Don't require the delegate to implement collectionView:itemAtIndexPath:willMoveToIndexPath:

Author: Zach Drayer

LXReorderableCollectionViewFlowLayout/LXReorderableCollectionViewFlowLayout.h

@@ -20,10 +20,9 @@
 
 @protocol LXReorderableCollectionViewDataSource <UICollectionViewDataSource>
 
-- (void)collectionView:(UICollectionView *)collectionView itemAtIndexPath:(NSIndexPath *)fromIndexPath willMoveToIndexPath:(NSIndexPath *)toIndexPath;
-
 @optional
 
+- (void)collectionView:(UICollectionView *)collectionView itemAtIndexPath:(NSIndexPath *)fromIndexPath willMoveToIndexPath:(NSIndexPath *)toIndexPath;
 - (BOOL)collectionView:(UICollectionView *)collectionView canMoveItemAtIndexPath:(NSIndexPath *)indexPath;
 - (BOOL)collectionView:(UICollectionView *)collectionView itemAtIndexPath:(NSIndexPath *)fromIndexPath canMoveToIndexPath:(NSIndexPath *)toIndexPath;
 

LXReorderableCollectionViewFlowLayout/LXReorderableCollectionViewFlowLayout.m

@@ -154,8 +154,10 @@ - (void)invalidateLayoutIfNecessary {
 
     self.selectedItemIndexPath = newIndexPath;
 
-    [self.dataSource collectionView:self.collectionView itemAtIndexPath:previousIndexPath willMoveToIndexPath:newIndexPath];
-    
+    if ([self.dataSource respondsToSelector:@selector(collectionView:itemAtIndexPath:willMoveToIndexPath:)]) {
+        [self.dataSource collectionView:self.collectionView itemAtIndexPath:previousIndexPath willMoveToIndexPath:newIndexPath];
+    }
+
     __weak typeof(self) weakSelf = self;
     [self.collectionView performBatchUpdates:^{
         __strong typeof(self) strongSelf = weakSelf;