Fix re-entrance + caching when using IGListCollectionViewLayout

Summary:
The issue I've been seeing is that list kit would not recalculate layout for when collection view bounds would change

IGListCollectionViewLayout has code to invalidate layout when collection view size chances however it would not happen without explicitly calling `….collectionViewLayout invalidateLayout];`

It appears that in the loop of calculating the layout we would accidentally call the code that would start caching existing layout data (not calculated at the point of call) and cache the value

This does not seem to happen all the time, however moving the cache purge to the end of layout calculation loop seems reasonable since at this point we have the new values that can't be known to the cache

As long as the consumer uses `IGListCollectionViewLayout` explicitly or implicitly through convenience initialize of `IGListCollectionView` this fix should work as is

Reviewed By: fabiomassimo

Differential Revision: D50197736

fbshipit-source-id: cb6d818f8be965f2e6ba494ffea083a6ab35682d

Source/IGListKit/IGListCollectionViewLayout.mm

@@ -474,11 +474,7 @@ - (NSString *)_classNameForDelegate:(id<UICollectionViewDelegateFlowLayout>)dele
 - (void)_calculateLayoutIfNeeded {
     if (_minimumInvalidatedSection == NSNotFound) {
         return;
-    }
-
-    // purge attribute caches so they are rebuilt
-    [_attributesCache removeAllObjects];
-    [self _resetSupplementaryAttributesCache];
+    }    
 
     UICollectionView *collectionView = self.collectionView;
     id<UICollectionViewDelegateFlowLayout> delegate = (id<UICollectionViewDelegateFlowLayout>)collectionView.delegate;
@@ -540,6 +536,8 @@ - (void)_calculateLayoutIfNeeded {
 
         for (NSInteger item = 0; item < itemCount; item++) {
             NSIndexPath *indexPath = [NSIndexPath indexPathForItem:item inSection:section];
+            // Following method subsequentally calls -layoutAttributesForItemAtIndexPath: and caches attributes that are not ready yet (we only calculate them at the end of this for loop)
+            // This results in _attributesCache[indexPath] being set to incorrect value. If we end up calling prepareLayout in response to frame change we
             const CGSize size = [delegate collectionView:collectionView layout:self sizeForItemAtIndexPath:indexPath];
 
             IGAssert(CGSizeGetLengthInDirection(size, fixedDirection) <= paddedLengthInFixedDirection
@@ -656,6 +654,12 @@ - (void)_calculateLayoutIfNeeded {
         _sectionData[section].lastNextRowCoordInScrollDirection = nextRowCoordInScrollDirection;
     }
 
+    // Reason we are purging attributes at the end is because in some circumstances calling
+    // -[delegate collectionView: layout: sizeForItemAtIndexPath:] results in creating the cache with incorrect values
+    // See the comment next to the call for more information
+    [_attributesCache removeAllObjects];
+    [self _resetSupplementaryAttributesCache];
+
     _minimumInvalidatedSection = NSNotFound;
 }