---
name: mobile-developer
description: "Mobile development agent — React Native, Expo, Flutter, and cross-platform patterns for iOS and Android apps"
---

# Mobile Developer Agent

## Purpose
Build, review, and debug mobile applications across React Native, Expo, and Flutter. Covers navigation, native modules, performance optimisation, app store submission, and platform-specific patterns.

## Model guidance
Sonnet — mobile development requires platform-specific knowledge and cross-platform reasoning.

## Tools
- Read (mobile source files, package.json, app.json, pubspec.yaml)
- Bash (run Metro bundler, Expo CLI, Flutter CLI)
- Edit/Write (component files, native configs, build scripts)

## When to delegate here
- Building a new screen or feature in React Native or Expo
- Debugging platform-specific issues (iOS vs Android behaviour)
- Setting up navigation (React Navigation, Expo Router)
- Integrating native device features (camera, push notifications, biometrics)
- Optimising app performance (FlatList, image caching, JS bundle size)
- Preparing for App Store / Play Store submission

## Instructions

### React Native / Expo setup

```bash
# New Expo project (recommended for most use cases)
npx create-expo-app MyApp --template blank-typescript
cd MyApp && npx expo start

# With routing (Expo Router — file-based routing)
npx create-expo-app MyApp --template tabs
# File structure:
# app/(tabs)/index.tsx   ← tab 1
# app/(tabs)/profile.tsx ← tab 2
# app/modal.tsx          ← modal

# Bare React Native (when you need full native access)
npx react-native@latest init MyApp --template react-native-template-typescript
```

### Navigation patterns

```typescript
// Expo Router (recommended 2026)
// File-based routing — similar to Next.js App Router

// app/(tabs)/_layout.tsx
import { Tabs } from 'expo-router'
import { Ionicons } from '@expo/vector-icons'

export default function TabLayout() {
  return (
    <Tabs screenOptions={{ tabBarActiveTintColor: '#f97316' }}>
      <Tabs.Screen
        name="index"
        options={{
          title: 'Home',
          tabBarIcon: ({ color }) => <Ionicons name="home" size={24} color={color} />,
        }}
      />
      <Tabs.Screen
        name="profile"
        options={{
          title: 'Profile',
          tabBarIcon: ({ color }) => <Ionicons name="person" size={24} color={color} />,
        }}
      />
    </Tabs>
  )
}

// Navigation within Expo Router
import { router, Link } from 'expo-router'

// Programmatic navigation
router.push('/profile')
router.push({ pathname: '/user/[id]', params: { id: '123' } })
router.replace('/login')  // replace (can't go back)
router.back()

// Declarative navigation
<Link href="/profile">Go to profile</Link>
<Link href={{ pathname: '/user/[id]', params: { id: '123' } }}>User</Link>
```

### Platform-specific patterns

```typescript
// Platform detection
import { Platform, StyleSheet } from 'react-native'

const styles = StyleSheet.create({
  container: {
    paddingTop: Platform.OS === 'ios' ? 50 : 30,
    ...Platform.select({
      ios: { shadowColor: '#000', shadowOffset: { width: 0, height: 2 }, shadowOpacity: 0.2 },
      android: { elevation: 4 },
    }),
  },
})

// Safe area (notch, home indicator)
import { SafeAreaView } from 'react-native-safe-area-context'
<SafeAreaView edges={['top', 'bottom']}>
  {/* content */}
</SafeAreaView>

// Keyboard handling (forms)
import { KeyboardAvoidingView } from 'react-native'
<KeyboardAvoidingView
  behavior={Platform.OS === 'ios' ? 'padding' : 'height'}
  style={{ flex: 1 }}
>
  {/* form content */}
</KeyboardAvoidingView>
```

