Implement annotation syncing to and from server

lib/core/services/annotation_sync_service.dart

@@ -0,0 +1,118 @@
+import 'package:logging/logging.dart';
+
+import 'api_client.dart';
+import 'storage_service.dart';
+
+/// Service for synchronizing annotations between local storage and server.
+///
+/// Handles downloading annotations on sheet open and uploading on save,
+/// comparing timestamps to determine which version is newer.
+class AnnotationSyncService {
+  final _log = Logger('AnnotationSyncService');
+  final ApiClient _apiClient;
+  final StorageService _storageService;
+
+  AnnotationSyncService({
+    required ApiClient apiClient,
+    required StorageService storageService,
+  }) : _apiClient = apiClient,
+       _storageService = storageService;
+
+  /// Downloads annotations from server and merges with local storage.
+  ///
+  /// For each page, compares server's lastModified with local lastModified.
+  /// If server is newer, overwrites local. Local annotations that are newer
+  /// are preserved.
+  Future<void> syncFromServer(String sheetUuid) async {
+    try {
+      _log.info('Syncing annotations from server for sheet $sheetUuid');
+
+      // Fetch all annotations from server
+      final serverAnnotations = await _apiClient.fetchAnnotations(sheetUuid);
+
+      // Get all local annotations with metadata
+      final localAnnotations = await _storageService
+          .readAllAnnotationsWithMetadata(sheetUuid);
+
+      int updatedCount = 0;
+
+      // Process each server annotation
+      for (final serverAnnotation in serverAnnotations) {
+        final page = serverAnnotation.page;
+        final localAnnotation = localAnnotations[page];
+
+        bool shouldUpdate = false;
+
+        if (localAnnotation == null) {
+          // No local annotation - use server version
+          shouldUpdate = true;
+          _log.fine('Page $page: No local annotation, using server version');
+        } else if (serverAnnotation.lastModified.isAfter(
+          localAnnotation.lastModified,
+        )) {
+          // Server is newer - overwrite local
+          shouldUpdate = true;
+          _log.fine(
+            'Page $page: Server is newer '
+            '(server: ${serverAnnotation.lastModified}, '
+            'local: ${localAnnotation.lastModified})',
+          );
+        } else {
+          _log.fine(
+            'Page $page: Local is newer or same, keeping local version',
+          );
+        }
+
+        if (shouldUpdate) {
+          await _storageService.writeAnnotationsWithMetadata(
+            sheetUuid,
+            page,
+            serverAnnotation.annotationsJson,
+            serverAnnotation.lastModified,
+          );
+          updatedCount++;
+        }
+      }
+
+      _log.info(
+        'Sync complete: $updatedCount pages updated from server '
+        '(${serverAnnotations.length} total on server)',
+      );
+    } on ApiException catch (e) {
+      _log.warning('Failed to sync annotations from server: $e');
+    } catch (e) {
+      _log.warning('Unexpected error syncing annotations: $e');
+    }
+  }
+
+  /// Uploads a single page's annotation to the server.
+  ///
+  /// Called when annotations are saved (e.g., exiting paint mode).
+  /// Silently fails if upload fails (allows offline usage).
+  Future<bool> uploadAnnotation({
+    required String sheetUuid,
+    required int page,
+    required String annotationsJson,
+    required DateTime lastModified,
+  }) async {
+    try {
+      _log.info('Uploading annotation for sheet $sheetUuid page $page');
+
+      await _apiClient.uploadAnnotation(
+        sheetUuid: sheetUuid,
+        page: page,
+        lastModified: lastModified,
+        annotationsJson: annotationsJson,
+      );
+
+      _log.info('Upload successful');
+      return true;
+    } on ApiException catch (e) {
+      _log.warning('Failed to upload annotation: $e');
+      return false;
+    } catch (e) {
+      _log.warning('Unexpected error uploading annotation: $e');
+      return false;
+    }
+  }
+}

