Consumer-facing iOS and Android apps built from a single codebase -- React Native for teams with JavaScript/TypeScript expertise or those needing extensive npm ecosystem access, Flutter for teams requiring pixel-perfect custom UI with consistent cross-platform rendering. Authentication layer: Firebase Auth or Auth0 for social login (Google, Apple, Facebook) plus email/password, with refresh token management and biometric re-authentication via react-native-biometrics (Face ID, Touch ID, Android Biometric API) so returning users authenticate in under 2 seconds. Payments integration: Stripe with the Stripe React Native SDK for card payments and Apple Pay/Google Pay, react-native-iap for App Store and Google Play in-app purchases with server-side receipt validation preventing receipt replay attacks. Push notifications via Firebase Cloud Messaging (@react-native-firebase/messaging) for cross-platform delivery with APNs for iOS, with deep link handling for notification taps routing to the correct in-app screen rather than the app home. Performance tuning: Hermes JavaScript engine enabled for React Native (faster startup, lower memory); FlatList with getItemLayout for long scrolling lists; image loading via react-native-fast-image with progressive loading and memory management. App Store and Google Play submission: metadata, screenshots, privacy policy, age rating, and review compliance documentation prepared as part of the delivery; initial App Store review submission handled with 2-3 revision cycles budgeted.

Mobile tools for business users that replace the spreadsheet, paper form, or laptop-dependent process with a purpose-built app designed for how field workers, sales reps, and operations staff actually work. Field service app architecture: offline-first data model using @op-engineering/op-sqlite or WatermelonDB so technicians continue working in areas with no connectivity, with a background sync queue that pushes captured data when connectivity returns and a conflict resolution strategy for edits made offline. Sales enablement apps: product catalogue browsing from locally cached data, customer account lookup via CRM API (Salesforce, HubSpot), quote generation with configurable pricing rules, and e-signature capture on a mobile screen using react-native-signature-canvas for on-the-spot order sign-off. Inventory management: barcode and QR scanning via react-native-vision-camera with vision-camera-code-scanner, stock level updates via ERP API with optimistic UI so the scan registers immediately even before the API call completes, and cycle count workflows that guide the user through a warehouse zone systematically. SSO integration for corporate identity: SAML 2.0 via an IdP bridge or OAuth 2.0 with PKCE for Okta, Azure AD, and Google Workspace -- enterprise users log in with their corporate credentials without creating a new account. MDM enrollment compatibility with Jamf (iOS), Intune (iOS and Android), and VMware Workspace ONE for apps deployed to company-managed devices with remote wipe and policy enforcement.

Cross-platform apps in regulated categories where compliance requirements must be designed into the architecture from the start rather than retrofitted after the core feature work is complete. Healthcare (HIPAA) architecture: no PHI stored on device without encryption (AES-256 at rest using the iOS Keychain and Android Keystore for encryption keys); JWT tokens stored in the secure keychain rather than AsyncStorage; automatic session timeout after configurable inactivity period with biometric re-authentication; all API communication over TLS 1.2+; audit logging of every PHI access event with user ID, timestamp, and the specific data accessed. Financial services (PCI-DSS) architecture for apps that process or display payment card data: scope reduction by ensuring the app never directly handles raw card numbers (Stripe.js or Stripe React Native SDK handles card capture in an iframe/native component that keeps card data off the app's JavaScript layer); network security configuration that disallows cleartext traffic on Android; certificate pinning for production API endpoints. FDA Class II Software as a Medical Device (SaMD) considerations for diagnostic and clinical decision support apps: design documentation structured for 510(k) predicate device submissions, risk management documentation per ISO 14971, software development lifecycle documentation per IEC 62304, and usability engineering per IEC 62366. Compliance documentation package delivered alongside the app: architecture diagrams, data flow diagrams annotated with PHI/PCI data elements, security controls list, and penetration testing scope definition.

Apps that maintain persistent connections to live data -- fleet tracking, field monitoring, real-time messaging, live order status, and hardware-connected experiences -- built with the connection management and battery optimisation that makes real-time mobile viable in the field rather than draining batteries and failing on poor networks. WebSocket connection management: reconnection logic with exponential backoff, connection state indicator in the UI, and a message queue that buffers outgoing events during disconnection and replays them on reconnect -- the patterns that make real-time features resilient to the intermittent connectivity mobile users encounter. Background location tracking for fleet and field apps: @react-native-community/geolocation configured for significant-changes mode (lower battery impact than continuous GPS) or react-native-background-geolocation for continuous tracking with battery optimisation; geofence entry/exit events handled via the platform's native geofencing APIs (CLLocationManager on iOS, Geofencing API on Android) and reported to the backend via background app refresh. Bluetooth LE device integration using react-native-ble-plx: device discovery, characteristic subscription for continuous sensor readings, connection state management with auto-reconnect, and data parsing from the device's proprietary byte format into your app's data model. IoT sensor dashboard: real-time charting of incoming sensor data via WebSocket using a lightweight charting library (Victory Native or Recharts adapted for React Native) optimised for continuous data updates without re-rendering the entire chart on each incoming reading.

Apps where the camera is a primary interaction surface -- document scanning, barcode workflows, inspection capture, and media creation -- built with react-native-vision-camera for full camera API access including custom frame processors that run native code on every camera frame without JavaScript bridge overhead. Document scanning with perspective correction: vision-camera-code-scanner for real-time barcode and QR detection; custom frame processor running OpenCV-based edge detection to identify document boundaries and apply perspective transform, producing a flat corrected scan from a photo taken at an angle -- the same result a hardware document scanner produces but from a phone camera. Inspection and field capture workflows: photo capture tied to a structured form (the technician photographs the defect, then fills in the defect type, severity, and location fields in the same screen); batch capture mode for multi-point inspections; automatic metadata attachment (GPS coordinates, device timestamp, user ID) to each photo before upload. On-device ML inference using TensorFlow Lite or Core ML via react-native-fast-tflite for real-time image classification -- object identification, defect detection, or document type classification running locally without a network round-trip. Video recording with quality configuration (resolution, bitrate, frame rate) and upload via multipart streaming to S3 or a CDN origin, with upload progress displayed and resume-on-reconnect for large files on mobile networks. AR features using Expo Camera or ViroReact for ARKit (iOS) and ARCore (Android) integrations where the requirements stay within the cross-platform layer's capabilities.

Migration of existing native iOS or Android apps to a cross-platform React Native or Flutter codebase -- reducing the ongoing cost of maintaining two separate implementations and enabling both platforms to share every future improvement with a single code change. Migration approach: feature parity audit as the first deliverable, documenting every screen, interaction, and edge case in the existing apps before writing a line of migration code -- preventing the category of bugs where a feature existed in the old app but was overlooked in the new one. Phased porting strategy: new feature development begins in the new cross-platform codebase while the old native apps remain in production; the cross-platform app is shipped alongside the native app initially; once a configurable percentage of users have migrated, the native app is retired. Native module preservation: custom native code (proprietary hardware integrations, platform-specific cryptography, third-party SDK integrations) wrapped in React Native Native Modules using JSI for zero-overhead bridging, preserving the native functionality without requiring it to be rewritten in JavaScript. Architecture improvement during migration: the migration is the opportunity to introduce offline-first data management, structured state management (Redux Toolkit or Zustand), and a proper API layer that the original native codebase may have lacked. Feature parity test suite: every screen and interaction in the original app covered by Detox E2E tests that run against the migrated app, producing a pass/fail parity report before each release during the transition period.