': name, 'price': price}; } // For larger apps use json_serializable or freezed to generate this.

sealed class ProductState {} class ProductLoading extends ProductState {} class ProductLoaded extends ProductState { ProductLoaded(this.items); final List<Product> items; } class ProductError extends ProductState { ProductError(this.message); final String message; } class ProductCubit extends Cubit<ProductState> { ProductCubit(this._repo) : super(ProductLoading()); final ProductRepository _repo; Future<void> load() async { emit(ProductLoading()); try { emit(ProductLoaded(await _repo.fetchProducts())); } catch (_) { emit(ProductError('Could not load products')); } } }

final productsProvider = FutureProvider.autoDispose<List<Product>>((ref) { final repo = ref.watch(productRepositoryProvider); return repo.fetchProducts(); }); class ProductsView extends ConsumerWidget { const ProductsView({super.key}); @override Widget build(BuildContext context, WidgetRef ref) { final products = ref.watch(productsProvider); return products.when( loading: () => const Center(child: CircularProgressIndicator()), error: (e, _) => const Center(child: Text('Failed to load')), data: (items) => ListView(children: [for (final p in items) ListTile(title: Text(p.name))]), ); } }

final router = GoRouter( initialLocation: '/', refreshListenable: authNotifier, redirect: (context, state) { final loggedIn = authNotifier.isLoggedIn; final goingToLogin = state.matchedLocation == '/login'; if (!loggedIn && !goingToLogin) return '/login'; if (loggedIn && goingToLogin) return '/'; return null; }, routes: [ GoRoute(path: '/', builder: (_, __) => const HomePage()), GoRoute(path: '/login', builder: (_, __) => const LoginPage()), GoRoute( path: '/product/:id', builder: (_, state) => ProductPage(id: state.pathParameters['id']!), ), ], );

function ProductList({ items, loading, onRefresh, onEndReached }) { const renderItem = useCallback(({ item }) => <ProductRow product={item} />, []); return ( <FlatList data={items} keyExtractor={(item) => item.id} renderItem={renderItem} refreshing={loading} onRefresh={onRefresh} onEndReached={onEndReached} onEndReachedThreshold={0.5} ListEmptyComponent={!loading ? <Text>No products</Text> : null} /> ); }

function useFetch(url) { const [state, setState] = useState({ data: null, loading: true, error: null }); useEffect(() => { const controller = new AbortController(); fetch(url, { signal: controller.signal }) .then((r) => { if (!r.ok) throw new Error('HTTP ' + r.status); return r.json(); }) .then((data) => setState({ data, loading: false, error: null })) .catch((e) => { if (e.name !== 'AbortError') setState({ data: null, loading: false, error: e.message }); }); return () => controller.abort(); }, [url]); return state; }

import * as Location from 'expo-location'; export function MyLocation() { const [coords, setCoords] = useState(null); const [error, setError] = useState(null); useEffect(() => { (async () => { const { status } = await Location.requestForegroundPermissionsAsync(); if (status !== 'granted') { setError('Location permission denied'); return; } const pos = await Location.getCurrentPositionAsync({}); setCoords(pos.coords); })(); }, []); if (error) return <Text>{error}</Text>; if (!coords) return <ActivityIndicator />; return <Text>{coords.latitude}, {coords.longitude}</Text>; }

const Tab = createBottomTabNavigator(); const Stack = createNativeStackNavigator(); function HomeTabs() { return ( <Tab.Navigator> <Tab.Screen name="Feed" component={FeedScreen} /> <Tab.Screen name="Profile" component={ProfileScreen} /> </Tab.Navigator> ); } export default function App() { return ( <NavigationContainer> <Stack.Navigator> <Stack.Screen name="Home" component={HomeTabs} options={{ headerShown: false }} /> <Stack.Screen name="Details" component={DetailsScreen} /> </Stack.Navigator> </NavigationContainer> ); }

const cartSlice = createSlice({ name: 'cart', initialState: { items: [] }, reducers: { addItem(state, action) { const existing = state.items.find((i) => i.id === action.payload.id); if (existing) existing.qty += 1; else state.items.push({ ...action.payload, qty: 1 }); }, removeItem(state, action) { state.items = state.items.filter((i) => i.id !== action.payload); }, }, }); export const { addItem, removeItem } = cartSlice.actions; export const selectTotal = (s) => s.cart.items.reduce((t, i) => t + i.price * i.qty, 0);

final dio = Dio(BaseOptions( baseUrl: 'https://api.example.com', connectTimeout: const Duration(seconds: 10), receiveTimeout: const Duration(seconds: 15), )); dio.interceptors.add(InterceptorsWrapper( onRequest: (options, handler) async { final token = await secureStorage.read(key: 'access_token'); if (token != null) options.headers['Authorization'] = 'Bearer $token'; handler.next(options); }, onError: (e, handler) { if (e.response?.statusCode == 401) authNotifier.logout(); handler.next(e); }, ));

import * as FileSystem from 'expo-file-system'; export async function uploadImage(uri, token, onProgress) { const task = FileSystem.createUploadTask( 'https://api.example.com/upload', uri, { httpMethod: 'POST', uploadType: FileSystem.FileSystemUploadType.MULTIPART, fieldName: 'file', headers: { Authorization: `Bearer ${token}` }, }, ({ totalBytesSent, totalBytesExpectedToSend }) => onProgress(totalBytesSent / totalBytesExpectedToSend) ); const res = await task.uploadAsync(); if (res.status !== 200) throw new Error('Upload failed'); return JSON.parse(res.body); }

testWidgets('increments counter on tap', (tester) async { await tester.pumpWidget(const MaterialApp(home: Scaffold(body: Counter()))); expect(find.text('Count: 0'), findsOneWidget); await tester.tap(find.text('Add')); await tester.pump(); expect(find.text('Count: 1'), findsOneWidget); });

import { render, screen, fireEvent } from '@testing-library/react-native'; test('shows error when email is empty', () => { render(<LoginScreen onSubmit={jest.fn()} />); fireEvent.press(screen.getByRole('button', { name: 'Login' })); expect(screen.getByText('Email is required')).toBeTruthy(); });

name: flutter-ci on: pull_request: branches: [main] jobs: test: runs-on: ubuntu-latest steps: - uses: actions/checkout@v4 - uses: subosito/flutter-action@v2 with: channel: stable cache: true - run: flutter pub get - run: dart format --set-exit-if-changed . - run: flutter analyze - run: flutter test --coverage

flutter build appbundle --release --obfuscate --split-debug-info=build/symbols # Output: build/app/outputs/bundle/release/app-release.aab flutter build ipa --release --obfuscate --split-debug-info=build/symbols # Output: build/ios/ipa/*.ipa, upload with Transporter or xcrun altool # Version comes from pubspec.yaml, for example: # version: 2.3.1+45 (name 2.3.1, build number 45)

{ "cli": { "version": ">= 12.0.0" }, "build": { "development": { "developmentClient": true, "distribution": "internal" }, "preview": { "distribution": "internal", "channel": "preview" }, "production": { "autoIncrement": true, "channel": "production" } }, "submit": { "production": {} } }

1. Bump version name and build number on both platforms. 2. Run full automated tests and smoke tests on release builds. 3. Verify signing, obfuscation and debug symbol upload. 4. Check new permissions and update store privacy declarations. 5. Update release notes and screenshots if UI changed. 6. Upload to internal track and TestFlight; QA sign-off. 7. Submit for review with reviewer notes and demo account. 8. Start staged rollout at 5 to 10 percent. 9. Monitor crashes, ANRs, reviews and key metrics. 10. Expand rollout or halt and hotfix.