lib/core/services/api_client.dart

@@ -183,6 +183,61 @@ class ApiClient {
     _log.info('PDF cached at: ${cachedFile.path}');
     return cachedFile;
   }
+
+  // ---------------------------------------------------------------------------
+  // Annotation Operations
+  // ---------------------------------------------------------------------------
+
+  /// Fetches all annotations for a sheet from the server.
+  ///
+  /// Returns a list of [ServerAnnotation] objects containing page number,
+  /// lastModified timestamp, and the annotations JSON string.
+  Future<List<ServerAnnotation>> fetchAnnotations(String sheetUuid) async {
+    final response = await get('/api/sheets/$sheetUuid/annotations');
+    final data = jsonDecode(response.body) as List<dynamic>;
+
+    return data
+        .map((item) => ServerAnnotation.fromJson(item as Map<String, dynamic>))
+        .toList();
+  }
+
+  /// Uploads annotations for a specific page of a sheet.
+  ///
+  /// The [lastModified] should be the current time when the annotation was saved.
+  Future<void> uploadAnnotation({
+    required String sheetUuid,
+    required int page,
+    required DateTime lastModified,
+    required String annotationsJson,
+  }) async {
+    await post('/api/sheets/$sheetUuid/annotations', {
+      'page': page,
+      'lastModified': lastModified.toIso8601String(),
+      'annotations': annotationsJson,
+    });
+    _log.info('Annotation uploaded for sheet $sheetUuid page $page');
+  }
+}
+
+/// Represents an annotation from the server.
+class ServerAnnotation {
+  final int page;
+  final DateTime lastModified;
+  final String annotationsJson;
+
+  ServerAnnotation({
+    required this.page,
+    required this.lastModified,
+    required this.annotationsJson,
+  });
+
+  factory ServerAnnotation.fromJson(Map<String, dynamic> json) {
+    return ServerAnnotation(
+      page: json['page'] as int,
+      lastModified: DateTime.parse(json['lastModified'] as String),
+      annotationsJson: json['annotations'] as String,
+    );
+  }
 }
 
 /// Exception thrown when an API request fails.

lib/core/services/storage_service.dart

@@ -6,6 +6,29 @@ import 'package:sheetless/core/models/config.dart';
 /// Keys for secure storage (credentials and tokens).
 enum SecureStorageKey { url, jwt, email }
 
+/// Data class for storing annotations with metadata.
+class StoredAnnotation {
+  final String annotationsJson;
+  final DateTime lastModified;
+
+  StoredAnnotation({
+    required this.annotationsJson,
+    required this.lastModified,
+  });
+
+  Map<String, dynamic> toMap() => {
+        'annotationsJson': annotationsJson,
+        'lastModified': lastModified.toIso8601String(),
+      };
+
+  factory StoredAnnotation.fromMap(Map<dynamic, dynamic> map) {
+    return StoredAnnotation(
+      annotationsJson: map['annotationsJson'] as String,
+      lastModified: DateTime.parse(map['lastModified'] as String),
+    );
+  }
+}
+
 /// Service for managing local storage operations.
 ///
 /// Uses [FlutterSecureStorage] for sensitive data (credentials, tokens)
@@ -134,16 +157,70 @@ class StorageService {
   /// Returns the JSON string of annotations, or null if none exist.
   Future<String?> readAnnotations(String sheetUuid, int pageNumber) async {
     final box = await Hive.openBox(_annotationsBox);
-    return box.get(_annotationKey(sheetUuid, pageNumber));
+    final value = box.get(_annotationKey(sheetUuid, pageNumber));
+
+    // Handle legacy format (plain string) and new format (map with metadata)
+    if (value == null) return null;
+    if (value is String) return value;
+    if (value is Map) {
+      final stored = StoredAnnotation.fromMap(value);
+      return stored.annotationsJson;
+    }
+    return null;
+  }
+
+  /// Reads annotations with metadata for a specific sheet page.
+  ///
+  /// Returns [StoredAnnotation] with annotations and lastModified, or null if none exist.
+  Future<StoredAnnotation?> readAnnotationsWithMetadata(
+    String sheetUuid,
+    int pageNumber,
+  ) async {
+    final box = await Hive.openBox(_annotationsBox);
+    final value = box.get(_annotationKey(sheetUuid, pageNumber));
+
+    if (value == null) return null;
+
+    // Handle legacy format (plain string) - treat as very old
+    if (value is String) {
+      return StoredAnnotation(
+        annotationsJson: value,
+        lastModified: DateTime.fromMillisecondsSinceEpoch(0),
+      );
+    }
+
+    if (value is Map) {
+      return StoredAnnotation.fromMap(value);
+    }
+
+    return null;
   }
 
