add favorite

docs/favorites_loading_fix.md

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+# Favorites Page - Loading State Fix
+
+## Problem
+Users were seeing the empty state ("Chưa có sản phẩm yêu thích") flash briefly before the actual favorites data loaded, even when they had favorites. This created a poor user experience.
+
+## Root Cause
+The `favoriteProductsProvider` is an async provider that:
+1. Loads favorites from API/cache
+2. Fetches all products
+3. Filters to get favorite products
+
+During this process, the provider goes through these states:
+- **Loading** → Returns empty list [] → Shows loading skeleton
+- **Data** → If products.isEmpty → Shows empty state ❌ **FLASH**
+- **Data** → Returns actual products → Shows grid
+
+The flash happened because when the provider rebuilt, it momentarily returned an empty list before the actual data arrived.
+
+## Solution
+
+### 1. Added `ref.keepAlive()` to Providers
+```dart
+@riverpod
+class Favorites extends _$Favorites {
+  @override
+  Future<Set<String>> build() async {
+    ref.keepAlive(); // ← Prevents provider disposal
+    // ... rest of code
+  }
+}
+
+@riverpod
+Future<List<Product>> favoriteProducts(Ref ref) async {
+  ref.keepAlive(); // ← Keeps previous data in memory
+  // ... rest of code
+}
+```
+
+**Benefits**:
+- Prevents state from being disposed when widget rebuilds
+- Keeps previous data available via `favoriteProductsAsync.valueOrNull`
+- Reduces unnecessary API calls
+
+### 2. Smart Loading State Logic
+
+```dart
+loading: () {
+  // 1. Check for previous data first
+  final previousValue = favoriteProductsAsync.valueOrNull;
+
+  // 2. If we have previous data, show it while loading
+  if (previousValue != null && previousValue.isNotEmpty) {
+    return Stack([
+      _FavoritesGrid(products: previousValue), // Show old data
+      LoadingIndicator(), // Small loading badge on top
+    ]);
+  }
+
+  // 3. Use favoriteCount as a hint
+  if (favoriteCount > 0) {
+    return LoadingState(); // Show skeleton
+  }
+
+  // 4. No data, show skeleton (not empty state)
+  return LoadingState();
+}
+```
+
+### 3. Data State - Only Show Empty When Actually Empty
+
+```dart
+data: (products) {
+  // Only show empty state when data is actually empty
+  if (products.isEmpty) {
+    return const _EmptyState();
+  }
+
+  return RefreshIndicator(
+    child: _FavoritesGrid(products: products),
+  );
+}
+```
+
+## User Experience Flow
+
+### Before Fix ❌
+```
+User opens favorites page
+  ↓
+Loading skeleton shows (0.1s)
+  ↓
+Empty state flashes (0.2s) ⚡ BAD UX
+  ↓
+Favorites grid appears
+```
+
+### After Fix ✅
+```
+User opens favorites page
+  ↓
+Loading skeleton shows (0.3s)
+  ↓
+Favorites grid appears smoothly
+```
+
+**OR** if returning to page:
+```
+User opens favorites page (2nd time)
+  ↓
+Previous favorites show immediately
+  ↓
+Small "Đang tải..." badge appears at top
+  ↓
+Updated favorites appear (if changed)
+```
+
+## Additional Benefits
+
+### 1. Better Offline Support
+- When offline, previous data stays visible
+- Shows error banner on top instead of hiding content
+- User can still browse cached favorites
+
+### 2. Faster Perceived Performance
+- Instant display of previous data
+- Users don't see empty states during reloads
+- Smoother transitions
+
+### 3. Error Handling
+```dart
+error: (error, stackTrace) {
+  final previousValue = favoriteProductsAsync.valueOrNull;
+
+  // Show previous data with error message
+  if (previousValue != null && previousValue.isNotEmpty) {
+    return Stack([
+      _FavoritesGrid(products: previousValue),
+      ErrorBanner(onRetry: ...),
+    ]);
+  }
+
+  // No previous data, show full error state
+  return _ErrorState();
+}
+```
+
+## Files Modified
+
+1. **lib/features/favorites/presentation/providers/favorites_provider.dart**
+   - Added `ref.keepAlive()` to `Favorites` class (line 81)
+   - Added `ref.keepAlive()` to `favoriteProducts` provider (line 271)
+
+2. **lib/features/favorites/presentation/pages/favorites_page.dart**
+   - Enhanced loading state logic (lines 138-193)
+   - Added previous value checking
+   - Added favoriteCount hint logic
+
+## Testing Checklist
+
+- [x] No empty state flash on first load
+- [x] Smooth loading with skeleton
+- [x] Previous data shown on subsequent visits
+- [x] Loading indicator overlay when refreshing
+- [ ] Test with slow network (3G)
+- [ ] Test with offline mode
+- [ ] Test with errors during load
+
+## Performance Impact
+
+✅ **Positive**:
+- Reduced state rebuilds
+- Better memory management with keepAlive
+- Fewer API calls on navigation
+
+⚠️ **Watch**:
+- Memory usage (keepAlive keeps data in memory)
+- Can manually dispose with `ref.invalidate()` if needed
+
+## Future Improvements
+
+1. **Add shimmer duration control**
+   - Minimum shimmer display time to prevent flash
+   - Smooth fade transition from skeleton to content
+
+2. **Progressive loading**
+   - Show cached data first
+   - Overlay with "Updating..." badge
+   - Fade in updated items
+
+3. **Prefetch on app launch**
+   - Load favorites in background
+   - Data ready before user navigates to page
+
+---
+
+**Status**: ✅ Implemented
+**Impact**: High - Significantly improves perceived performance
+**Breaking Changes**: None