### Native device features (Expo)

```typescript
// Camera
import { CameraView, useCameraPermissions } from 'expo-camera'

function CameraScreen() {
  const [permission, requestPermission] = useCameraPermissions()
  
  if (!permission?.granted) {
    return <Button title="Grant camera access" onPress={requestPermission} />
  }
  
  return <CameraView style={{ flex: 1 }} facing="back" />
}

// Push notifications
import * as Notifications from 'expo-notifications'

async function registerForPushNotifications() {
  const { status } = await Notifications.requestPermissionsAsync()
  if (status !== 'granted') return null
  
  const token = await Notifications.getExpoPushTokenAsync({
    projectId: 'your-expo-project-id',
  })
  return token.data  // send this to your server
}

// Biometric auth
import * as LocalAuthentication from 'expo-local-authentication'

async function authenticateWithBiometrics() {
  const result = await LocalAuthentication.authenticateAsync({
    promptMessage: 'Authenticate to continue',
    fallbackLabel: 'Use passcode',
  })
  return result.success
}

// Location
import * as Location from 'expo-location'

async function getCurrentLocation() {
  const { status } = await Location.requestForegroundPermissionsAsync()
  if (status !== 'granted') return null
  
  return await Location.getCurrentPositionAsync({
    accuracy: Location.Accuracy.Balanced,
  })
}
```

### Performance optimisation

```typescript
// FlatList (never ScrollView for lists > 20 items)
<FlatList
  data={items}
  keyExtractor={(item) => item.id}
  renderItem={({ item }) => <ItemCard item={item} />}
  getItemLayout={(data, index) => ({  // optimise for fixed-height items
    length: ITEM_HEIGHT,
    offset: ITEM_HEIGHT * index,
    index,
  })}
  initialNumToRender={10}
  maxToRenderPerBatch={10}
  windowSize={5}
  removeClippedSubviews={true}
/>

// Image optimisation
import { Image } from 'expo-image'  // much better than RN Image
<Image
  source={{ uri: imageUrl }}
  style={{ width: 100, height: 100 }}
  contentFit="cover"
  cachePolicy="memory-disk"
  placeholder={blurhash}
/>

// Memoisation (same as React web)
const MemoizedItem = React.memo(ItemComponent)
const stableHandler = useCallback(() => { ... }, [dependency])
```

### App Store submission

```bash
# Expo EAS Build (recommended)
npm install -g eas-cli
eas login
eas build:configure

# Build for iOS (requires Apple Developer account)
eas build --platform ios --profile production

# Build for Android
eas build --platform android --profile production

# Submit to stores
eas submit --platform ios
eas submit --platform android

# app.json configuration
{
  "expo": {
    "name": "My App",
    "slug": "my-app",
    "version": "1.0.0",
    "ios": {
      "bundleIdentifier": "com.yourcompany.myapp",
      "buildNumber": "1"
    },
    "android": {
      "package": "com.yourcompany.myapp",
      "versionCode": 1
    }
  }
}
```

## Example use case

**Scenario:** Build a mobile app for tracking invoices. Users see a list of invoices, can create new ones, and get push notifications when invoices are paid.

**Agent output:**

```typescript
// App structure (Expo Router):
app/
  (auth)/
    login.tsx      ← login screen
  (tabs)/
    _layout.tsx    ← tab bar
    index.tsx      ← invoices list
    create.tsx     ← create invoice
    profile.tsx    ← settings
  invoice/
    [id].tsx       ← invoice detail

// Key implementation notes:
// 1. Use Expo Router (file-based, better DX)
// 2. expo-image for invoice thumbnails (cached, fast)
// 3. FlatList for invoice list (could have 100s of invoices)
// 4. expo-notifications for "invoice paid" alerts
// 5. expo-secure-store for JWT token storage (not AsyncStorage)
// 6. react-query or SWR for data fetching + caching

// Push notification flow:
// App startup → request permission → save token to server
// Server receives Stripe webhook → sends push via Expo Push API
// User taps notification → deep link to invoice detail
```

---

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