   /// Writes annotations for a specific sheet page.
   ///
   /// Pass null or empty string to delete annotations for that page.
+  /// Automatically sets lastModified to current time.
   Future<void> writeAnnotations(
     String sheetUuid,
     int pageNumber,
     String? annotationsJson,
+  ) async {
+    await writeAnnotationsWithMetadata(
+      sheetUuid,
+      pageNumber,
+      annotationsJson,
+      DateTime.now(),
+    );
+  }
+
+  /// Writes annotations with a specific lastModified timestamp.
+  ///
+  /// Used when syncing from server to preserve server's timestamp.
+  Future<void> writeAnnotationsWithMetadata(
+    String sheetUuid,
+    int pageNumber,
+    String? annotationsJson,
+    DateTime lastModified,
   ) async {
     final box = await Hive.openBox(_annotationsBox);
     final key = _annotationKey(sheetUuid, pageNumber);
@@ -151,7 +228,11 @@ class StorageService {
     if (annotationsJson == null || annotationsJson.isEmpty) {
       await box.delete(key);
     } else {
-      await box.put(key, annotationsJson);
+      final stored = StoredAnnotation(
+        annotationsJson: annotationsJson,
+        lastModified: lastModified,
+      );
+      await box.put(key, stored.toMap());
     }
   }
 
@@ -169,8 +250,54 @@ class StorageService {
         final pageNumber = int.tryParse(pageStr);
         if (pageNumber != null) {
           final value = box.get(key);
-          if (value != null && value is String && value.isNotEmpty) {
+          if (value != null) {
+            // Handle legacy format (plain string) and new format (map)
+            if (value is String && value.isNotEmpty) {
               result[pageNumber] = value;
+            } else if (value is Map) {
+              final stored = StoredAnnotation.fromMap(value);
+              if (stored.annotationsJson.isNotEmpty) {
+                result[pageNumber] = stored.annotationsJson;
+              }
+            }
+          }
+        }
+      }
+    }
+
+    return result;
+  }
+
+  /// Reads all annotations with metadata for a sheet (all pages).
+  ///
+  /// Returns a map of page number to [StoredAnnotation].
+  Future<Map<int, StoredAnnotation>> readAllAnnotationsWithMetadata(
+    String sheetUuid,
+  ) async {
+    final box = await Hive.openBox(_annotationsBox);
+    final prefix = '${sheetUuid}_page_';
+    final result = <int, StoredAnnotation>{};
+
+    for (final key in box.keys) {
+      if (key is String && key.startsWith(prefix)) {
+        final pageStr = key.substring(prefix.length);
+        final pageNumber = int.tryParse(pageStr);
+        if (pageNumber != null) {
+          final value = box.get(key);
+          if (value != null) {
+            StoredAnnotation? stored;
+            // Handle legacy format (plain string) and new format (map)
+            if (value is String && value.isNotEmpty) {
+              stored = StoredAnnotation(
+                annotationsJson: value,
+                lastModified: DateTime.fromMillisecondsSinceEpoch(0),
+              );
+            } else if (value is Map) {
+              stored = StoredAnnotation.fromMap(value);
+            }
+            if (stored != null && stored.annotationsJson.isNotEmpty) {
+              result[pageNumber] = stored;
+            }
           }
         }
       }

