[offline] Expose last successful sync date

Author: Clément Le Provost

ChangeLog.md

@@ -1,6 +1,13 @@
 Change Log
 ==========
 
+## Next
+
+### Offline mode
+
+- Expose last successful sync date (property `MirroredIndex.lastSuccessfulSyncDate`)
+
+
 ## 3.5 (2016-07-29)
 
 - New `multipleQueries()` method at index level

Source/Offline/MirroredIndex.swift

@@ -145,6 +145,11 @@ import Foundation
     /// Minimum delay between two syncs.
     @objc public var delayBetweenSyncs: NSTimeInterval = DefaultDelayBetweenSyncs
 
+    /// Date of the last successful sync, or nil if the index has never been successfully synced.
+    @objc public var lastSuccessfulSyncDate: NSDate? {
+        return mirrorSettings.lastSyncDate
+    }
+    
     /// Error encountered by the current/last sync (if any).
     @objc public private(set) var syncError : NSError?