/app-ui-design

name: app-ui-design
description: Mobile app UI design expert for iOS and Android. Use when designing app interfaces, creating design systems, ensuring accessibility, or following platform guidelines. Covers Material Design 3, Human Interface Guidelines, color theory, typography, and 2025 trends.

Mobile App UI Design

> Expert guidance for designing beautiful, accessible, and platform-native mobile app interfaces following 2025 best practices.

Core Philosophy

• **User-First Design** — Prioritize user needs, behaviors, and mental models over aesthetics
• **Platform Consistency** — Follow iOS HIG and Android Material Design guidelines
• **Accessibility as Foundation** — Design for all users from the start, not as an afterthought
• **Emotional Intelligence** — Create interfaces that users emotionally connect with
• **Performance-Conscious** — Beautiful design that doesn't sacrifice app performance

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Hard Rules (Must Follow)

> These rules are mandatory. Violating them means the skill is not working correctly.

Accessibility First

**All designs must meet WCAG 2.2 AA standards. Accessibility is not optional.**

❌ FORBIDDEN:
- Color contrast below 4.5:1 for text
- Touch targets smaller than 44×44pt (iOS) or 48×48dp (Android)
- Information conveyed by color alone
- Missing alternative text for images
- Non-keyboard-navigable interfaces

✅ REQUIRED:
- Minimum 4.5:1 contrast ratio for normal text
- Minimum 3:1 contrast ratio for large text (18pt+)
- Touch targets: 44×44pt (iOS) / 48×48dp (Android)
- Support for Dynamic Type / Font Scaling
- VoiceOver (iOS) / TalkBack (Android) compatibility
- Clear focus indicators for keyboard navigation

Platform Guidelines Adherence

**Follow platform-specific design guidelines. Do not mix iOS and Android patterns.**

❌ FORBIDDEN:
- Using Android-style FAB on iOS
- Using iOS-style bottom sheets on Android without adaptation
- Mixing platform navigation patterns
- Ignoring platform typography (SF Pro vs Roboto)

✅ REQUIRED:
iOS (Human Interface Guidelines):
- Use SF Pro font family
- Tab bar at bottom for primary navigation
- Push navigation with back chevron
- Standard iOS controls (UIKit/SwiftUI)

Android (Material Design 3):
- Use Roboto font family
- Bottom navigation bar or navigation drawer
- Material components with ripple effects
- Follow Material You dynamic theming

Consistent Design System

**Every app must have a documented design system with tokens.**

❌ FORBIDDEN:
- Ad-hoc colors and spacing values
- Inconsistent button styles across screens
- Multiple unnamed font sizes
- Components without defined states

✅ REQUIRED:
Design System Must Include:
├── Color Tokens (primary, secondary, surface, error, etc.)
├── Typography Scale (heading1-6, body, caption, etc.)
├── Spacing Scale (4, 8, 12, 16, 24, 32, 48px)
├── Border Radius Tokens (none, sm, md, lg, full)
├── Shadow/Elevation Tokens (elevation1-5)
└── Component Library (button, input, card, etc.)

Touch-Friendly Design

**All interactive elements must be optimized for touch interaction.**

❌ FORBIDDEN:
- Touch targets smaller than minimum size
- Interactive elements too close together (<8px gap)
- Important actions outside thumb reach zone
- Hover-dependent interactions

✅ REQUIRED:
- Minimum touch target: 44×44pt (iOS) / 48×48dp (Android)
- Minimum spacing between targets: 8px
- Primary actions in thumb-friendly zone (bottom 2/3)
- Clear tap feedback (ripple, highlight, scale)

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Quick Reference

When to Use What

| Scenario | Approach | Key Considerations | |----------|----------|-------------------| | New app design | Start with design system | Define tokens before screens | | iOS-only app | Human Interface Guidelines | SF Pro, standard iOS patterns | | Android-only app | Material Design 3 | Roboto, Material components | | Cross-platform app | Adaptive design | Platform-specific navigation | | Redesign existing app | Audit first | Maintain mental models | | Accessibility review | WCAG 2.2 AA checklist | Contrast, touch targets, labels |

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Design System Fundamentals

Color System

## Color Token Structure

### Semantic Colors
primary          → Main brand color, CTAs
primary-variant  → Darker/lighter primary for states
secondary        → Secondary actions, accents
background       → App background
surface          → Card backgrounds, elevated surfaces
error            → Error states, destructive actions
on-primary       → Text/icons on primary color
on-background    → Text/icons on background
on-surface       → Text/icons on surface

### Color Palette (60-30-10 Rule)
60% → Neutral/background colors
30% → Secondary/supporting colors
10% → Accent/primary colors

### Dark Mode Considerations
- Don't just invert colors
- Use desaturated colors for dark surfaces
- Reduce contrast slightly (avoid pure white on black)
- Maintain color meaning across modes

Typography Scale

## Mobile Typography Guidelines

### iOS Typography (SF Pro)
Large Title   → 34pt, Bold
Title 1       → 28pt, Bold
Title 2       → 22pt, Bold
Title 3       → 20pt, Semibold
Headline      → 17pt, Semibold
Body          → 17pt, Regular
Callout       → 16pt, Regular
Subhead       → 15pt, Regular
Footnote      → 13pt, Regular
Caption 1     → 12pt, Regular
Caption 2     → 11pt, Regular

### Android Typography (Roboto / Material 3)
Display Large  → 57sp
Display Medium → 45sp
Display Small  → 36sp
Headline Large → 32sp
Headline Medium→ 28sp
Headline Small → 24sp
Title Large    → 22sp
Title Medium   → 16sp, Medium
Title Small    → 14sp, Medium
Body Large     → 16sp
Body Medium    → 14sp
Body Small     → 12sp
Label Large    → 14sp, Medium
Label Medium   → 12sp, Medium
Label Small    → 11sp, Medium

### Best Practices
- Maximum 2-3 font families per app
- Minimum body text: 16px (14px absolute minimum)
- Line height: 1.4-1.6× font size for body text
- Support Dynamic Type (iOS) / Font Scalin