# Flutter Mobile Development Rules

This document defines Flutter mobile development constraints.

## Design System

### Design Tokens

All UI styling must use design tokens from `apps/lib/core/theme/design_tokens.dart`:

| Type | Usage |
|------|-------|
| Colors | `AppColors.primary`, `AppColors.slate500`, `AppColors.background` |
| Spacing | `AppSpacing.xs`, `AppSpacing.sm`, `AppSpacing.md` |
| Radius | `AppRadius.sm`, `AppRadius.md`, `AppRadius.lg` |

**NEVER hardcode colors, sizes, or spacing values.**

### Reuse Existing Components

Use pre-built components instead of creating custom ones:
- Buttons: Use `AppButton` widget from `apps/lib/shared/widgets/app_button.dart`
- Input fields: Use standard Flutter `TextField` with `InputDecoration`
- Loading states: Use built-in loading indicators

## New Page Design Workflow

1. **Analyze existing pages**: Study login, register, home screens for:
   - Layout structure (centered form, padding, spacing)
   - Typography hierarchy (title 28px bold, label 13px, hint 14px)
   - Component usage (AppButton, TextField style)
   - Color and spacing tokens

2. **Use frontend-design skill for mockups**:
   ```
   Use the `frontend-design` skill to create HTML/CSS mockups for review
   Match colors to `apps/lib/core/theme/design_tokens.dart`
   Match spacing to `AppSpacing` values
   Match radius to `AppRadius` values
   ```

3. **Verify design tokens**:
   - All colors from `AppColors`
   - All spacing from `AppSpacing`
   - All radius from `AppRadius`
   - NO hardcoded values

4. **Code review checklist**:
   - [ ] All colors/spacing/radius use design tokens
   - [ ] Reuses existing components (AppButton)
   - [ ] Consistent with existing page patterns
   - [ ] No magic numbers

## Layout Mapping Rules

Map design layout properties to Flutter explicitly:

1. **Always set `crossAxisAlignment` on `Row`/`Column`**:
   - `alignItems: center` -> `CrossAxisAlignment.center`
   - `alignItems: start` -> `CrossAxisAlignment.start`
   - `alignItems: stretch` -> `CrossAxisAlignment.stretch`
2. **Map full container chain**: From root to leaf, ensure each `alignItems` and `justifyContent` has a Flutter equivalent.
3. **Analyze before coding**: Verify each container's alignment settings.

## Centering and Visual Balance

1. Centering must be evaluated inside **`SafeArea`** bounds, not full-screen bounds.
2. Avoid relying on proportional `Spacer` values as the only centering mechanism for critical content.
3. For layouts with persistent top/bottom regions (e.g., headers or footers), center the primary content in the remaining available region.
4. Distinguish geometric centering from visual centering; validate final visual balance with screenshot review.

## Quality Gate

For important screens, add widget tests that reduce layout-regression risk:

1. Verify primary content remains centered relative to the usable viewport.
2. Add at least one constrained viewport scenario (small height or large text scale).

## Prohibitions

- DO NOT use colors not defined in design tokens
- DO NOT skip design container layers
- DO NOT start implementation before retrieving design variables
- DO NOT hardcode colors; use design variables

## UI Feedback System

**MUST use the Toast system for all user feedback messages.**

### Components

| Component | Use Case | Example |
|-----------|----------|---------|
| `Toast.show()` | Global temporary notifications | Success/error feedback after action |
| `AppBanner` | Inline form validation errors | Login form error message |

### Toast Types

```dart
enum ToastType { info, success, warning, error }
```

### Usage Examples

**Global Toast (auto-dismiss):**
```dart
Toast.show(context, '保存成功', type: ToastType.success);
Toast.show(context, '网络错误', type: ToastType.error);
Toast.show(context, '正在加载...', type: ToastType.info, duration: Duration(seconds: 3));
```

**Inline Banner (persistent):**
```dart
AppBanner(message: '邮箱或密码错误', type: ToastType.error)
AppBanner(message: '请检查输入', type: ToastType.warning)
```

### Rules

- Use `Toast` for transient feedback that auto-dismisses
- Use `AppBanner` for persistent inline messages (form errors)
- DO NOT create custom SnackBar, Dialog, or Banner components
- DO NOT use raw `ScaffoldMessenger`