flutter-ui
Build performant, accessible UIs with strict design tokens and reusable widget patterns. Use when implementing layouts, responsive breakpoints, theming, widget extraction, or fixing common rendering issues like overflow errors.
What this skill does
# 1. Performance & Rendering
- **Const-First**: Every widget that can be `const` MUST be `const`.
- **Lazy Rendering**: Use `SliverList.builder` or `SliverGrid.builder` for lists > 10 items.
- **Repaint Boundaries**: Wrap complex animations in `RepaintBoundary`.
- **Isolate Parsing**: Use `compute()` or `Isolate` for JSON > 1MB.
- **BuildContext Safety**: Check `mounted` before using `context` across async gaps.
# 2. Design Tokens (Theming)
Use `AppColors`, `AppSpacing`, `AppRadius`, and `AppTypography`. NEVER hardcode raw values.
- **Colors**: Use `context.colorScheme.primary` or `AppColors`. Support light/dark modes.
- **Spacing**: Use `AppSpacing.sm` (8), `AppSpacing.md` (16), etc. Use `SizedBox` for gaps.
- **Radius**: Use `AppRadius.md` (12) for consistent rounding.
- **Typography**: Use `context.textTheme.bodyMedium`. Support text scaling.
# 3. Reusable Components
- **Single Responsibility**: Each component has one clear purpose.
- **Parameterization**: Expose parameters for customization.
- **Complexity**: Extract widgets or code blocks used multiple times into `core/views/widgets`.
- **Keys**: Assign `Key('feature_action_id')` to interactive widgets for test access.
# 4. Widget & Interaction Patterns
- **Extraction**: STRICTLY prohibit private `_build*()` methods. Extract into separate widget classes.
- **Slivers**: Prefer `CustomScrollView` with Slivers for non-trivial scrollable layouts.
- **FAB**: Use Floating Action Buttons for primary positive actions (Add, Create).
- **Scroll Padding**: Add dynamic bottom padding when a FAB or BottomBar is present to prevent overlap.
- **Sheets vs Screens**: Prefer full `Scaffold` screens over `ModalBottomSheet` for complex forms.
# 5. Adaptive & Responsive Design and Common Fixes
- **Mobile First**: Design for mobile, then adapt for tablet (`600-840dp`) and desktop (`>840dp`). Use `MediaQuery.sizeOf(context).width`.
- **Layout Builder**: Use `LayoutBuilder` when widget rendering depends on the immediate parent's constraints, not the whole screen.
- **Safe Area**: Always wrap main layouts in `SafeArea` to avoid device notches and system UI.
- **Overflow Fixes**:
- `A RenderFlex overflowed...`: Typically means a widget is demanding more space than available in a `Row` or `Column`. Wrap the offending widget in `Expanded` or `Flexible`.
- `Vertical viewport was given unbounded height`: Often happens when nesting scrollable views (like `ListView` inside `Column`). Use `Expanded` on the `ListView` or set `shrinkWrap: true`.
- **Rules**: Never lock orientation unless strictly required by a unique feature. Support keyboard navigation and hover effects for desktop users.
# 6. Responsive Layout Patterns
## LayoutBuilder vs MediaQuery
| Tool | Use When | Returns |
|---|---|---|
| `MediaQuery.sizeOf(context)` | Layout depends on **screen size** (app-level breakpoints) | `Size` of the screen |
| `LayoutBuilder` | Layout depends on **parent constraints** (widget-level) | `BoxConstraints` from parent |
**Rule**: Use `MediaQuery` for page-level layouts. Use `LayoutBuilder` for reusable components that adapt to their container.
## Breakpoint Conventions
| Window Class | Width | Layout | Columns |
|---|---|---|---|
| Compact | < 600dp | Single column, bottom nav | 4 |
| Medium | 600–840dp | Two-pane, navigation rail | 8 |
| Expanded | > 840dp | Multi-pane, side navigation | 12 |
## Adaptive Layout Pattern
```dart
Widget build(BuildContext context) {
final width = MediaQuery.sizeOf(context).width;
return switch (width) {
< 600 => const MobileLayout(),
< 840 => const TabletLayout(),
_ => const DesktopLayout(),
};
}
```
## Constraint Widgets
| Widget | Behavior | Use When |
|---|---|---|
| `Expanded` | Fill ALL remaining space in `Row`/`Column` | Child should stretch |
| `Flexible` | Allow child to be SMALLER than remaining space | Child has natural size |
| `FractionallySizedBox` | Size as fraction of parent (e.g., 0.5 = 50%) | Proportional layouts |
| `ConstrainedBox` | Set min/max constraints | Bounded flexibility |
| `SizedBox` | Fixed absolute dimensions | Known exact size |
# 7. UI States & Accessibility
- **States**: Always handle Loading, Error, and Empty states with clear messaging.
- **Accessibility**: Include `Semantics` labels. Ensure 48x48 dp touch targets. WCAG AA contrast.
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