Fix CollectionView scroll behaviour when section is empty

CHANGELOG.md

@@ -14,6 +14,7 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 - Prevent a crash when update queued immediately after item batch update. [Ryan Nystrom](https://github.com/rnystrom) (tbd)
 - Return correct `-[IGListAdapter visibleSectionControllers]` when section has no items, but has supplementary views. [Mani Ghasemlou](https://github.com/manicakes) [(#643)](https://github.com/Instagram/IGListKit/issues/643)
 - Call `[CATransaction commit]` before calling completion block in IGListAdapterUpdater to prevent animation issues. [Maxime Ollivier](https://github.com/maxoll) (tbd)
+- Fix `scrollToObject:supplementaryKinds:...` not scrolling when section is empty but does have supplymentary views
 
 ### Enhancements
 

Source/IGListAdapter.m

@@ -172,21 +172,33 @@ - (void)scrollToObject:(id)object
     }
 
     UICollectionView *collectionView = self.collectionView;
-    const NSInteger numberOfItems = [collectionView numberOfItemsInSection:section];
-    if (numberOfItems == 0) {
-        return;
-    }
-
+    UICollectionViewLayout *layout = self.collectionView.collectionViewLayout;
+
     // force layout before continuing
     // this method is typcially called before pushing a view controller
     // thus, before the layout process has actually happened
     [collectionView layoutIfNeeded];
 
+    NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:section];
+
     // collect the layout attributes for the cell and supplementary views for the first index
     // this will break if there are supplementary views beyond item 0
-    NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:section];
-    NSArray *attributes = [self layoutAttributesForIndexPath:indexPath supplementaryKinds:supplementaryKinds];
-
+    NSArray *attributes = nil;
+
+    const NSInteger numberOfItems = [collectionView numberOfItemsInSection:section];
+    if (numberOfItems > 0) {
+        attributes = [self layoutAttributesForIndexPath:indexPath supplementaryKinds:supplementaryKinds];
+    } else {
+        NSMutableArray *supplementaryAttributes = [NSMutableArray new];
+        for (NSString* supplementaryKind in supplementaryKinds) {
+            UICollectionViewLayoutAttributes *supplementaryAttribute = [layout layoutAttributesForSupplementaryViewOfKind:supplementaryKind atIndexPath:indexPath];
+            if (supplementaryAttribute != nil) {
+                [supplementaryAttributes addObject: supplementaryAttribute];
+            }
+        }
+        attributes = supplementaryAttributes;
+    }
+
     CGFloat offsetMin = 0.0;
     CGFloat offsetMax = 0.0;
     for (UICollectionViewLayoutAttributes *attribute in attributes) {
@@ -715,7 +727,6 @@ - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL
     }
 }
 
-
 #pragma mark - IGListCollectionContext
 
 - (CGSize)containerSize {

Tests/IGListAdapterTests.m

@@ -652,6 +652,43 @@ - (void)test_whenScrollVerticallyToItem {
     IGAssertEqualPoint([self.collectionView contentOffset], 0, 60);
 }
 
+- (void)test_whenScrollVerticallyToItemInASectionWithNoCellsAndNoSupplymentaryView {
+    self.dataSource.objects = @[@1, @0, @300];
+    [self.adapter reloadDataWithCompletion:nil];
+    XCTAssertEqual([self.collectionView numberOfSections], 3);
+    [self.adapter scrollToObject:@1 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 0);
+    [self.adapter scrollToObject:@0 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 0);
+    [self.adapter scrollToObject:@300 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 10);
+}
+
+- (void)test_whenScrollVerticallyToItemInASectionWithNoCellsButAHeaderSupplymentaryView {
+    self.dataSource.objects = @[@1, @0, @300];
+    [self.adapter reloadDataWithCompletion:nil];
+
+    IGTestSupplementarySource *supplementarySource = [IGTestSupplementarySource new];
+    supplementarySource.collectionContext = self.adapter;
+    supplementarySource.supportedElementKinds = @[UICollectionElementKindSectionHeader];
+
+    IGListSectionController *controller = [self.adapter sectionControllerForObject:@0];
+    controller.supplementaryViewSource = supplementarySource;
+    supplementarySource.sectionController = controller;
+
+    [self.adapter performUpdatesAnimated:NO completion:nil];
+
+    XCTAssertEqual([self.collectionView numberOfSections], 3);
+    [self.adapter scrollToObject:@1 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 0);
+    [self.adapter scrollToObject:@0 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 0);
+    [self.adapter scrollToObject:@0 supplementaryKinds:@[UICollectionElementKindSectionHeader] scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionTop animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 10);
+    [self.adapter scrollToObject:@300 supplementaryKinds:nil scrollDirection:UICollectionViewScrollDirectionVertical scrollPosition:UICollectionViewScrollPositionNone animated:NO];
+    IGAssertEqualPoint([self.collectionView contentOffset], 0, 20);
+}
+
 - (void)test_whenScrollHorizontallyToItem {
     // # of items for each object == [item integerValue], so @2 has 2 items (cells)
     IGListTestAdapterHorizontalDataSource *dataSource = [[IGListTestAdapterHorizontalDataSource alloc] init];