Adds viewForSupplementaryElementOfKind method to IGListCollectionContext

Summary:
Returns the supplementary view in the collection at the specified index for the section controller.

 param `elementKind` The element kind of the supplementary view.
 param `index` The index of the desired cell.
 param `sectionController` The section controller requesting this information.

 return The collection reusable view, or `nil` if not found.

 warning This method may return `nil` if the cell is offscreen.

Reviewed By: maxolls

Differential Revision: D42726375

fbshipit-source-id: d9873440bd94140b88e7d2d56c0737ecf94925a8

CHANGELOG.md

@@ -60,6 +60,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 ```
 
 ### Enhancements
+- Added `viewForSupplementaryElementOfKind:atIndex:sectionController:` to `IGListAdapter`. [ryanmathews]
+
 - Added `shouldDeselectItemAtIndex:` to `IGListSectionController` . [bladeofky](https://github.com/bladeofky)
 
 - Added `shouldSelectItemAtIndex:` to `IGListSectionController` . [dirtmelon](https://github.com/dirtmelon)

Source/IGListKit/IGListAdapter.m

@@ -1051,6 +1051,37 @@ typedef struct OffsetRange {
     return nil;
 }
 
+- (__kindof UICollectionReusableView *)viewForSupplementaryElementOfKind:(NSString *)elementKind
+                                                                 atIndex:(NSInteger)index
+                                                       sectionController:(IGListSectionController *)sectionController {
+    IGAssertMainThread();
+    IGParameterAssert(sectionController != nil);
+
+    // if this is accessed while a cell is being dequeued or displaying working range elements, just return nil
+    if (_isDequeuingSupplementaryView || _isSendingWorkingRangeDisplayUpdates) {
+        return nil;
+    }
+
+    NSIndexPath *indexPath = [self indexPathForSectionController:sectionController index:index usePreviousIfInUpdateBlock:YES];
+    // prevent querying the collection view if it isn't fully reloaded yet for the current data set
+    if (indexPath != nil
+        && indexPath.section < [self.collectionView numberOfSections]) {
+        // only return a supplementary view if it belongs to the section controller
+        UICollectionReusableView *view = [self.collectionView supplementaryViewForElementKind:elementKind atIndexPath:indexPath];
+
+        if (IGListExperimentEnabled(_experiments, IGListExperimentSkipViewSectionControllerMap)) {
+            if ([self sectionControllerForSection:indexPath.section] == sectionController) {
+                return view;
+            }
+        } else {
+            if ([self sectionControllerForView:view] == sectionController) {
+                return view;
+            }
+        }
+    }
+    return nil;
+}
+
 - (NSArray<UICollectionViewCell *> *)fullyVisibleCellsForSectionController:(IGListSectionController *)sectionController {
     const NSInteger section = [self sectionForSectionController:sectionController];
     if (section == NSNotFound) {

Source/IGListKit/IGListCollectionContext.h

@@ -94,6 +94,21 @@ NS_SWIFT_NAME(ListCollectionContext)
 - (nullable __kindof UICollectionViewCell *)cellForItemAtIndex:(NSInteger)index
                                              sectionController:(IGListSectionController *)sectionController;
 
+/**
+ Returns the supplementary view in the collection at the specified index for the section controller.
+
+ @param elementKind The element kind of the supplementary view.
+ @param index The index of the desired cell.
+ @param sectionController The section controller requesting this information.
+
+ @return The collection reusable view, or `nil` if not found.
+
+ @warning This method may return `nil` if the cell is offscreen.
+ */
+- (nullable __kindof UICollectionReusableView *)viewForSupplementaryElementOfKind:(NSString *)elementKind
+                                                                          atIndex:(NSInteger)index
+                                                                sectionController:(IGListSectionController *)sectionController;
+
 /**
  Returns the fully visible cells for the given section controller.
 

Source/IGListKit/Internal/IGListAdapter+UICollectionView.m

@@ -61,7 +61,12 @@
 - (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
     IGListSectionController *sectionController = [self sectionControllerForSection:indexPath.section];
     id <IGListSupplementaryViewSource> supplementarySource = [sectionController supplementaryViewSource];
+
+    // flag that a supplementary view is being dequeued in case it tries to access a supplementary view in the process
+    _isDequeuingSupplementaryView = YES;
     UICollectionReusableView *view = [supplementarySource viewForSupplementaryElementOfKind:kind atIndex:indexPath.item];
+    _isDequeuingSupplementaryView = NO;
+
     IGAssert(view != nil, @"Returned a nil supplementary view at indexPath <%@> from section controller: <%@>, supplementary source: <%@>", indexPath, sectionController, supplementarySource);
 
     // associate the section controller with the cell so that we know which section controller is using it

Source/IGListKit/Internal/IGListAdapterInternal.h

@@ -30,6 +30,8 @@ IGListBatchContext
 {
     __weak UICollectionView *_collectionView;
     BOOL _isDequeuingCell;
+    BOOL _isDequeuingSupplementaryView;
+
     BOOL _isSendingWorkingRangeDisplayUpdates;
 }
 

Tests/IGListAdapterTests.m

@@ -2146,4 +2146,20 @@
     [sectionController.collectionContext invalidateLayoutForSectionController:sectionController completion:nil];
 }
 
+- (void)test_whenSettingSupplementaryView_thatViewForSupplementaryElementExists {
+    self.dataSource.objects = @[@0];
+    [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];
+    XCTAssertNotNil([self.adapter viewForSupplementaryElementOfKind:UICollectionElementKindSectionHeader atIndex:0 sectionController:controller]);
+}
+
 @end