Pier_v1/.hmcmkit/knowledge/skills/mobile-app/SKILL.md

---
name: mobile-app
classification: capability
classification-reason: Specialized domain knowledge with limited model overlap
deprecation-risk: low
description: |
  Mobile app development guide — React Native, Flutter, Expo for cross-platform.
  Triggers: mobile app, React Native, Flutter, Expo, iOS, Android, 모바일 앱.
agent: hmcmkit:pipeline-guide
allowed-tools:
  - Read
  - Write
  - Edit
  - Glob
  - Grep
  - Bash
  - WebSearch
user-invocable: true
---

# Mobile App Development Expertise

## Overview

A guide for developing mobile apps based on web development experience.
Develop for iOS and Android simultaneously using cross-platform frameworks.

---

## Framework Selection Guide

### Framework Selection by Tier (v1.3.0)

| Framework | Tier | Recommendation | Use Case |
|-----------|------|----------------|----------|
| **React Native (Expo)** | Tier 1 | ⭐ Primary | TypeScript ecosystem, AI tools |
| **React Native CLI** | Tier 1 | Recommended | Native module needs |
| **Flutter** | Tier 2 | Supported | Multi-platform (6 OS), performance |

> **AI-Native Recommendation**: React Native with TypeScript
> - Full Copilot/AI Agent support
> - Extensive npm ecosystem
> - 20:1 developer availability vs Dart

> **Performance Recommendation**: Flutter
> - Impeller rendering engine
> - Single codebase for 6 platforms
> - Smaller bundles

### Level-wise Recommendations

```
Starter → Expo (React Native) [Tier 1]
  - Simple setup, can leverage web knowledge
  - Full AI tool support

Dynamic → Expo + EAS Build [Tier 1] or Flutter [Tier 2]
  - Includes server integration, production build support
  - Choose Flutter for multi-platform needs

Enterprise → React Native CLI [Tier 1] or Flutter [Tier 2]
  - Complex native features, performance optimization needed
  - Flutter for consistent cross-platform UI
```

---

## Expo (React Native) Guide

### Project Creation

```bash
# Install Expo CLI
npm install -g expo-cli

# Create new project
npx create-expo-app my-app
cd my-app

# Start development server
npx expo start
```

### Folder Structure

```
my-app/
├── app/                    # Expo Router pages
│   ├── (tabs)/            # Tab navigation
│   │   ├── index.tsx      # Home tab
│   │   ├── explore.tsx    # Explore tab
│   │   └── _layout.tsx    # Tab layout
│   ├── _layout.tsx        # Root layout
│   └── +not-found.tsx     # 404 page
├── components/            # Reusable components
├── hooks/                 # Custom hooks
├── constants/             # Constants
├── assets/               # Images, fonts, etc.
├── app.json              # Expo configuration
└── package.json
```

### Navigation Patterns

```typescript
// app/_layout.tsx - Stack navigation
import { Stack } from 'expo-router';

export default function RootLayout() {
  return (
    <Stack>
      <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
      <Stack.Screen name="modal" options={{ presentation: 'modal' }} />
    </Stack>
  );
}
```

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

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

### Styling Patterns

```typescript
// Basic StyleSheet
import { StyleSheet, View, Text } from 'react-native';

export function MyComponent() {
  return (
    <View style={styles.container}>
      <Text style={styles.title}>Hello</Text>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    padding: 16,
    backgroundColor: '#fff',
  },
  title: {
    fontSize: 24,
    fontWeight: 'bold',
  },
});
```

```typescript
// NativeWind (Tailwind for RN) - Recommended
import { View, Text } from 'react-native';

export function MyComponent() {
  return (
    <View className="flex-1 p-4 bg-white">
      <Text className="text-2xl font-bold">Hello</Text>
    </View>
  );
}
```

### API Integration

```typescript
// hooks/useApi.ts
import { useState, useEffect } from 'react';

export function useApi<T>(endpoint: string) {
  const [data, setData] = useState<T | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<Error | null>(null);

  useEffect(() => {
    const fetchData = async () => {
      try {
        const response = await fetch(`${process.env.EXPO_PUBLIC_API_URL}${endpoint}`);
        if (!response.ok) throw new Error('API Error');
        const json = await response.json();
        setData(json);
      } catch (e) {
        setError(e as Error);
      } finally {
        setLoading(false);
      }
    };
    fetchData();
  }, [endpoint]);

  return { data, loading, error };
}
```

### Authentication Pattern

```typescript
// context/AuthContext.tsx
import { createContext, useContext, useState, useEffect } from 'react';
import * as SecureStore from 'expo-secure-store';

interface AuthContextType {
  user: User | null;
  signIn: (email: string, password: string) => Promise<void>;
  signOut: () => Promise<void>;
}

const AuthContext = createContext<AuthContextType | null>(null);

export function AuthProvider({ children }: { children: React.ReactNode }) {
  const [user, setUser] = useState<User | null>(null);

  useEffect(() => {
    // Check for stored token on app start
    const loadToken = async () => {
      const token = await SecureStore.getItemAsync('authToken');
      if (token) {
        // Load user info with token
      }
    };
    loadToken();
  }, []);

  const signIn = async (email: string, password: string) => {
    const response = await fetch('/api/auth/login', {
      method: 'POST',
      body: JSON.stringify({ email, password }),
    });
    const { token, user } = await response.json();
    await SecureStore.setItemAsync('authToken', token);
    setUser(user);
  };

  const signOut = async () => {
    await SecureStore.deleteItemAsync('authToken');
    setUser(null);
  };

  return (
    <AuthContext.Provider value={{ user, signIn, signOut }}>
      {children}
    </AuthContext.Provider>
  );
}

export const useAuth = () => useContext(AuthContext)!;
```

