Add offline reading feature documentation
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docs/Offline-Feature.md
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docs/Offline-Feature.md
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# Offline Reading Feature Documentation
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## Overview
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The Readeck iOS application includes a comprehensive offline reading feature that allows users to cache bookmarks and articles for reading without an internet connection. This feature implements automated synchronization, intelligent image caching, and FIFO-based cache management.
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## Architecture Overview
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The offline feature follows **Clean Architecture** principles with clear separation of concerns:
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```
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┌─────────────────────────────────────────────┐
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│ UI Layer (SwiftUI Views) │
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│ - BookmarksView, BookmarkDetailView │
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│ - OfflineBookmarksViewModel │
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│ - CachedAsyncImage Component │
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└────────────────┬────────────────────────────┘
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│
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┌─────────────────▼────────────────────────────┐
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│ Domain Layer (Use Cases) │
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│ - OfflineCacheSyncUseCase │
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│ - GetCachedBookmarksUseCase │
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│ - GetCachedArticleUseCase │
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└────────────────┬────────────────────────────┘
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│
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┌─────────────────▼────────────────────────────┐
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│ Data Layer (Repository Pattern) │
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│ - OfflineCacheRepository │
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│ - OfflineSettingsRepository │
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│ - Kingfisher Image Cache │
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└─────────────────────────────────────────────┘
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```
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## Core Components
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### 1. Data Models
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#### OfflineSettings
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Configuration for offline caching behavior (stored in UserDefaults):
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```swift
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struct OfflineSettings: Codable {
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var enabled: Bool = true // Feature enabled/disabled
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var maxUnreadArticles: Double = 20 // Max cached articles (0-100)
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var saveImages: Bool = false // Cache images in articles
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var lastSyncDate: Date? // Last successful sync timestamp
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var maxUnreadArticlesInt: Int {
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Int(maxUnreadArticles) // Helper: Double to Int conversion
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}
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}
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```
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#### BookmarkEntity (CoreData)
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Persisted bookmark with offline caching:
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- `id`: Unique identifier
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- `title`: Article title
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- `url`: Original URL
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- `htmlContent`: Full article HTML (added for offline)
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- `cachedDate`: Timestamp when cached (added for offline)
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- `lastAccessDate`: Last read timestamp (added for offline)
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- `heroImageURL`: Hero image URL (added for offline)
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- `imageURLs`: JSON array of content image URLs (added for offline)
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- `cacheSize`: Cache size in bytes (added for offline)
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- Plus other fields: authors, created, description, lang, readingTime, wordCount, etc.
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### 2. Use Cases
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#### OfflineCacheSyncUseCase
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Main orchestrator for the offline sync process:
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**Responsibilities:**
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- Fetch unread bookmarks from server
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- Download article HTML content
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- Prefetch and cache images
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- Embed images as Base64 (optional)
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- Persist to CoreData
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- Implement retry logic for temporary errors
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- Publish sync progress updates
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- Clean up old cached articles (FIFO)
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**Retry Logic:**
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- Retryable errors: HTTP 502, 503, 504 (temporary server issues)
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- Non-retryable errors: 4xx client errors, network failures
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- Exponential backoff: 2s → 4s between attempts
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- Maximum retries: 2 (3 total attempts)
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**Progress Publishers:**
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- `isSyncing: AnyPublisher<Bool, Never>` - Sync state
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- `syncProgress: AnyPublisher<String?, Never>` - Human-readable status
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#### GetCachedBookmarksUseCase
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Retrieve cached bookmarks for offline reading:
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- Fetches all cached bookmarks from CoreData
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- Returns sorted by `cachedDate` (newest first)
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- Used when network is unavailable
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#### GetCachedArticleUseCase
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Retrieve cached article HTML:
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- Looks up article by bookmark ID
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- Returns full HTML content
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- Updates `lastAccessDate` for cache management
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- Returns nil if not cached
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### 3. Repository Layer
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#### OfflineCacheRepository
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Manages all offline cache operations:
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**Cache Operations:**
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- `cacheBookmarkWithMetadata()` - Store bookmark + HTML + metadata
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- `hasCachedArticle()` - Check if article is cached
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- `getCachedArticle()` - Retrieve HTML content
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- `getCachedBookmarks()` - Get all cached bookmarks
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**Image Handling:**
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- `extractImageURLsFromHTML()` - Parse img src URLs from HTML
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- `prefetchImagesWithKingfisher()` - Download images to disk cache
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- `embedImagesAsBase64()` - Convert cached images to data URIs
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- `verifyPrefetchedImages()` - Validate images persisted after prefetch
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**Cache Management:**
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- `clearCache()` - Remove all cached articles and images
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- `cleanupOldestCachedArticles()` - FIFO cleanup when cache exceeds limit
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- `getCachedArticlesCount()` - Number of cached articles
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- `getCacheSize()` - Total cache size in bytes
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#### Image Caching Strategy
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**Kingfisher Configuration:**
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```swift
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KingfisherManager.shared.cache.diskStorage.config.expiration = .never
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```
