Defer invalidatation until all batch updates are finished

Summary:
Added a mechanism to `IGListAdapter` to defer (_queue or execute_) blocks around asking the updater to do batch updates. I discovered a crash where the `_updateCompletionHandler` (private ivar) of `UICollectionView` was not being niled when two batch updates collide. There is an assert in `-[UICollectionView dealloc]` that fires if this block != nil. Took some reverse eng to track down what is happening.

Sadly I cannot reproduce the issue in a unit test _or_ sample app. However I can 100% repro this in my GitHawk app. Applying this patch fixes the problem. I added a unit test that got me _close_ to the state of the crash. Consider this new test future-proofing (also covers the enter/exit/defer stuff).

Also dumping the `UICollectionView` internals confirms that the bad state (`_updateCompletionHandler != nil`) is no longer true.

Issue fixed: #929

Depends on #930

- [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 https://github.com/Instagram/IGListKit/pull/931

Differential Revision: D5863761

Pulled By: rnystrom

fbshipit-source-id: 496aa939e3a5e83472b26bfb5a4cdc872f58cc0e

CHANGELOG.md

@@ -5,13 +5,15 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 3.2.0 (upcoming release)
 -----
 
+### Enhancements
+
+- Added `-[IGListSectionController didHighlightItemAtIndex:]` and `-[IGListSectionController didUnhighlightItemAtIndex:]` APIs to support `UICollectionView` cell highlighting. [Kevin Delannoy](https://github.com/delannoyk) [(#933)](https://github.com/Instagram/IGListKit/pull/933)
+
 ### 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)
 
-### Enhancements
-
-- Added `-[IGListSectionController didHighlightItemAtIndex:]` and `-[IGListSectionController didUnhighlightItemAtIndex:]` APIs to support `UICollectionView` cell highlighting. [Kevin Delannoy](https://github.com/delannoyk) [(#933)](https://github.com/Instagram/IGListKit/pull/933)
+- Avoid crash when invalidating the layout while inside `-[UICollectionView performBatchUpdates:completion:]. [Ryan Nystrom](https://github.com/rnystrom) [(#tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
 
 3.1.1
 -----

Source/IGListAdapter.m

@@ -18,6 +18,8 @@
 
 @implementation IGListAdapter {
     NSMapTable<UICollectionReusableView *, IGListSectionController *> *_viewSectionControllerMap;
+    // An array of blocks to execute once batch updates are finished
+    NSMutableArray<void (^)()> *_queuedCompletionBlocks;
 }
 
 - (void)dealloc {
@@ -317,6 +319,8 @@
     NSArray *fromObjects = self.sectionMap.objects;
     NSArray *newObjects = [dataSource objectsForListAdapter:self];
 
+    [self enterBatchUpdates];
+
     __weak __typeof__(self) weakSelf = self;
     [self.updater performUpdateWithCollectionView:collectionView
                                       fromObjects:fromObjects
@@ -335,6 +339,8 @@
                                 if (completion) {
                                     completion(finished);
                                 }
+
+                                [weakSelf exitBatchUpdates];
                             }];
 }
 
@@ -702,6 +708,26 @@
     [_viewSectionControllerMap removeObjectForKey:view];
 }
 
+- (void)deferBlockBetweenBatchUpdates:(void (^)())block {
+    IGAssertMainThread();
+    if (_queuedCompletionBlocks == nil) {
+        block();
+    } else {
+        [_queuedCompletionBlocks addObject:block];
+    }
+}
+
+- (void)enterBatchUpdates {
+    _queuedCompletionBlocks = [NSMutableArray new];
+}
+
+- (void)exitBatchUpdates {
+    for (void (^block)() in _queuedCompletionBlocks) {
+        block();
+    }
+    _queuedCompletionBlocks = nil;
+}
+
 #pragma mark - UIScrollViewDelegate
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView {
@@ -964,6 +990,8 @@
     IGParameterAssert(updates != nil);
     UICollectionView *collectionView = self.collectionView;
     IGAssert(collectionView != nil, @"Performing batch updates without a collection view.");
+
+    [self enterBatchUpdates];
     
     __weak __typeof__(self) weakSelf = self;
     [self.updater performUpdateWithCollectionView:collectionView animated:animated itemUpdates:^{
@@ -976,6 +1004,8 @@
         if (completion) {
             completion(finished);
         }
+
+        [weakSelf exitBatchUpdates];
     }];
 }
 
@@ -1004,11 +1034,14 @@
     UICollectionViewLayoutInvalidationContext *context = [[[layout.class invalidationContextClass] alloc] init];
     [context invalidateItemsAtIndexPaths:indexPaths];
     
-    void (^block)() = ^{
-        [layout invalidateLayoutWithContext:context];
-    };
-    
-    [_collectionView performBatchUpdates:block completion:completion];
+    __weak __typeof__(_collectionView) weakCollectionView = _collectionView;
+
+    // do not call -[UICollectionView performBatchUpdates:completion:] while already updating. defer it until completed.
+    [self deferBlockBetweenBatchUpdates:^{
+        [weakCollectionView performBatchUpdates:^{
+            [layout invalidateLayoutWithContext:context];
+        } completion:completion];
+    }];
 }
 
 #pragma mark - IGListBatchContext

Tests/IGListAdapterE2ETests.m

@@ -1552,4 +1552,53 @@
     [self waitForExpectationsWithTimeout:30 handler:nil];
 }
 
+- (void)test_whenInvalidatingInsideBatchUpdate_withSystemReleased_thatSystemNil_andCollectionViewDoesntCrashOnDealloc {
+    __weak id weakAdapter = nil;
+    __block BOOL executedItemUpdate = NO;
+    XCTestExpectation *expectation = genExpectation;
+
+    @autoreleasepool {
+        self.dataSource.objects = @[
+                                    genTestObject(@1, @"Bar"),
+                                    genTestObject(@0, @"Foo")
+                                    ];
+
+        UICollectionView *collectionView = [[UICollectionView alloc] initWithFrame:self.window.frame collectionViewLayout:[UICollectionViewFlowLayout new]];
+        [self.window addSubview:collectionView];
+        IGListAdapterUpdater *updater = [IGListAdapterUpdater new];
+        IGListAdapter *adapter = [[IGListAdapter alloc] initWithUpdater:updater viewController:nil];
+        adapter.dataSource = self.dataSource;
+        adapter.collectionView = collectionView;
+        [collectionView layoutIfNeeded];
+
+        IGTestDelegateController *section = [adapter sectionControllerForObject:self.dataSource.objects.firstObject];
+
+        __weak typeof(section) weakSection = section;
+        section.itemUpdateBlock = ^{
+            executedItemUpdate = YES;
+            [weakSection.collectionContext invalidateLayoutForSectionController:weakSection completion:nil];
+        };
+
+        self.dataSource.objects = @[
+                                    genTestObject(@1, @"Bar"),
+                                    genTestObject(@0, @"Foo")
+                                    ];
+
+        [adapter performUpdatesAnimated:YES completion:^(BOOL finished) {
+            XCTAssertNotNil(collectionView);
+            XCTAssertNotNil(adapter);
+            [collectionView removeFromSuperview];
+            [expectation fulfill];
+        }];
+
+        weakAdapter = adapter;
+        XCTAssertNotNil(weakAdapter);
+    }
+
+    [self waitForExpectationsWithTimeout:30 handler:^(NSError * _Nullable error) {
+        XCTAssertTrue(executedItemUpdate);
+        XCTAssertNil(weakAdapter);
+    }];
+}
+
 @end