Merge pull request #13 from scribd/stephane/safety_check

Add delta safety check.

Author: Stephane Magne

Sources/LiveCollections/Classes/Internal/ItemDataCalculator.swift

@@ -204,8 +204,14 @@ private extension ItemDataCalculator {
             }
             self?._performNextCalculation()
         }
-        
-        guard delta.hasChanges else {
+
+        let isDeltaAccurate: Bool = (itemProvider.items.count + delta.insertions.count - delta.deletions.count) == updatedItems.count
+
+        if isDeltaAccurate == false {
+            calculationDelegate?.inaccurateDeltaDetected(delta)
+        }
+
+        guard delta.hasChanges, isDeltaAccurate else {
             updateData()
             calculationCompletion()
             return

Sources/LiveCollections/Classes/Internal/SectionDataCalculator.swift

@@ -274,8 +274,16 @@ private extension SectionDataCalculator {
                 }
                 return
             }
-            
-            guard sectionDelta.hasChanges || itemDelta.hasChanges else {
+
+            let currentItemCount: Int = sectionProvider.sections.reduce(0) { $0 + $1.items.count }
+            let updatedItemCount: Int = updatedSections.reduce(0) { $0 + $1.items.count }
+            let isDeltaAccurate: Bool = (currentItemCount + itemDelta.insertions.count - itemDelta.deletions.count) == updatedItemCount
+
+            if isDeltaAccurate == false {
+                self.calculationDelegate?.inaccurateDeltaDetected(itemDelta)
+            }
+
+            guard (sectionDelta.hasChanges || itemDelta.hasChanges), isDeltaAccurate else {
                 sectionProvider.calculatingSections = nil
                 calculationCompletion()
                 return // don't need to update with no changes

Sources/LiveCollections/Classes/Public/Delegate/CollectionDataDelegate.swift

@@ -65,6 +65,7 @@ public protocol CollectionDataCalculationNotificationDelegate: AnyObject {
 
     func collectionDataDidBeginCalculating()
     func collectionDataDidEndCalculating()
+    func inaccurateDeltaDetected(_ delta: IndexDelta)
 }
 
 // MARK: CollectionDataAnimationDelegate