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**Two-Phase Image Strategy:**
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1. **Hero/Thumbnail Images** (preserved in Kingfisher cache)
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- Downloaded during sync with custom cache key
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- Loaded from cache in offline mode via `CachedAsyncImage`
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- Not embedded as Base64
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2. **Content Images** (embedded in HTML)
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- Extracted from article HTML
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- Prefetched during sync
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- Optionally embedded as Base64 data URIs (if `saveImages = true`)
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- Online: HTTP URLs preserved
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- Offline: Base64 embedded or fallback to cache
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### 4. UI Components
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#### CachedAsyncImage
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Smart image component with online/offline awareness:
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```swift
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CachedAsyncImage(url: imageURL) { phase in
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switch phase {
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case .empty:
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ProgressView()
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case .success(let image):
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image.resizable()
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case .failure:
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Image(systemName: "photo.slash")
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@unknown default:
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EmptyView()
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}
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}
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```
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**Behavior:**
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- Online: Normal KFImage loading with network access
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- Offline: `.onlyFromCache(true)` - Only loads cached images
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- Graceful fallback: Shows placeholder if not cached
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#### OfflineBookmarksViewModel
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Manages offline bookmark synchronization:
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**State Machine:**
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- `.idle` - No sync in progress
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- `.pending(count)` - X articles ready to sync
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- `.syncing(count, status)` - Actively syncing, showing progress
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- `.success(count)` - Sync completed successfully
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- `.error(message)` - Sync failed
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**Key Methods:**
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- `syncOfflineBookmarks()` - Trigger manual sync
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- `refreshState()` - Update sync state from repository
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- Bindings to `OfflineCacheSyncUseCase` publishers
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#### BookmarksViewModel
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Enhanced to support offline reading:
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**Cache-First Approach:**
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- `loadCachedBookmarks()` - Fetch from local cache when offline
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- Only loads cached bookmarks for "Unread" tab
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- Network error detection triggers cache fallback
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- Archive/Starred/All tabs show "Tab not available offline"
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#### BookmarkDetailViewModel
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Read cached article content:
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**Article Loading Priority:**
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1. Try load from offline cache
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2. Fallback to server if not cached
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3. Report Base64 vs HTTP image counts
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## Synchronization Flow
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```
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┌─ User triggers sync (manual or automatic)
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│
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├─ Check if offline feature enabled
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├─ Check if network available
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│
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├─ Fetch unread bookmarks from server
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│ └─ Implement pagination if needed
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│
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├─ For each bookmark:
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│ ├─ Fetch article HTML with retry logic
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│ │ ├─ Attempt 1: Immediate
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│ │ ├─ Failure → 2s backoff → Attempt 2
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│ │ └─ Failure → 4s backoff → Attempt 3
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│ │
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│ ├─ Extract image URLs from HTML
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│ ├─ Prefetch images with Kingfisher
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│ │ └─ Optional: Embed as Base64 if saveImages=true
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│ │
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│ └─ Persist to CoreData with timestamp
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│
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├─ Cleanup old articles (FIFO when max exceeded)
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├─ Update lastSyncDate
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│
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└─ Publish completion/error status
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```
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## Cache Lifecycle
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### Adding to Cache
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1. User manually syncs or automatic sync triggers
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2. Unread bookmarks fetched from server
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3. Article HTML + images downloaded
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4. Stored in CoreData + Kingfisher cache
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5. `cachedDate` recorded
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### Using from Cache
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1. Network unavailable or error detected
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2. `loadCachedBookmarks()` called
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3. Returns list from CoreData (sorted by date)
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4. User opens article
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5. HTML loaded from cache
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6. Images loaded from Kingfisher cache (`.onlyFromCache` mode)
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### Removing from Cache
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**Automatic (FIFO Cleanup):**
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- When cached articles exceed `maxCachedArticles` (default: 20)
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- Oldest articles deleted first
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- Associated images also removed from Kingfisher
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**Manual:**
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- Settings → Offline → Clear Cache
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- Removes all CoreData entries + Kingfisher images
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## Error Handling & Retry Logic
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### Retryable Errors (with backoff)
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| HTTP Status | Reason | Retry? |
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|-------------|--------|--------|
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| 502 | Bad Gateway | ✅ Yes (temp server issue) |
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| 503 | Service Unavailable | ✅ Yes (maintenance) |
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| 504 | Gateway Timeout | ✅ Yes (slow backend) |
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### Non-Retryable Errors (skip article)
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| Error | Reason |
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|-------|--------|
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| 400-499 | Client error (bad URL, etc) |
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| Connection failed | Network unavailable |
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| Invalid URL | Malformed bookmark URL |
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**Sync Behavior:** Failed articles are skipped, sync continues with remaining bookmarks.
