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Building Native UI SKILL.md Source

Full source content for verification and traceability.

Expo UI Guidelines

For routes, links, stacks, tabs, modals, sheets, and headers, use the expo-router skill.

References

Consult these resources as needed:

references/
  animations.md          Reanimated: entering, exiting, layout, scroll-driven, gestures
  controls.md            Native iOS: Switch, Slider, SegmentedControl, DateTimePicker, Picker
  gradients.md           CSS gradients via experimental_backgroundImage (New Arch only)
  icons.md               SF Symbols via expo-image (sf: source), names, animations, weights
  media.md               Camera, audio, video, and file saving
  storage.md             SQLite, AsyncStorage, SecureStore
  visual-effects.md      Blur (expo-blur) and liquid glass (expo-glass-effect)
  webgpu-three.md        3D graphics, games, GPU visualizations with WebGPU and Three.js

Running the App

CRITICAL: Always try Expo Go first before creating custom builds.

Most Expo apps work in Expo Go without any custom native code. Before running npx expo run:ios or npx expo run:android:

  1. Start with Expo Go: Run npx expo start and scan the QR code with Expo Go
  2. Check if features work: Test your app thoroughly in Expo Go
  3. Only create custom builds when required - see below

When Custom Builds Are Required

You need npx expo run:ios/android or eas build ONLY when using:

  • Local Expo modules (custom native code in modules/)
  • Apple targets (widgets, app clips, extensions via @bacons/apple-targets)
  • Third-party native modules not included in Expo Go
  • Custom native configuration that can't be expressed in app.json

When Expo Go Works

Expo Go supports a huge range of features out of the box:

  • All expo-* packages (camera, location, notifications, etc.)
  • Expo Router navigation
  • Most UI libraries (reanimated, gesture handler, etc.)
  • Push notifications, deep links, and more

If you're unsure, try Expo Go first. Creating custom builds adds complexity, slower iteration, and requires Xcode/Android Studio setup.

Code Style

  • Be cautious of unterminated strings. Ensure nested backticks are escaped; never forget to escape quotes correctly.
  • Always use import statements at the top of the file.
  • Always use kebab-case for file names, e.g. comment-card.tsx
  • Never use special characters in file names
  • Configure tsconfig.json with path aliases, and prefer aliases over relative imports for refactors.

Library Preferences

  • Never use modules removed from React Native such as Picker, WebView, SafeAreaView, or AsyncStorage
  • Never use legacy expo-permissions
  • expo-audio not expo-av
  • expo-video not expo-av
  • expo-image with source="sf:name" for SF Symbols, not expo-symbols or @expo/vector-icons
  • react-native-safe-area-context not react-native SafeAreaView
  • process.env.EXPO_OS not Platform.OS
  • React.use not React.useContext
  • expo-image Image component instead of intrinsic element img
  • expo-glass-effect for liquid glass backdrops
  • Color from expo-router for native semantic colors, not raw PlatformColor (type-safe, auto-adapts to light/dark)
  • In SDK 56+, never import from @react-navigation/* directly — use expo-router/react-navigation instead (covers @react-navigation/native, /core, /elements, /routers)

Responsiveness

  • Always wrap root component in a scroll view for responsiveness
  • Use <ScrollView contentInsetAdjustmentBehavior="automatic" /> instead of <SafeAreaView> for smarter safe area insets
  • contentInsetAdjustmentBehavior="automatic" should be applied to FlatList and SectionList as well
  • Use flexbox instead of Dimensions API
  • ALWAYS prefer useWindowDimensions over Dimensions.get() to measure screen size

Behavior

  • Use expo-haptics conditionally on iOS to make more delightful experiences
  • Use views with built-in haptics like <Switch /> from React Native and @react-native-community/datetimepicker
  • When a route belongs to a Stack, its first child should almost always be a ScrollView with contentInsetAdjustmentBehavior="automatic" set
  • When adding a ScrollView to the page it should almost always be the first component inside the route component
  • Use the <Text selectable /> prop on text containing data that could be copied
  • Consider formatting large numbers like 1.4M or 38k
  • Never use intrinsic elements like 'img' or 'div' unless in a webview or Expo DOM component

Styling

Follow Apple Human Interface Guidelines.

General Styling Rules

  • Prefer flex gap over margin and padding styles
  • Prefer padding over margin where possible
  • Always account for safe area, either with stack headers, tabs, or ScrollView/FlatList contentInsetAdjustmentBehavior="automatic"
  • Ensure both top and bottom safe area insets are accounted for
  • Inline styles not StyleSheet.create unless reusing styles is faster
  • Add entering and exiting animations for state changes
  • Use { borderCurve: 'continuous' } for rounded corners unless creating a capsule shape
  • ALWAYS use a navigation stack title instead of a custom text element on the page
  • When padding a ScrollView, use contentContainerStyle padding and gap instead of padding on the ScrollView itself (reduces clipping)
  • CSS and Tailwind are not supported - use inline styles

Colors

Use the Color API from expo-router for native semantic colors. It is a type-safe wrapper over PlatformColor that exposes iOS UIKit colors through Color.ios.* and Android Material 3 colors through Color.android.material.* (static) or Color.android.dynamic.* (adapts to the user's wallpaper on Android 12+). These resolve on-device and automatically adapt to light/dark mode and accessibility settings, so you no longer maintain separate light/dark hex tables or a colors.web.ts file.

Color is platform-specific, so wrap each value in Platform.select with a default hex fallback for web. Centralize the palette in theme/colors.ts and import colors everywhere:

// theme/colors.ts
import { Platform } from "react-native";
import { Color } from "expo-router";

export const colors = {
  label: Platform.select({
    ios: Color.ios.label,
    android: Color.android.dynamic.onSurface,
    default: "#000000",
  })!,
  secondaryLabel: Platform.select({
    ios: Color.ios.secondaryLabel,
    android: Color.android.dynamic.onSurfaceVariant,
    default: "#3c3c43",
  })!,
  separator: Platform.select({
    ios: Color.ios.separator,
    android: Color.android.dynamic.outlineVariant,
    default: "#c6c6c8",
  })!,
  systemBackground: Platform.select({
    ios: Color.ios.systemBackground,
    android: Color.android.dynamic.surface,
    default: "#ffffff",
  })!,
  systemBlue: Platform.select({
    ios: Color.ios.systemBlue,
    android: Color.android.dynamic.primary,
    default: "#007aff",
  })!,
};

import { colors } from "@/theme/colors";

<View style={{ backgroundColor: colors.systemBackground }}>
  <Text style={{ color: colors.label }}>Title</Text>
</View>;
  • iOS re-resolves these colors automatically when the system theme changes. On Android, call useColorScheme() inside any component that renders them so it re-renders when the theme flips (required when React Compiler memoizes the component).
  • Don't pass Color / PlatformColor values into Reanimated styles — use static colors there (see references/animations.md).
  • Platform.select({...})! returns string | OpaqueColorValue. Most React Native style props accept ColorValue (string | OpaqueColorValue) so this works fine. But some third-party props only accept string (e.g. tintColor on expo-image). Cast when needed: colors.label as string.

Text Styling

  • Add the selectable prop to every <Text/> element displaying important data or error messages
  • Counters should use { fontVariant: 'tabular-nums' } for alignment

Shadows

Use CSS boxShadow style prop. NEVER use legacy React Native shadow or elevation styles.

<View style={{ boxShadow: "0 1px 2px rgba(0, 0, 0, 0.05)" }} />

'inset' shadows are supported.