Fix NSInternalInconsistencyException related to the number of items in the collection view when using FUIFirestoreCollectionViewDataSource

Author: tejasd

FirebaseFirestoreUI/FUIFirestoreCollectionViewDataSource.m

@@ -24,6 +24,15 @@ @interface FUIFirestoreCollectionViewDataSource () <FUIBatchedArrayDelegate>
 
 @property (nonatomic, readonly, nonnull) FUIBatchedArray *collection;
 
+// We need to maintain our own count property because the self.collection.count is sometimes
+// inconsistent with how the -[UICollectionView performBatchUpdates:] method expects it to work.
+// -[UICollectionView performBatchUpdates:] calls -[UICollectionViewDataSource count] at the
+// beginning and at the end of the method, and if the value of -[UICollectionViewDataSource count]
+// after -[UICollectionView performBatchUpdates:] is not equal to the value of
+// -[UICollectionViewDataSource count] before + (rows added - rows deleted) it throws an
+// NSInternalInconsistencyException.
+@property (nonatomic) NSUInteger count;
+
 /**
  * The callback to populate a subclass of UICollectionViewCell with an object
  * provided by the datasource.
@@ -46,6 +55,7 @@ - (instancetype)initWithCollection:(FUIBatchedArray *)collection
  _collection = collection;
  _collection.delegate = self;
  _populateCellAtIndexPath = populateCell;
+ _count = 0;
  }
  return self;
 }
@@ -58,10 +68,6 @@ - (instancetype)initWithQuery:(FIRQuery *)query
  return [self initWithCollection:array populateCell:populateCell];
 }
 
-- (NSUInteger)count {
- return self.collection.count;
-}
-
 - (NSArray<FIRDocumentSnapshot *> *)items {
  return self.collection.items;
 }
@@ -131,7 +137,9 @@ - (void)batchedArray:(FUIBatchedArray *)array
  [insertedIndexPaths addObject:inserted];
  }
  [self.collectionView insertItemsAtIndexPaths:insertedIndexPaths];
-
+ 
+ self.count += insertedIndexPaths.count;
+ self.count -= deletedIndexPaths.count;
  } completion:^(BOOL finished) {
  // Reload paths that have been moved.
  NSMutableArray *movedIndexPaths =
@@ -170,7 +178,7 @@ - (NSInteger)numberOfSectionsInCollectionView:(nonnull UICollectionView *)collec
 
 - (NSInteger)collectionView:(nonnull UICollectionView *)collectionView
  numberOfItemsInSection:(NSInteger)section {
- return self.collection.count;
+ return self.count;
 }
 
 @end