Fixes crash when accessing a cell within working range updates

Summary:
I was able to build a unit test that reproduces the issue. We can avoid the crash by simply returning `nil` when accessing a cell while working range events are being vended.

There is definitely something weird going on here though. When debugging `cellForItemAtIndexPath:` I found:

```
(lldb) po indexPath
<NSIndexPath: 0xc000000000000516> {length = 2, path = 5 - 0}
(lldb) po [[self collectionView] numberOfSections]
11
```

So in theory we should be fine, right? But when I continue I get

```
*** Assertion failure in -[UICollectionViewData numberOfItemsBeforeSection:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit_Sim/UIKit-3600.5.2/UICollectionViewData.m:611
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'request for number of items before section 5 when there are only 3 sections in the collection view'
```

There _were_ 3 sections in the UICV before the update, but the data source and structure powering
Closes #216

Differential Revision: D4204625

Pulled By: rnystrom

fbshipit-source-id: 455ed199dfc115077e4294e2843016a50e179015

CHANGELOG.md

@@ -28,6 +28,10 @@ This release closes the [2.0.0 milestone](https://github.com/Instagram/IGListKit
 - Added new `-[IGListAdapter visibleObjects]` API. [Ryan Nystrom](https://github.com/rnystrom) [(386ae07)](https://github.com/Instagram/IGListKit/commit/386ae0786445c06e1eabf074a4181614332f155f)
 - Added new `-[IGListAdapter objectForSectionController:]` API. [Ayush Saraswat](https://github.com/saraswatayu) [(#204)](https://github.com/Instagram/IGListKit/pull/204)
 
+### Fixes
+
+- Prevent `UICollectionView` bug when accessing a cell during working range updates. [Ryan Nystrom](https://github.com/rnystrom) [(#216)](https://github.com/Instagram/IGListKit/pull/216)
+
 1.0.0
 -----
 

Source/IGListAdapter.m

@@ -19,6 +19,7 @@
 @implementation IGListAdapter {
     NSMapTable<UICollectionViewCell *, IGListSectionController<IGListSectionType> *> *_cellSectionControllerMap;
     BOOL _isDequeuingCell;
+    BOOL _isSendingWorkingRangeDisplayUpdates;
 }
 
 - (void)dealloc {
@@ -613,7 +614,10 @@ - (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICol
 
     id object = [self.sectionMap objectForSection:indexPath.section];
     [self.displayHandler willDisplayCell:cell forListAdapter:self sectionController:sectionController object:object indexPath:indexPath];
+
+    _isSendingWorkingRangeDisplayUpdates = YES;
     [self.workingRangeHandler willDisplayItemAtIndexPath:indexPath forListAdapter:self];
+    _isSendingWorkingRangeDisplayUpdates = NO;
 }
 
 - (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath {
@@ -691,8 +695,8 @@ - (__kindof UICollectionViewCell *)cellForItemAtIndex:(NSInteger)index
     IGAssertMainThread();
     IGParameterAssert(sectionController != nil);
 
-    // if this is accessed while a cell is being dequeued, just return nil
-    if (_isDequeuingCell) {
+    // if this is accessed while a cell is being dequeued or displaying working range elements, just return nil
+    if (_isDequeuingCell || _isSendingWorkingRangeDisplayUpdates) {
         return nil;
     }
 

Tests/IGListAdapterE2ETests.m

@@ -1022,4 +1022,37 @@ - (void)test_whenBatchUpdating_withDuplicateIdentifiers_thatHaveDifferentValues_
     [self waitForExpectationsWithTimeout:15 handler:nil];
 }
 
