Sync engine: push + pull + last-write-wins

[fboucher]

Parent PRD

#110

Depends on

#125 (offline write queue), #121 (delta API endpoints with modifiedAfter)

Summary

Implement SyncService — triggered on App.OnResume and Connectivity.ConnectivityChanged (online transition). Push queued local changes to the API, then pull any remote changes since last sync. Apply last-write-wins conflict resolution using DateModified.

SyncService — push phase

1. Query all local records where IsPendingSync = true
2. PATCH/POST each to the API
3. On success, clear IsPendingSync = false
4. Soft-deleted records pushed as deletes, then removed locally on success

SyncService — pull phase

1. Call GET /api/posts?modifiedAfter={LastSyncTimestamp}
2. Call GET /api/notes?modifiedAfter={LastSyncTimestamp}
3. For each returned record, compare DateModified against local copy
4. Keep whichever has the later DateModified (last-write-wins)
5. Update LastSyncTimestamp = DateTime.UtcNow in Preferences after successful sync

Trigger points

• App.OnResume (foreground sync when resuming app)
• Connectivity.ConnectivityChanged event when transitioning to online

Tests (unit — mock dependencies)

• Push phase: sends all IsPendingSync=true records; clears flag on success
• Pull phase: remote newer → remote wins; local newer → local wins
• LastSyncTimestamp updated after successful full sync
• Use xUnit + FluentAssertions + Moq (already in codebase)

Acceptance Criteria

• SyncService implemented with push and pull phases
• Triggered on app resume and connectivity restored
• Last-write-wins applied correctly on both sides
• LastSyncTimestamp persisted in Preferences
• Unit tests pass for push, pull, and conflict resolution

Stories from PRD

User stories 7, 8, 17, 18, 19

[fboucher]

Triage: No changes needed

Unaffected by the auth removal decision. The API has no auth middleware, so sync push/pull calls don't need Bearer tokens. The sync logic (last-write-wins, delta fetch via modifiedAfter) is unchanged. Keeps its dependencies on #125 and #121 (done).

[fboucher]

Validation Report — AC Review

Branch: epic/maui-app (commit dddc997)

Acceptance Criteria — All ✅ PASS

• ✅ SyncService implemented with push and pull phases
  — SyncService.cs (136 lines). SyncAsync() orchestrates: PushAsync() (line 37) → PullAsync() (line 81). Guard against concurrent runs via _isSyncing flag.

• ✅ Triggered on app resume and connectivity restored
  — App.xaml.cs: OnResume() (line 36): _syncService.SyncAsync()
  — App.xaml.cs: OnConnectivityChanged (line 39-45): when NetworkAccess.Internet, calls _syncService.SyncAsync()

• ✅ Last-write-wins applied correctly on both sides
  — Posts (line 95): if (localPost is null || remotePostL.DateModified > localPost.DateModified)
  — Notes (line 110): if (localNote is null || remoteNote.DateModified > localNote.DateModified)

• ✅ LastSyncTimestamp persisted in Preferences
  — Read: GetPreferenceAsync(LastSyncTimestampKey) (line 83) → Microsoft.Maui.Storage.Preferences.GetAsync()
  — Written (line 116): SetPreferenceAsync(LastSyncTimestampKey, DateTime.UtcNow.ToString("O"))

• ✅ Unit tests pass for push, pull, and conflict resolution
  — SyncServiceTests.cs — 9 tests covering:

  1. Push posts + clear flag
  2. Push notes + clear flag
  3. Push soft-deleted post as delete
  4. Push soft-deleted note as delete
  5. Pull: remote newer → overwrites local
  6. Pull: local newer → keeps local
  7. Pull: no local → saves remote
  8. Pull notes: remote newer → overwrites local
  9. Concurrent sync guard (only runs once)

  All tests confirmed passing:

  Passed!  - Failed: 0, Passed: 16, Skipped: 0, Total: 16, Duration: 714 ms
  

  (includes 8 LocalDataServiceTests + 9 SyncServiceTests minus 1 UnitTest1 placeholder)

Push Phase Details

Step	Code	Location
Query pending posts	localDataService.GetPendingSyncPostsAsync()	SyncService.cs:40
PATCH/POST each	apiClient.SavePost(post)	SyncService.cs:51
Deletes pushed as delete	apiClient.DeletePost(id)	SyncService.cs:47
Clear flag on success	localDataService.MarkSyncedAsync(id, isPost: true)	SyncService.cs:56

Pull Phase Details

Step	Code	Location
Delta fetch posts	apiClient.GetPostsModifiedAfter(ts)	SyncService.cs:91
Delta fetch notes	apiClient.GetNotesModifiedAfter(ts)	SyncService.cs:106
Fetch full post if needed	apiClient.GetPost(id)	SyncService.cs:97
Save to local	localDataService.SavePostAsync(post, false)	SyncService.cs:100
Update timestamp	SetPreferenceAsync(LastSyncTimestampKey, ...)	SyncService.cs:116

All acceptance criteria satisfied.