Weak reference collection view when queueing update

Summary:
Found a "bug" when adding some new unit tests. The queue method holds a strong ref to the collection view, and doesn't release it until that block is executed. The adapter may have already been released, holding onto the collection view is wasteful.

- [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.
Closes #930

Differential Revision: D5861975

Pulled By: rnystrom

fbshipit-source-id: 986ead725839d39b30daadf92675b397f8794520

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 The changelog for `IGListKit`. Also see the [releases](https://github.com/instagram/IGListKit/releases) on GitHub.
 
+3.2.0 (upcoming release)
+-----
+
+### Fixes
+
+- Weakly reference the `UICollectionView` in coalescence so that it can be released if the rest of system is destroyed. [Ryan Nystrom](https://github.com/rnystrom) [(#tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
+
+
 3.1.1
 -----
 

Source/IGListAdapterUpdater.m

@@ -380,16 +380,22 @@ - (void)queueUpdateWithCollectionView:(UICollectionView *)collectionView {
     }
 
     __weak __typeof__(self) weakSelf = self;
+    __weak __typeof__(collectionView) weakCollectionView = collectionView;
+
+    // dispatch_async to give the main queue time to collect more batch updates so that a minimum amount of work
+    // (diffing, etc) is done on main. dispatch_async does not garauntee a full runloop turn will pass though.
+    // see -performUpdateWithCollectionView:fromObjects:toObjects:animated:]objectTransitionBlock:completion: for more
+    // details on how coalescence is done.
     dispatch_async(dispatch_get_main_queue(), ^{
         if (weakSelf.state != IGListBatchUpdateStateIdle
             || ![weakSelf hasChanges]) {
             return;
         }
 
         if (weakSelf.hasQueuedReloadData) {
-            [weakSelf performReloadDataWithCollectionView:collectionView];
+            [weakSelf performReloadDataWithCollectionView:weakCollectionView];
         } else {
-            [weakSelf performBatchUpdatesWithCollectionView:collectionView];
+            [weakSelf performBatchUpdatesWithCollectionView:weakCollectionView];
         }
     });
 }

Tests/IGListAdapterE2ETests.m

@@ -1124,6 +1124,9 @@ - (void)test_whenDataSourceDeallocatedAfterUpdateQueued_thatUpdateSuccesfullyCom
 
 - (void)test_whenQueuingUpdate_withSectionControllerBatchUpdate_thatSectionControllerNotRetained {
     __weak id weakSectionController = nil;
+    __weak id weakAdapter = nil;
+    __weak id weakCollectionView = nil;
+
     @autoreleasepool {
         IGListAdapter *adapter = [[IGListAdapter alloc] initWithUpdater:[IGListAdapterUpdater new] viewController:nil];
         IGTestDelegateDataSource *dataSource = [IGTestDelegateDataSource new];
@@ -1146,9 +1149,16 @@ - (void)test_whenQueuingUpdate_withSectionControllerBatchUpdate_thatSectionContr
         dataSource.objects = @[object, genTestObject(@2, @2)];
         [adapter performUpdatesAnimated:YES completion:^(BOOL finished) {}];
 
+        weakAdapter = adapter;
+        weakCollectionView = collectionView;
         weakSectionController = section;
+
+        XCTAssertNotNil(weakAdapter);
+        XCTAssertNotNil(weakCollectionView);
         XCTAssertNotNil(weakSectionController);
     }
+    XCTAssertNil(weakAdapter);
+    XCTAssertNil(weakCollectionView);
     XCTAssertNil(weakSectionController);
 }