engineering-mobile-app-builder

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---
name: engineering-mobile-app-builder
description: "Build native and cross-platform mobile applications for iOS and Android with optimized performance and platform integration. Use when you need SwiftUI or Jetpack Compose development, React Native or Flutter cross-platform apps, offline-first architecture, biometric authentication, push notifications, deep linking, app startup optimization, or mobile-specific UX patterns and gesture handling."
metadata:
  version: "1.0.1"
---

# Mobile Development Guide

## Overview
This guide covers native iOS/Android development (SwiftUI, Jetpack Compose) and cross-platform frameworks (React Native, Flutter) with patterns for offline-first architecture, platform integrations, and performance optimization. Use it when choosing a platform strategy, building mobile UI, or integrating device capabilities.

## Platform Selection Guide

- Use **native** (SwiftUI/Compose) when the app requires deep platform integration (widgets, extensions, AR, custom camera pipelines).
- Use **React Native or Flutter** when shipping to both platforms with a small team and the app is primarily data display and forms.
- For iOS, use SwiftUI with `@Observable` (iOS 17+) or `@StateObject`/`@ObservedObject` (iOS 15+); fall back to UIKit only for unsupported features.
- For Android, use Jetpack Compose with Hilt for DI and `StateFlow` for reactive state; avoid XML layouts in new screens.
- For navigation, use `NavigationStack` (iOS) or Navigation Compose (Android) with typed routes.

## Performance and UX Rules

- Cold start must be under 2 seconds on mid-range devices; defer initialization not needed for first frame. If cold start exceeds 1.5s, profile with Instruments (iOS) or `reportFullyDrawn()` (Android) and move heavy work to background.
- Maintain 60fps scrolling on devices two generations behind current. If frame drops occur: on iOS, check for off-main-thread image decoding and use `prefetchDataSource`; on Android, enable `compositionStrategy = ViewCompositionStrategy.DisposeOnViewTreeLifecycleDestroyed` and check for unnecessary recomposition with Layout Inspector.
- Animations: use SwiftUI `.animation()` / Compose `animateFloatAsState`; keep durations 200-350ms. Never animate layout properties (frame size, padding) — animate opacity and offset only.
- Offline-first: local database (Core Data, Room, SQLite/Drift) as single source of truth, background server sync. If the app has >3 entity types with relationships, use Core Data (iOS) or Room (Android) — never raw SQLite.
- Profile battery with Xcode Energy Diagnostics or Android Battery Historian; fix any operation keeping CPU awake >2s without user interaction.
- Batch network requests; use background sessions (URLSession / WorkManager) for large transfers. If payload >1MB, use background transfer; if >10MB, add resumable upload support.
- Incremental sync with timestamps or change tokens instead of full-collection fetches. If collection has >500 items, paginate sync with cursor; never fetch all.
- Lazy-load images with caching (Kingfisher/SDWebImage on iOS, Coil on Android); downscale to display size. If list shows >20 images, implement memory cache cap (50MB iOS, 100MB Android) and disk cache cap (200MB).
- App binary size: target <50MB iOS, <30MB APK (or use AAB for Android). If over threshold, audit with `scripts/check_app_size.sh`, remove unused assets, and enable app thinning (iOS) or dynamic feature modules (Android).
- Network timeout: 10s for API calls, 30s for file uploads. If a request fails, retry with exponential backoff (1s, 2s, 4s) max 3 attempts. Never retry non-idempotent requests (POST without idempotency key).
- Memory: monitor with `os_signpost` (iOS) / `Debug.getNativeHeapAllocatedSize()` (Android). If memory exceeds 200MB on mid-range device, investigate with Instruments Allocations or Android Studio Memory Profiler. Fix retain cycles (iOS) or leaked ViewModels (Android) before shipping.

## Platform Integration Rules

- Biometric auth: