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

Name: app-ui-design
Author: majiayu000

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

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)

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

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 Scaling (Android)

Spacing System

## 8-Point Grid System

Base Unit: 8px

### Spacing Scale
spacing-0    → 0px
spacing-1    → 4px   (half unit)
spacing-2    → 8px   (1 unit)
spacing-3    → 12px  (1.5 units)
spacing-4    → 16px  (2 units)
spacing-5    → 24px  (3 units)
spacing-6    → 32px  (4 units)
spacing-7    → 48px  (6 units)
spacing-8    → 64px  (8 units)

### Component Spacing
Button padding      → 12px vertical, 24px horizontal
Card padding        → 16px
List item padding   → 16px horizontal, 12px vertical
Section spacing     → 24px-32px
Screen edge margin  → 16px (phones), 24px (tablets)

### Touch Target Spacing
Minimum gap between interactive elements: 8px
Recommended gap: 12-16px

Platform Guidelines

iOS Human Interface Guidelines

## iOS Design Principles

### Core Principles
1. Clarity    → Text legible, icons precise, purpose obvious
2. Deference  → UI helps understanding, not competing with content
3. Depth      → Visual layers and realistic motion convey hierarchy

### Navigation Patterns
- Tab Bar (bottom) → 3-5 primary destinations
- Navigation Bar (top) → Title, back button, actions
- Modal sheets → Temporary focused tasks
- Popovers (iPad) → Contextual options

### 2025 Updates: Liquid Glass
- Translucent materials for controls
- Floating navigation elements
- Dynamic depth and hierarchy
- Background blur effects

### Safe Areas
- Top: Status bar + Dynamic Island/Notch
- Bottom: Home indicator (34pt on Face ID devices)
- Use safeAreaInsets for proper content placement

### Standard Dimensions
- Navigation bar height: 44pt (96pt with large title)
- Tab bar height: 49pt (83pt on Face ID devices)
- Toolbar height: 44pt

Android Material Design 3

## Material Design 3 Principles

### Core Concepts
1. Material You    → Personal, adaptive, dynamic color
2. Expressiveness  → Emotional connection through design
3. Accessibility   → Inclusive design for all users

### Navigation Patterns
- Bottom Navigation → 3-5 primary destinations
- Navigation Drawer → 5+ destinations or secondary nav
- Navigation Rail → Tablets and large screens
- Top App Bar → Title, navigation, actions

### Material 3 Expressive (2025)
- Dynamic color from user wallpaper
- Springy, physics-based motion
- Bolder, more expressive typography
- Rounded, approachable shapes

### Component Shapes
- Extra Small: 4dp
- Small: 8dp
- Medium: 12dp
- Large: 16dp
- Extra Large: 28dp
- Full: 50% (pills/circles)

### Standard Dimensions
- App bar height: 64dp
- Bottom navigation height: 80dp
- FAB size: 56dp (standard), 40dp (small), 96dp (large)

2025 Design Trends

AI-Driven Personalization

## Adaptive UI Patterns

### Personalization Strategies
- Content recommendations based on behavior
- Adaptive UI layouts based on usage patterns
- Smart defaults and predictive actions
- Contextual feature suggestions

### Implementation Guidelines
- Always provide manual override options
- Explain why content is recommended
- Respect user privacy preferences
- Gradual personalization (don't overwhelm new users)

Dark Mode Excellence

## Dark Mode Best Practices

### Color Adaptation
Light Mode          →    Dark Mode
White (#FFFFFF)     →    Dark gray (#121212)
Black (#000000)     →    White (#FFFFFF)
Primary (vibrant)   →    Primary (desaturated)
Shadows             →    Subtle shadows or none

### Surface Elevation (Material 3)
Elevation 0  → #121212
Elevation 1  → #1E1E1E (+ 5% white overlay)
Elevation 2  → #232323 (+ 7% white overlay)
Elevation 3  → #272727 (+ 8% white overlay)
Elevation 4  → #2C2C2C (+ 9% white overlay)
Elevation 5  → #323232 (+ 11% white overlay)

### Recommendations
- Auto-switch based on system setting
- Separate color tokens for light/dark
- Test contrast in both modes
- Consider OLED optimization (true blacks)

Microinteractions

## Meaningful Motion

### Types of Microinteractions
- Feedback → Button press, form submission
- State → Loading, success, error
- Guidance → Onboarding hints, tooltips
- Delight → Celebration animations, easter eggs

### Motion Principles
- Duration: 200-500ms for most interactions
- Easing: Use natural easing curves
- Purpose: Every animation should have a reason
- Performance: Use hardware-accelerated properties

### 2025 Trends
- Springy, physics-based animations
- Morphing transitions between states
- Gesture-driven interactions
- Reduced motion support for accessibility

Glassmorphism & Neumorphism

## Modern Visual Styles

### Glassmorphism
Properties:
- Background blur (backdrop-filter: blur)
- Transparency (10-40% opacity)
- Subtle border (1px white at 20% opacity)
- Soft shadow

Use Cases:
- Overlays and modals
- Cards on image backgrounds
- Navigation bars (iOS Liquid Glass)

Caution:
- Ensure text readability
- Consider performance impact
- Provide fallback for older devices

### Neumorphism (Soft UI)
Properties:
- Soft shadows (light and dark)
- Subtle elevation effect
- Muted color palette

Use Cases:
- Toggle switches
- Input fields
- Cards

Caution:
- Often fails accessibility contrast tests
- Use sparingly, not as primary style
- Always test with actual content

Accessibility Checklist

Visual Accessibility

## Vision Accessibility Checklist

### Color & Contrast
- [ ] Text contrast ≥ 4.5:1 (normal), ≥ 3:1 (large)
- [ ] UI component contrast ≥ 3:1
- [ ] Color is not the only indicator (use icons, text)
- [ ] Works for colorblind users (test with simulators)

### Text & Typography
- [ ] Base font size ≥ 16px
- [ ] Supports Dynamic Type (iOS) / Font Scaling (Android)
- [ ] Line height ≥ 1.4× for body text
- [ ] Text can scale to 200% without loss of content

### Focus & Navigation
- [ ] Visible focus indicators
- [ ] Logical tab order
- [ ] Skip links for repetitive content
- [ ] Keyboard-accessible (external keyboards)

Motor Accessibility

## Motor Accessibility Checklist

### Touch Targets
- [ ] Minimum size: 44×44pt (iOS) / 48×48dp (Android)
- [ ] Spacing between targets ≥ 8px
- [ ] No time-limited interactions (or extendable)
- [ ] Alternative to drag gestures (WCAG 2.2)

### Interaction
- [ ] Single-tap for all actions (no double-tap required)
- [ ] Undo available for destructive actions
- [ ] No precision requirements (fine motor)
- [ ] Support for Switch Control / Voice Access

Screen Reader

## Screen Reader Checklist

### Content
- [ ] All images have descriptive alt text
- [ ] Decorative images have empty alt (alt="")
- [ ] Buttons/links have descriptive labels
- [ ] Form inputs have associated labels
- [ ] Headings used for structure (not styling)

### Navigation
- [ ] Logical reading order
- [ ] Meaningful link text (not "click here")
- [ ] State changes announced
- [ ] Modal focus properly trapped
- [ ] Custom components have ARIA roles

Component Patterns

Button Design

## Button Hierarchy

### Primary Button
- Most important action on screen
- Filled/solid style
- High contrast
- Only ONE per view section

### Secondary Button
- Alternative actions
- Outlined or tonal style
- Medium contrast

### Tertiary Button
- Low-priority actions
- Text-only or ghost style
- Subtle appearance

### Destructive Button
- Delete, remove, cancel
- Red/error color
- Requires confirmation for important data

### Button States
- Default → Normal appearance
- Pressed → Scale down, darker
- Disabled → 50% opacity, no interaction
- Loading → Spinner, disabled

Form Design

## Form Best Practices

### Input Fields
- Clear labels (above input, not placeholder-only)
- Helpful placeholder text
- Error states with specific messages
- Success validation feedback
- Appropriate keyboard types

### Layout
- Single column for mobile
- Group related fields
- Logical tab order
- Primary action at bottom
- Sticky submit button if long form

### Validation
- Inline validation (on blur or after input)
- Clear error messages with fix suggestions
- Don't clear fields on error
- Mark required fields clearly

Navigation Patterns

## Mobile Navigation

### Bottom Navigation (Primary)
- 3-5 destinations
- Icons + labels
- Active state indicator
- Consistent across app

### Tab Bar (Content Switching)
- Horizontal tabs for related content
- Swipe between tabs
- Indicator for active tab

### Navigation Stack
- Clear back navigation
- Meaningful titles
- Breadcrumbs for deep hierarchies

### Gestures
- Swipe back (iOS standard)
- Pull to refresh
- Swipe actions on list items
- Long press for context menus

Design Process

Workflow

## App Design Workflow

1. Research & Discovery
   ├── User research and personas
   ├── Competitor analysis
   ├── Platform guidelines review
   └── Technical constraints

2. Information Architecture
   ├── User flows
   ├── Navigation structure
   ├── Content hierarchy
   └── Sitemap

3. Design System Setup
   ├── Color tokens
   ├── Typography scale
   ├── Spacing system
   ├── Component library
   └── Icon set

4. Wireframing
   ├── Low-fidelity layouts
   ├── Flow validation
   ├── Stakeholder review
   └── Iteration

5. Visual Design
   ├── High-fidelity mockups
   ├── Interaction design
   ├── Prototype creation
   └── Accessibility audit

6. Handoff & Implementation
   ├── Design specs documentation
   ├── Asset export
   ├── Developer collaboration
   └── QA review

Anti-Patterns to Avoid

## Common Mistakes

### Visual Design
❌ Using more than 3 fonts
❌ Inconsistent spacing values
❌ Low contrast text
❌ Overly complex gradients/effects
❌ Ignoring safe areas

### Interaction Design
❌ Hidden navigation
❌ Gesture-only actions without alternatives
❌ Tiny touch targets
❌ No loading states
❌ Confusing back navigation

### Accessibility
❌ Placeholder-only labels
❌ Color-only information
❌ Auto-playing media
❌ Timed interactions without extension
❌ Missing screen reader labels

### Platform
❌ iOS patterns on Android (or vice versa)
❌ Custom controls when standard works
❌ Ignoring platform conventions
❌ Not supporting dark mode

Checklist

Before Design

Reviewed platform guidelines (iOS HIG / Material Design 3)
Defined target users and their needs
Analyzed competitor apps
Identified accessibility requirements
Set up design system tokens

During Design

Following 8-point grid
Using defined color tokens
Typography from scale only
Touch targets meet minimum size
Contrast ratios verified
All states designed (default, pressed, disabled, error)
Dark mode variant created

Before Handoff

Accessibility audit completed
Tested with screen reader
Verified on multiple screen sizes
Motion/animation documented
Design specs annotated
Assets exported correctly

Install Skill

SKILL.md