/mobile-framework-expo

name: mobile-framework-expo
description: Expo managed workflow

Expo Development Patterns

> **Quick Guide:** Build production-ready React Native apps with Expo. Use managed workflow with Continuous Native Generation for most projects, Expo Router for file-based navigation, and EAS for builds/updates. Development builds replace Expo Go for production testing.

---

<critical_requirements>

CRITICAL: Before Using This Skill

**(You MUST use development builds for production testing - Expo Go is for prototyping only)**

**(You MUST update runtimeVersion when making native dependency changes to prevent OTA update crashes)**

**(You MUST use config plugins for native customization - NEVER manually edit android/ios directories in managed workflow)**

**(You MUST use `EXPO_PUBLIC_` prefix for client-side environment variables - NEVER store secrets in these variables)**

</critical_requirements>

---

**Auto-detection:** Expo, expo-router, EAS Build, EAS Update, expo-dev-client, app.config.js, app.json, expo prebuild, npx expo, eas.json, expo-constants, expo-notifications, Continuous Native Generation, CNG

**When to use:**

• Starting new React Native projects with rapid development needs
• Building apps that need OTA (over-the-air) updates
• Using file-based routing with convention-over-configuration
• Managing native code without maintaining android/ios directories
• Deploying to app stores with cloud builds

**Key patterns covered:**

• Managed workflow with Continuous Native Generation (CNG)
• Expo Router file-based navigation
• EAS Build, Submit, and Update workflows
• Development builds vs Expo Go
• Config plugins for native customization
• Environment configuration and secrets
• Push notifications setup

**When NOT to use:**

• Apps requiring complex custom native code beyond Expo Modules API
• When app size must be under 15MB (Expo adds overhead)
• Legacy React Native projects not ready for migration

---

<philosophy>

Philosophy

Expo transforms React Native development from "write once, debug everywhere" to "write once, deploy confidently." The key insight is that **most apps don't need direct native access** - they need well-maintained native modules with consistent APIs.

**Core principles:**

1. **Managed by default** - Let Expo handle native complexity; prebuild only when necessary 2. **Continuous Native Generation** - Treat android/ios as build artifacts, not source code 3. **Development builds for truth** - Expo Go is for learning; development builds show production reality 4. **OTA for velocity** - Ship JavaScript updates without app store delays 5. **Config plugins over ejection** - Customize native code declaratively when needed

**Mental model:**

Expo is NOT a limitation on React Native - it's a professional-grade abstraction. You can always drop down to native code via Expo Modules API or prebuild, but most apps never need to.

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Dynamic Configuration with `app.config.ts`

Use `app.config.ts` for environment-specific builds. Use named constants for SDK versions and build numbers.

// app.config.ts - Environment-aware config
const IS_PRODUCTION = process.env.APP_ENV === "production";
const BUILD_NUMBER = 1;

export default ({ config }: ConfigContext): ExpoConfig => ({
  ...config,
  name: IS_PRODUCTION ? "MyApp" : "MyApp (Dev)",
  ios: {
    bundleIdentifier: IS_PRODUCTION ? "com.app" : "com.app.dev",
    buildNumber: String(BUILD_NUMBER),
  },
  android: {
    package: IS_PRODUCTION ? "com.app" : "com.app.dev",
    versionCode: BUILD_NUMBER,
  },
});

> Full examples: [examples/core.md](examples/core.md) - App Configuration section

---

Pattern 2: Config Plugins for Native Customization

Modify native code declaratively -- changes survive `expo prebuild --clean`. Use config plugins for permissions, SDK versions, and native settings.

// app.config.ts plugins array
plugins: [
  [
    "expo-camera",
    { cameraPermission: "Allow $(PRODUCT_NAME) to access your camera." },
  ],
  [
    "expo-build-properties",
    { android: { minSdkVersion: 24 }, ios: { deploymentTarget: "15.1" } },
  ],
];

> Full examples: [examples/core.md](examples/core.md) - Config Plugins section

---

Pattern 3: Environment Variables

Use `EXPO_PUBLIC_` prefix for client-side variables. Metro requires direct property access -- destructuring and bracket notation don't work.

// MUST use direct access - Metro static analysis requirement
const API_URL = process.env.EXPO_PUBLIC_API_URL; // Works
// const { EXPO_PUBLIC_API_URL } = process.env;  // BROKEN - undefined at runtime

> Full examples: [examples/core.md](examples/core.md) - Environment Variables section

---

Pattern 4: Development Builds

Use `expo-dev-client` for production-accurate testing. Expo Go is for prototyping only -- it lacks your native dependencies, push notifications, and accurate splash screens.

# Cloud build
eas build --profile development --platform ios
# Local build
npx expo run:ios

> Full configuration: [examples/eas.md](examples/eas.md) - Development Builds section

---

Pattern 5: Asset Management

Block splash screen while loading fonts, use `expo-image` for remote images with blur hash placeholders and disk caching.

SplashScreen.preventAutoHideAsync();
// Load fonts, then call SplashScreen.hideAsync() when ready

> Full examples: [examples/core.md](examples/core.md) - Font Loading and Image Handling sections

</patterns>

---

<red_flags>

RED FLAGS

• **Expo Go for production testing** -- missing native modules, push notifications, accurate splash screens. Always use development builds.
• **Not updating runtimeVersion after native changes** -- OTA updates crash on apps with incompatible native code. Use `"fingerprint"` policy for automatic detection.
• **Storing secrets in `EXPO_PUBLIC_` variables** -- embedded in JS bundle, visible to any