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.

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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

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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.

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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.

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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

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🚫 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

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🚫 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

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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

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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

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7. Performance Doctrine

React Native (Required Pattern)

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)

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

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

• const everywhere possible
• Targeted rebuilds only

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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.

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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.

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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

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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

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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.

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