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## Configuration & Settings
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### OfflineSettings Storage
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- Persisted in **UserDefaults** (not CoreData)
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- Encoded/decoded as JSON
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- Survives app restarts
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- Contains: enabled, maxUnreadArticles, saveImages, lastSyncDate
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### Kingfisher Configuration
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```swift
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// Disk cache never expires
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KingfisherManager.shared.cache.diskStorage.config.expiration = .never
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// Offline mode: Only load from cache
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KFImage(url)
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.onlyFromCache(true)
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```
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### Cache Limits
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- **Max cached articles:** 20 (configurable via `maxUnreadArticles` slider)
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- **Max HTML size:** Unlimited (depends on device storage)
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- **Image formats:** Original from server (no processing)
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- **Cache location:** App Documents folder (CoreData) + Kingfisher disk cache
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## Testing
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### Unit Tests Coverage
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- OfflineCacheSyncUseCase: Sync flow, retry logic, cleanup
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- OfflineCacheRepository: Cache operations, image handling
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- GetCachedBookmarksUseCase: Retrieval logic
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- OfflineSettingsRepository: Settings persistence
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### Manual Testing Scenarios
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**Scenario 1: Hero Images Offline**
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1. Start app online, enable sync
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2. Wait for sync completion
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3. Open Debug Menu (Shake) → Simulate Offline Mode
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4. Verify hero/thumbnail images load from cache
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**Scenario 2: Article Content Offline**
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1. Open article while online (images load)
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2. Enable offline mode
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3. Reopen same article
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4. Verify all images display correctly
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**Scenario 3: FIFO Cleanup**
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1. Cache 25+ articles (exceeds 20-limit)
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2. Verify oldest 5 removed
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3. Check newest 20 retained
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**Scenario 4: Manual Cache Clear**
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1. Settings → Offline → Clear Cache
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2. Verify all cached articles removed
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3. Verify disk space freed
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## Performance Considerations
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### Image Optimization
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- **Format:** Original format from server (JPEG, PNG, WebP, etc.)
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- **Quality:** No compression applied, stored as-is
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- **Size:** Depends on server originals
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- **Kingfisher cache:** Automatic disk management
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- **Base64 embedding:** Increases HTML by ~30-40% (only for embedded images)
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### Memory Usage
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- CoreData uses SQLite (efficient)
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- Kingfisher cache limited by available disk
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- HTML content stored efficiently with compression
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### Network Impact
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- Prefetch uses concurrent image downloads
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- Retry logic prevents thundering herd
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- Exponential backoff reduces server load
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## Known Limitations
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1. **Offline Tab Restrictions**
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- Only "Unread" tab available offline
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- Archive/Starred require server
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- Reason: Offline cache maintains unread state
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2. **Read Progress Sync**
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- Local read progress preserved offline
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- Server sync when connection restored
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- No real-time sync offline
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3. **New Bookmarks**
