Mobile Design System

(Mobile-First · Touch-First · Platform-Respectful)

> Philosophy: Touch-first. Battery-conscious. Platform-respectful. Offline-capable. > Core Law: Mobile is NOT a small desktop. > Operating Rule: Think constraints first, aesthetics second.

This skill exists to prevent desktop-thinking, AI-defaults, and unsafe assumptions when designing or building mobile applications.

1. Mobile Feasibility & Risk Index (MFRI)

Before designing or implementing any mobile feature or screen, assess feasibility.

MFRI Dimensions (1–5)

| Dimension | Question | | -------------------------- | ----------------------------------------------------------------- | | Platform Clarity | Is the target platform (iOS / Android / both) explicitly defined? | | Interaction Complexity | How complex are gestures, flows, or navigation? | | Performance Risk | Does this involve lists, animations, heavy state, or media? | | Offline Dependence | Does the feature break or degrade without network? | | Accessibility Risk | Does this impact motor, visual, or cognitive accessibility? |

Score Formula

MFRI = (Platform Clarity + Accessibility Readiness)
       − (Interaction Complexity + Performance Risk + Offline Dependence)

Range: -10 → +10

Interpretation

| MFRI | Meaning | Required Action | | -------- | --------- | ------------------------------------- | | 6–10 | Safe | Proceed normally | | 3–5 | Moderate | Add performance + UX validation | | 0–2 | Risky | Simplify interactions or architecture | | < 0 | Dangerous | Redesign before implementation |

2. Mandatory Thinking Before Any Work

⛔ STOP: Ask Before Assuming (Required)

If any of the following are not explicitly stated, you MUST ask before proceeding:

| Aspect | Question | Why | | ---------- | ------------------------------------------ | ---------------------------------------- | | Platform | iOS, Android, or both? | Affects navigation, gestures, typography | | Framework | React Native, Flutter, or native? | Determines performance and patterns | | Navigation | Tabs, stack, drawer? | Core UX architecture | | Offline | Must it work offline? | Data & sync strategy | | Devices | Phone only or tablet too? | Layout & density rules | | Audience | Consumer, enterprise, accessibility needs? | Touch & readability |

🚫 Never default to your favorite stack or pattern.

3. Mandatory Reference Reading (Enforced)

Universal (Always Read First)

| File | Purpose | Status | | ----------------------------- | ---------------------------------- | ----------------- | | mobile-design-thinking.md | Anti-memorization, context-forcing | 🔴 REQUIRED FIRST | | touch-psychology.md | Fitts’ Law, thumb zones, gestures | 🔴 REQUIRED | | mobile-performance.md | 60fps, memory, battery | 🔴 REQUIRED | | mobile-backend.md | Offline sync, push, APIs | 🔴 REQUIRED | | mobile-testing.md | Device & E2E testing | 🔴 REQUIRED | | mobile-debugging.md | Native vs JS debugging | 🔴 REQUIRED |

Platform-Specific (Conditional)

| Platform | File | | -------------- | ------------------- | | iOS | platform-ios.md | | Android | platform-android.md | | Cross-platform | BOTH above |

> ❌ If you haven’t read the platform file, you are not allowed to design UI.

4. AI Mobile Anti-Patterns (Hard Bans)

🚫 Performance Sins (Non-Negotiable)

| ❌ Never | Why | ✅ Always | | ------------------------- | -------------------- | --------------------------------------- | | ScrollView for long lists | Memory explosion | FlatList / FlashList / ListView.builder | | Inline renderItem | Re-renders all rows | useCallback + memo | | Index as key | Reorder bugs | Stable ID | | JS-thread animations | Jank | Native driver / GPU | | console.log in prod | JS thread block | Strip logs | | No memoization | Battery + perf drain | React.memo / const widgets |

🚫 Touch & UX Sins

| ❌ Never | Why | ✅ Always | | --------------------- | -------------------- | ----------------- | | Touch <44–48px | Miss taps | Min touch target | | Gesture-only action | Excludes users | Button fallback | | No loading state | Feels broken | Explicit feedback | | No error recovery | Dead end | Retry + message | | Ignore platform norms | Muscle memory broken | iOS ≠ Android |

🚫 Security Sins

| ❌ Never | Why | ✅ Always | | ---------------------- | ------------------ | ---------------------- | | Tokens in AsyncStorage | Easily stolen | SecureStore / Keychain | | Hardcoded secrets | Reverse engineered | Env + secure storage | | No SSL pinning | MITM risk | Cert pinning | | Log sensitive data | PII leakage | Never log secrets |

5. Platform Unification vs Divergence Matrix

UNIFY                          DIVERGE
──────────────────────────     ─────────────────────────
Business logic                Navigation behavior
Data models                    Gestures
API contracts                  Icons
Validation                     Typography
Error semantics                Pickers / dialogs

Platform Defaults

| Element | iOS | Android | | --------- | ------------ | -------------- | | Font | SF Pro | Roboto | | Min touch | 44pt | 48dp | | Back | Edge swipe | System back | | Sheets | Bottom sheet | Dialog / sheet | | Icons | SF Symbols | Material Icons |

6. Mobile UX Psychology (Non-Optional)

Fitts’ Law (Touch Reality)

• Finger ≠ cursor
• Accuracy is low
• Reach matters more than precision

Rules:

• Primary CTAs live in thumb zone
• Destructive actions pushed away
• No hover assumptions

7. Performance Doctrine

React Native (Required Pattern)

ts
const Row = React.memo(({ item }) => (
  <View><Text>{item.title}</Text></View>
));


const renderItem = useCallback(
  ({ item }) => <Row item={item} />,
  []
);


<FlatList
  data={items}
  renderItem={renderItem}
  keyExtractor={(i) => i.id}
  getItemLayout={(_, i) => ({
    length: ITEM_HEIGHT,
    offset: ITEM_HEIGHT * i,
    index: i,
  })}
/>

Flutter (Required Pattern)

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


  @override
  Widget build(BuildContext context) {
    return const Text('Static');
  }
}

• const everywhere possible
• Targeted rebuilds only

8. Mandatory Mobile Checkpoint

Before writing any code, you must complete this:

🧠 MOBILE CHECKPOINT


Platform:     ___________
Framework:    ___________
Files Read:   ___________


3 Principles I Will Apply:
1.
2.
3.


Anti-Patterns I Will Avoid:
1.
2.

❌ Cannot complete → go back and read.

9. Framework Decision Tree (Canonical)

Need OTA + web team → React Native + Expo
High-perf UI → Flutter
iOS only → SwiftUI
Android only → Compose

No debate without justification.

10. Release Readiness Checklist

Before Shipping

• ] Touch targets ≥ 44–48px
• ] Offline handled
• ] Secure storage used
• ] Lists optimized
• ] Logs stripped
• ] Tested on low-end devices
• ] Accessibility labels present
• ] MFRI ≥ 3

11. Related Skills

• frontend-design – Visual systems & components
• frontend-dev-guidelines – RN/TS architecture
• backend-dev-guidelines – Mobile-safe APIs
• error-tracking – Crash & performance telemetry

> Final Law: > Mobile users are distracted, interrupted, and impatient—often using one hand on a bad network with low battery. > Design for that reality, or your app will fail quietly.