+- (void)test_whenPerformingUpdates_withWorkingRange_thatAccessingCellDoesntCrash {
+    [self setupWithObjects:@[
+                             genTestObject(@1, @1),
+                             genTestObject(@2, @1),
+                             genTestObject(@3, @1),
+                             ]];
+
+    // section controller try to access a cell in -listAdapter:sectionControllerWillEnterWorkingRange:
+    // add items beyond the 100x100 frame so they access unavailable cells
+    self.dataSource.objects = @[
+                                genTestObject(@1, @1),
+                                genTestObject(@2, @1),
+                                genTestObject(@3, @1),
+                                genTestObject(@4, @1),
+                                genTestObject(@5, @1),
+                                genTestObject(@6, @1),
+                                genTestObject(@7, @1),
+                                genTestObject(@8, @1),
+                                genTestObject(@9, @1),
+                                genTestObject(@10, @1),
+                                genTestObject(@11, @1),
+                                ];
+    XCTestExpectation *expectation = genExpectation;
+
+    // this will call -collectionView:performBatchUpdates:, trigger collectionView:willDisplayCell:forItemAtIndexPath:,
+    // which kicks off the working range logic
+    [self.adapter performUpdatesAnimated:YES completion:^(BOOL finished) {
+        [expectation fulfill];
+    }];
+
+    [self waitForExpectationsWithTimeout:15 handler:nil];
+}
+
 @end

Tests/IGListWorkingRangeHandlerTests.m

@@ -3,7 +3,7 @@
  * All rights reserved.
  *
  * This source code is licensed under the BSD-style license found in the
- * LICENSE file in the root directory of this source tree. An additional grant 
+ * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
@@ -48,7 +48,7 @@ - (UIView *)emptyViewForListAdapter:(IGListAdapter *)listAdapter {
 }
 
 - (IGListSectionController<IGListSectionType> *)listAdapter:(IGListAdapter *)listAdapter
-                            sectionControllerForObject:(id)object {
+                                 sectionControllerForObject:(id)object {
     return [_map objectForKey:object];
 }
 
@@ -211,7 +211,7 @@ - (void)test_whenDisplayingItemAtPath_withWorkingRangeSizeOne_thenEndDisplayingT
     // Act: Hide the first item, and watch for the second item to leave the working range.
     [[mockWorkingRangeDelegate expect] listAdapter:adapter sectionControllerDidExitWorkingRange:controller2];
     [adapter.workingRangeHandler didEndDisplayingItemAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0] forListAdapter:adapter];
-
+    
     [mockWorkingRangeDelegate verifyWithDelay:5];
 }
 

Tests/Objects/IGTestDelegateController.h

@@ -13,7 +13,7 @@
 
 @class IGTestObject;
 
-@interface IGTestDelegateController : IGListSectionController <IGListSectionType, IGListDisplayDelegate>
+@interface IGTestDelegateController : IGListSectionController <IGListSectionType, IGListDisplayDelegate, IGListWorkingRangeDelegate>
 
 @property (nonatomic, strong, readonly) IGTestObject *item;
 

Tests/Objects/IGTestDelegateController.m

@@ -18,6 +18,7 @@ - (instancetype)init {
     if (self = [super init]) {
         _willDisplayCellIndexes = [NSCountedSet new];
         _didEndDisplayCellIndexes = [NSCountedSet new];
+        self.workingRangeDelegate = self;
     }
     return self;
 }
@@ -81,4 +82,12 @@ - (void)listAdapter:(IGListAdapter *)listAdapter didEndDisplayingSectionControll
 
 - (void)listAdapter:(IGListAdapter *)listAdapter didScrollSectionController:(IGListSectionController <IGListSectionType> *)sectionController {}
 
+#pragma mark - IGListWorkingRangeDelegate
+
+- (void)listAdapter:(IGListAdapter *)listAdapter sectionControllerWillEnterWorkingRange:(IGListSectionController<IGListSectionType> *)sectionController {
+    __unused UICollectionViewCell *cell = [self.collectionContext cellForItemAtIndex:0 sectionController:self];
+}
+
+- (void)listAdapter:(IGListAdapter *)listAdapter sectionControllerDidExitWorkingRange:(IGListSectionController<IGListSectionType> *)sectionController {}
+
 @end