---

## Flutter Guide

### Project Creation

```bash
# After installing Flutter SDK
flutter create my_app
cd my_app

# Start development server
flutter run
```

### Folder Structure

```
my_app/
├── lib/
│   ├── main.dart           # App entry point
│   ├── app/
│   │   ├── app.dart        # MaterialApp setup
│   │   └── routes.dart     # Route definitions
│   ├── features/           # Feature-based folders
│   │   ├── auth/
│   │   │   ├── screens/
│   │   │   ├── widgets/
│   │   │   └── providers/
│   │   └── home/
│   ├── shared/
│   │   ├── widgets/        # Common widgets
│   │   ├── services/       # API services
│   │   └── models/         # Data models
│   └── core/
│       ├── theme/          # Theme settings
│       └── constants/      # Constants
├── assets/                 # Images, fonts
├── pubspec.yaml           # Dependency management
└── android/ & ios/        # Native code
```

### Basic Widget Patterns

```dart
// lib/features/home/screens/home_screen.dart
import 'package:flutter/material.dart';

class HomeScreen extends StatelessWidget {
  const HomeScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Home'),
      ),
      body: const Center(
        child: Text('Hello, Flutter!'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {},
        child: const Icon(Icons.add),
      ),
    );
  }
}
```

### State Management (Riverpod)

```dart
// lib/providers/counter_provider.dart
import 'package:flutter_riverpod/flutter_riverpod.dart';

final counterProvider = StateNotifierProvider<CounterNotifier, int>((ref) {
  return CounterNotifier();
});

class CounterNotifier extends StateNotifier<int> {
  CounterNotifier() : super(0);

  void increment() => state++;
  void decrement() => state--;
}
```

```dart
// Usage
class CounterScreen extends ConsumerWidget {
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final count = ref.watch(counterProvider);

    return Text('Count: $count');
  }
}
```

---

## Web vs Mobile Differences

### UI/UX Differences

| Element | Web | Mobile |
|---------|-----|--------|
| Click | onClick | onPress |
| Scroll | overflow: scroll | ScrollView / FlatList |
| Input | input | TextInput |
| Links | a href | Link / navigation |
| Layout | div + CSS | View + StyleSheet |

### Navigation Differences

```
Web: URL-based (browser back button)
Mobile: Stack-based (screen stacking)

Web: /users/123
Mobile: navigation.navigate('User', { id: 123 })
```

### Storage Differences

```
Web: localStorage, sessionStorage, Cookie
Mobile: AsyncStorage, SecureStore, SQLite

⚠️ SecureStore is required for sensitive info on mobile!
```

---

## Build & Deployment

### Expo EAS Build

```bash
# Install EAS CLI
npm install -g eas-cli

# Login
eas login

# Configure build
eas build:configure

# iOS build
eas build --platform ios

# Android build
eas build --platform android

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

### Environment Variables

```json
// app.json
{
  "expo": {
    "extra": {
      "apiUrl": "https://api.example.com"
    }
  }
}
```

```typescript
// Usage
import Constants from 'expo-constants';

const apiUrl = Constants.expoConfig?.extra?.apiUrl;
```

---

## Mobile PDCA Checklist

### Phase 1: Schema
```
□ Identify data that needs offline caching
□ Define sync conflict resolution strategy
```

### Phase 3: Mockup
```
□ Follow iOS/Android native UX guidelines
□ Consider gestures (swipe, pinch, etc.)
□ Layout for different screen sizes (phone, tablet)
```

### Phase 6: UI
```
□ Keyboard handling (screen adjustment during input)
□ Safe Area handling (notch, home button area)
□ Handle platform-specific UI differences
```

### Phase 7: Security
```
□ Store sensitive info with SecureStore
□ Certificate Pinning (if needed)
□ App obfuscation settings
```

### Phase 9: Deployment
```
□ Follow App Store review guidelines
□ Prepare Privacy Policy URL
□ Prepare screenshots, app description
```

---

## Frequently Asked Questions

### Q: Can I convert a web project to an app?
```
A: Recommend separate project over full conversion
   - APIs can be shared
   - UI needs to be rewritten (for native UX)
   - Business logic can be shared
```

### Q: Should I use Expo or React Native CLI?
```
A: Start with Expo!
   - 90%+ of apps are sufficient with Expo
   - Can eject later if needed
   - Use CLI only when native modules are absolutely required
```

### Q: How long does app review take?
```
A:
   - iOS: 1-7 days (average 2-3 days)
   - Android: Few hours ~ 3 days

   ⚠️ First submission has high rejection possibility → Follow guidelines carefully!
```

---

## Requesting from AI Agent

### Project Creation
```
"Set up a [app description] app project with React Native + Expo.
Configure with 3 tab navigation (Home, Search, Profile)."
```

### Screen Implementation
```
"Implement [screen name] screen.
- Display [content] at the top
- Display [list/form/etc.] in the middle
- [Button/navigation] at the bottom"
```

### API Integration
```
"Implement screen integrating with [API endpoint].
- Show loading state
- Handle errors
- Support pull-to-refresh"
```