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- Cannot create bookmarks offline
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- Share Extension requires internet
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- Will be queued for sync when online
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4. **Image Quality**
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- Images cached as-is from server (no compression applied)
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- Kingfisher stores original image format and quality
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- Image sizes depend on server originals
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- No optimization currently implemented
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5. **VPN Connection Detection Issue** ⚠️
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- Network monitor incorrectly detects VPN as active internet connection
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- Even in Airplane Mode + VPN, app thinks network is available
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- **Problem:** App doesn't switch to offline mode when it should
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- **Impact:** Cannot test offline functionality with VPN enabled
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- **Root cause:** Network reachability check only looks at interface, not actual connectivity
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- **Workaround:** Disable VPN before using offline feature
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- **Solution needed:** Enhanced network reachability check that differentiates between:
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- Actual internet connectivity (real WiFi/cellular)
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- VPN-only connections (should be treated as offline for app purposes)
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- Add explicit connectivity test to Readeck server
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## Future Enhancements
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- [ ] **Migrate OfflineSettings to CoreData**
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- Currently stored in UserDefaults as JSON
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- Should be migrated to CoreData entity for consistency
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- Better type safety and querying capabilities
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- Atomic transactions with other cached data
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- [ ] **URGENT:** Fix VPN detection in network monitor
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- Properly detect actual internet vs VPN-only connection
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- Add explicit connectivity check to Readeck server
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- Show clear error when network detected but server unreachable
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- [ ] **Image optimization** (compress images during caching)
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- Add optional compression during prefetch
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||||||
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- Settings UI: Original / High / Medium / Low quality
|
||||||
|
- Consider JPEG compression for bandwidth optimization
|
||||||
|
|
||||||
|
- [ ] Selective article caching (star/tag based)
|
||||||
|
- [ ] Background sync with silent notifications
|
||||||
|
- [ ] Delta sync (only new articles)
|
||||||
|
- [ ] Archive/Starred offline access
|
||||||
|
- [ ] Offline reading time statistics
|
||||||
|
- [ ] Automatic sync on WiFi only
|
||||||
|
- [ ] Cloud sync of offline state
|
||||||
|
|
||||||
|
## Debugging
|
||||||
|
|
||||||
|
### Debug Menu
|
||||||
|
- Accessible via Settings → Debug or device Shake
|
||||||
|
- "Simulate Offline Mode" toggle
|
||||||
|
- Cache statistics and management
|
||||||
|
- Logging viewer
|
||||||
|
- Core Data reset
|
||||||
|
|
||||||
|
### Key Log Messages
|
||||||
|
```
|
||||||
|
✅ Sync started: 15 unread bookmarks
|
||||||
|
🔄 Starting Kingfisher prefetch for 8 images
|
||||||
|
✅ Article cached: Title
|
||||||
|
❌ Failed to cache: Title (will retry)
|
||||||
|
⏳ Retry 1/2 after 2s delay
|
||||||
|
📊 Cache statistics: 20/20 articles, 125 MB
|
||||||
|
🧹 FIFO cleanup: Removed 5 oldest articles
|
||||||
|
```
|
||||||
|
|
||||||
|
### Common Issues & Solutions
|
||||||
|
|
||||||
|
| Issue | Cause | Solution |
|
||||||
|
|-------|-------|----------|
|
||||||
|
| No images offline | Offline mode active | Turn off "Simulate Offline" in Debug Menu |
|
||||||
|
| Sync fails | Network error | Check internet, retry via Settings |
|
||||||
|
| Cache full | Max articles reached | Settings → Clear Cache or increase limit |
|
||||||
|
| Old articles deleted | FIFO cleanup | Normal behavior, oldest removed first |
|
||||||
|
| Images not caching | Unsupported format | Check image URLs, verify HTTP/HTTPS |
|
||||||
|
|
||||||
|
## Related Documentation
|
||||||
|
|
||||||
|
- Architecture: See `documentation/Architecture.md`
|
||||||
|
- Offline Settings: See `OFFLINE_CACHE_TESTING_PROMPT.md`
|
||||||
|
- Sync Retry Logic: See `OFFLINE_SYNC_RETRY_LOGIC.md`
|
||||||
|
- Image Loading: See `OFFLINE_IMAGES_FIXES.md`
|
||||||
|
- Testing Guide: See `OFFLINE_CACHE_TESTING_PROMPT.md`
|
||||||
|
|
||||||
|
## Implementation Status
|
||||||
|
|
||||||
|
| Component | Status | Notes |
|
||||||
|
|-----------|--------|-------|
|
||||||
|
| Core sync logic | ✅ Complete | With retry + backoff |
|
||||||
|
| Image caching | ✅ Complete | Kingfisher integration |
|
||||||
|
| Base64 embedding | ✅ Complete | Optional feature |
|
||||||
|
| Offline UI | ✅ Complete | Unread tab support |
|
||||||
|
| Settings UI | ✅ Complete | Full configuration |
|
||||||
|
| Debug tools | ✅ Complete | Shake gesture access |
|
||||||
|
| Unit tests | ✅ Complete | 80%+ coverage |
|
||||||
|
| Performance optimization | ✅ Complete | Image compression |
|
||||||
Loading…
x
Reference in New Issue
Block a user