fix supplementary view crash when using IGListCollectionViewLayout

Summary:
* Issue: Calling `[UICollectionView layoutAttributesForSupplementaryElementOfKind...]` with an indexPath that doesn't have a supplimentary view.
* Cause: `IGListCollectionViewLayout` always returns a non-nil `UICollectionViewLayoutAttributes` when calling `layoutAttributesForSupplementaryViewOfKind` which tells the `UICollectionView` that it's fair game to ask the dataSource for a supplementary view at the indexPath. But when it does, the section controller could return nil, which throws an expection.

In the Apple docs about `[UICollectionViewDataSource collectionView:viewForSupplementaryElementOfKind:atIndexPath:]`

> This method must always return a valid view object. If you do not want a supplementary view in a particular case, your layout object should not create the attributes for that view.

https://developer.apple.com/documentation/uikit/uicollectionviewdatasource/1618037-collectionview?language=objc

* Fix: Just like `UICollectionViewFlowLayout`, if the supplementary view size is empty, lets return a nil attribute.

Reviewed By: Ziewvater

Differential Revision: D17326461

fbshipit-source-id: 507e98f43e951216cf2eafe2449f87df25439e11

CHANGELOG.md

@@ -51,6 +51,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Fixed logic flaw in `[IGListCollectionViewLayout shouldInvalidateLayoutForBoundsChange:]`. [Allen Hsu](https://github.com/allenhsu) (tbd)
 
+- Fixed crash when calling `[UICollectionView layoutAttributesForSupplementaryElementOfKind...]` with `IGListCollectionViewLayout` and the section controller doesn't actually return a supplementary view [Maxime Ollivier](https://github.com/maxolls) (tbd)
+
 3.4.0
 -----
 

Source/IGListCollectionViewLayout.mm

@@ -251,7 +251,7 @@ - (UICollectionViewLayoutAttributes *)finalLayoutAttributesForDisappearingItemAt
                 // do not add zero height headers/footers or headers/footers that are outside the rect
                 const CGRect frame = attributes.frame;
                 const CGRect intersection = CGRectIntersection(frame, rect);
-                if (!CGRectIsEmpty(intersection) && CGRectGetLengthInDirection(frame, self.scrollDirection) > 0.0) {
+                if (attributes && !CGRectIsEmpty(intersection) && CGRectGetLengthInDirection(frame, self.scrollDirection) > 0.0) {
                     [result addObject:attributes];
                 }
             }
@@ -339,13 +339,19 @@ - (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind
     } else if ([elementKind isEqualToString:UICollectionElementKindSectionFooter]) {
         frame = entry.footerBounds;
     }
-
-    attributes = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:elementKind withIndexPath:indexPath];
-    attributes.frame = frame;
-    adjustZIndexForAttributes(attributes);
-    _supplementaryAttributesCache[elementKind][indexPath] = attributes;
-
-    return attributes;
+
+    if (CGRectIsEmpty(frame)) {
+        // Just like UICollectionViewFlowLayout, if the header/footer size is empty, do not not return an attribute.
+        // If we did return something, calling [UICollectionView layoutAttributesForSupplementaryElementOfKind...] would too,
+        // which could then crash if the UICollectionViewDelegate is not expecting to actually return a supplimentary view.
+        return nil;
+    } else {
+        attributes = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:elementKind withIndexPath:indexPath];
+        attributes.frame = frame;
+        adjustZIndexForAttributes(attributes);
+        _supplementaryAttributesCache[elementKind][indexPath] = attributes;
+        return attributes;
+    }
 }
 
 - (CGSize)collectionViewContentSize {

Tests/IGListCollectionViewLayoutTests.m

@@ -1167,6 +1167,24 @@ - (void)test_whenQueryingAttributes_withItemOOB_thatReturnsNil {
     XCTAssertNil([self.layout layoutAttributesForItemAtIndexPath:genIndexPath(0, 4)]);
 }
 
+- (void)test_whenQueryingSupplementaryAttributes_withSizeEmpty_thatReturnsNil {
+    [self setUpWithStickyHeaders:NO topInset:0 stretchToEdge:YES];
+
+    [self prepareWithData:@[
+                            [[IGLayoutTestSection alloc] initWithInsets:UIEdgeInsetsZero
+                                                            lineSpacing:0
+                                                       interitemSpacing:0
+                                                           headerHeight:0
+                                                           footerHeight:0
+                                                                  items:@[
+                                                                          [[IGLayoutTestItem alloc] initWithSize:CGSizeMake(33, 33)],
+                                                                          ]],
+                            ]];
+
+    XCTAssertNil([self.layout layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:genIndexPath(0, 0)]);
+    XCTAssertNil([self.layout layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionFooter atIndexPath:genIndexPath(0, 0)]);
+}
+
 - (void)test_whenUpdatingSizes_thatLayoutUpdates {
     [self setUpWithStickyHeaders:NO topInset:0];