lib/features/sheet_viewer/sheet_viewer_page.dart

@@ -6,6 +6,7 @@ import 'package:logging/logging.dart';
 import 'package:pdfrx/pdfrx.dart';
 import 'package:sheetless/core/models/config.dart';
 import 'package:sheetless/core/models/sheet.dart';
+import 'package:sheetless/core/services/annotation_sync_service.dart';
 import 'package:sheetless/core/services/api_client.dart';
 import 'package:sheetless/core/services/storage_service.dart';
 
@@ -35,6 +36,7 @@ class _SheetViewerPageState extends State<SheetViewerPage>
     with FullScreenListener {
   final _log = Logger('SheetViewerPage');
   final _storageService = StorageService();
+  late final AnnotationSyncService _syncService;
 
   PdfDocument? _document;
   late Future<bool> _documentLoaded;
@@ -52,6 +54,12 @@ class _SheetViewerPageState extends State<SheetViewerPage>
   void initState() {
     super.initState();
 
+    // Initialize sync service
+    _syncService = AnnotationSyncService(
+      apiClient: widget.apiClient,
+      storageService: _storageService,
+    );
+
     // Initialize drawing controllers
     _leftDrawingController = DrawingController(maxHistorySteps: 50);
     _rightDrawingController = DrawingController(maxHistorySteps: 50);
@@ -94,6 +102,9 @@ class _SheetViewerPageState extends State<SheetViewerPage>
       _totalPages = _document!.pages.length;
     });
 
+    // Sync annotations from server (downloads newer versions)
+    await _syncService.syncAnnotationsFromServer(widget.sheet.uuid);
+
     // Load annotations for current page(s)
     await _loadAnnotationsForCurrentPages();
 
@@ -131,24 +142,52 @@ class _SheetViewerPageState extends State<SheetViewerPage>
     }
   }
 
-  /// Saves the current page(s) annotations to storage.
+  /// Saves the current page(s) annotations to storage and uploads to server.
   Future<void> _saveCurrentAnnotations() async {
+    final now = DateTime.now();
+
     // Save left page
     final leftJson = _leftDrawingController.toJsonString();
-    await _storageService.writeAnnotations(
+    final leftHasContent = leftJson.isNotEmpty && leftJson != '[]';
+
+    await _storageService.writeAnnotationsWithMetadata(
       widget.sheet.uuid,
       _currentPage,
-      leftJson.isEmpty || leftJson == '[]' ? null : leftJson,
+      leftHasContent ? leftJson : null,
+      now,
     );
 
+    // Upload left page to server
+    if (leftHasContent) {
+      _syncService.uploadAnnotation(
+        sheetUuid: widget.sheet.uuid,
+        page: _currentPage,
+        annotationsJson: leftJson,
+        lastModified: now,
+      );
+    }
+
     // Save right page (two-page mode)
     if (widget.config.twoPageMode && _currentPage < _totalPages) {
       final rightJson = _rightDrawingController.toJsonString();
-      await _storageService.writeAnnotations(
+      final rightHasContent = rightJson.isNotEmpty && rightJson != '[]';
+
+      await _storageService.writeAnnotationsWithMetadata(
         widget.sheet.uuid,
         _currentPage + 1,
-        rightJson.isEmpty || rightJson == '[]' ? null : rightJson,
+        rightHasContent ? rightJson : null,
+        now,
       );
+
+      // Upload right page to server
+      if (rightHasContent) {
+        _syncService.uploadAnnotation(
+          sheetUuid: widget.sheet.uuid,
+          page: _currentPage + 1,
+          annotationsJson: rightJson,
+          lastModified: now,
+        );
+      }
     }
   }