Fix visible section controllers bug (#643)

Summary:
Issue fixed: #643

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes #868

Reviewed By: manicakes

Differential Revision: D5553538

Pulled By: rnystrom

fbshipit-source-id: 25d98a28d47a15d494ba530c850e0335d87d758c

CHANGELOG.md

@@ -8,6 +8,7 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 ### Fixes
 
 - 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)
 
 ### Enhancements
 

Source/IGListAdapter.m

@@ -424,11 +424,12 @@ - (NSArray *)objects {
 
 - (NSArray<IGListSectionController *> *)visibleSectionControllers {
     IGAssertMainThread();
-    NSArray<UICollectionViewCell *> *visibleCells = [self.collectionView visibleCells];
     NSMutableSet *visibleSectionControllers = [NSMutableSet new];
-    for (UICollectionViewCell *cell in visibleCells) {
-        IGListSectionController *sectionController = [self sectionControllerForView:cell];
-        IGAssert(sectionController != nil, @"Section controller nil for cell %@", cell);
+    NSArray<UICollectionViewLayoutAttributes *> *attributes =
+    [self.collectionView.collectionViewLayout layoutAttributesForElementsInRect:self.collectionView.bounds];
+    for (UICollectionViewLayoutAttributes* attribute in attributes) {
+        IGListSectionController *sectionController = [self sectionControllerForSection:attribute.indexPath.section];
+        IGAssert(sectionController != nil, @"Section controller nil for cell in section %zd", attribute.indexPath.section);
         if (sectionController) {
             [visibleSectionControllers addObject:sectionController];
         }

Tests/IGListAdapterTests.m

@@ -262,6 +262,23 @@ - (void)test_whenCellsExtendBeyondBounds_thatVisibleSectionControllersAreLimited
     XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@4]]);
 }
 
+- (void) test_withEmptySectionPlusFooter_thatVisibleSectionControllersAreCorrect {
+    self.dataSource.objects = @[@0];
+    [self.adapter reloadDataWithCompletion:nil];
+    IGTestSupplementarySource *supplementarySource = [IGTestSupplementarySource new];
+    supplementarySource.dequeueFromNib = YES;
+    supplementarySource.collectionContext = self.adapter;
+    supplementarySource.supportedElementKinds = @[UICollectionElementKindSectionFooter];
+    IGListSectionController *controller = [self.adapter sectionControllerForObject:@0];
+    controller.supplementaryViewSource = supplementarySource;
+    supplementarySource.sectionController = controller;
+    [self.adapter performUpdatesAnimated:NO completion:nil];
+    NSArray<IGListSectionController *> *visibleSectionControllers = [self.adapter visibleSectionControllers];
+
+    XCTAssertTrue([visibleSectionControllers count] == 1);
+    XCTAssertTrue(visibleSectionControllers.firstObject.supplementaryViewSource == supplementarySource);
+}
+
 - (void)test_whenCellsExtendBeyondBounds_thatVisibleCellsExistForSectionControllers {
     self.dataSource.objects = @[@2, @3, @4, @5, @6];
     [self.adapter reloadDataWithCompletion:nil];