Fix some crash from IGListKit that nil dataSource might result in crash

Summary:
This happens when we set collectionView.dataSource to be nil before calling performUpdates:, the crash is easily reproduced by a simple unit test.

To make the infra more robust and we don't crash the app, let's add an early return before applying a collectionView update.

Reviewed By: rnystrom, calimarkus

Differential Revision: D12896196

fbshipit-source-id: ab024d0e7e9282d50c3be297e1e67cfccaff8242

CHANGELOG.md

@@ -22,6 +22,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 ### Fixes
 
+- Fixed crash when the data source is nil before calling `-[IGListAdapterUpdater performUpdateWithCollectionViewBlock:fromObjects:toObjectsBlock:animated:objectTransitionBlock:completion:]`. [Zhisheng Huang](https://github.com/lorixx) (tbd)
+
 - Experimental fix to get the `UICollectionView` for batch updating immediately before applying the update. [Ryan Nystrom](https://github.com/rnystrom) (tbd)
 
 - `[IGListAdapterUpdater performBatchUpdatesWithCollectionViewBlock:]` and `[IGListAdapterUpdater performReloadDataWithCollectionViewBlock:]` clean state and run completion blocks if their `UICollectionView` is nil. [Brandon Darin](https://github.com/jbd1030) (tbd)

Source/IGListAdapterUpdater.m

@@ -236,7 +236,10 @@ willPerformBatchUpdatesWithCollectionView:collectionView
                               fromObjects:fromObjects
                                 toObjects:toObjects
                        listIndexSetResult:result];
-            if (result.changeCount > 100 && IGListExperimentEnabled(experiments, IGListExperimentReloadDataFallback)) {
+            if (collectionView.dataSource == nil) {
+                // If the data source is nil, we should not call any collection view update.
+                batchUpdatesCompletionBlock(NO);
+            } else if (result.changeCount > 100 && IGListExperimentEnabled(experiments, IGListExperimentReloadDataFallback)) {
                 reloadDataFallback();
             } else if (animated) {
                 [collectionView performBatchUpdates:^{

Source/Internal/UICollectionView+IGListBatchUpdateData.m

@@ -11,7 +11,7 @@
 
 @implementation UICollectionView (IGListBatchUpdateData)
 
-- (void)ig_applyBatchUpdateData:(IGListBatchUpdateData *)updateData {
+- (void)ig_applyBatchUpdateData:(IGListBatchUpdateData *)updateData {    
     [self deleteItemsAtIndexPaths:updateData.deleteIndexPaths];
     [self insertItemsAtIndexPaths:updateData.insertIndexPaths];
     [self reloadItemsAtIndexPaths:updateData.updateIndexPaths];

Tests/IGListAdapterUpdaterTests.m

@@ -97,6 +97,17 @@
     XCTAssertEqual([self.collectionView numberOfSections], 0);
 }
 
+- (void)test_whenReloadingDataWithNilDataSourceBefore_thatCollectionViewNotCrash {
+    self.dataSource.sections = @[[IGSectionObject sectionWithObjects:@[@1]], [IGSectionObject sectionWithObjects:@[@2]]];
+    [self.updater performReloadDataWithCollectionViewBlock:[self collectionViewBlock]];
+    XCTAssertEqual([self.collectionView numberOfSections], 2);
+    
+    self.collectionView.dataSource = nil;
+    self.dataSource.sections = @[];
+    [self.updater performReloadDataWithCollectionViewBlock:[self collectionViewBlock]];
+    XCTAssertEqual([self.collectionView numberOfSections], 1); // Setting collectionView's dataSource to nil would yield a single section by default.
+}
+
 - (void)test_whenInsertingSection_thatCollectionViewUpdates {
     NSArray *from = @[
                       [IGSectionObject sectionWithObjects:@[]]
@@ -531,6 +542,28 @@
     [mockDelegate verify];
 }
 
+- (void)test_whenCollectionViewNotInWindow_andBackgroundReloadFlag_isDefaultYES_andDataSourceWasSetToNilBefore_thatCollectionViewNotCrash {
+    [self.collectionView removeFromSuperview];
+    
+    id mockDelegate = [OCMockObject niceMockForProtocol:@protocol(IGListAdapterUpdaterDelegate)];
+    self.updater.delegate = mockDelegate;
+    [[mockDelegate reject] listAdapterUpdater:self.updater willPerformBatchUpdatesWithCollectionView:self.collectionView fromObjects:@[] toObjects:@[] listIndexSetResult:OCMOCK_ANY];
+    [[mockDelegate reject] listAdapterUpdater:self.updater didPerformBatchUpdates:OCMOCK_ANY collectionView:self.collectionView];
+    
+    XCTestExpectation *expectation = genExpectation;
+    IGListToObjectBlock to = ^NSArray *{
+        return @[
+                 [IGSectionObject sectionWithObjects:@[]]
+                 ];
+    };
+    self.collectionView.dataSource = nil;
+    [self.updater performUpdateWithCollectionViewBlock:[self collectionViewBlock] fromObjects:self.dataSource.sections toObjectsBlock:to animated:NO objectTransitionBlock:self.updateBlock completion:^(BOOL finished) {
+        [expectation fulfill];
+    }];
+    waitExpectation;
+    [mockDelegate verify];
+}
+
 - (void)test_whenReloadBatchedWithUpdate_thatCompletionBlockStillExecuted {
     IGSectionObject *object = [IGSectionObject sectionWithObjects:@[@0, @1, @2]];
     self.dataSource.sections = @[object];
@@ -688,6 +721,36 @@
     [mockDelegate verify];
 }
 
+- (void)test_whenPerformUpdates_dataSourceWasSetToNil_shouldNotCrash {
+    NSArray<IGSectionObject *> *from = @[[IGSectionObject sectionWithObjects:@[@1] identifier:@"id1"]];
+    NSArray<IGSectionObject *> *to = @[[IGSectionObject sectionWithObjects:@[@2] identifier:@"id1"],
+                                       [IGSectionObject sectionWithObjects:@[@22] identifier:@"id2"]];
+    self.dataSource.sections = from;
+    id mockDelegate = [OCMockObject niceMockForProtocol:@protocol(IGListAdapterUpdaterDelegate)];
+    self.updater.delegate = mockDelegate;
+    [mockDelegate setExpectationOrderMatters:YES];
+    [[mockDelegate expect] listAdapterUpdater:self.updater willPerformBatchUpdatesWithCollectionView:self.collectionView fromObjects:from toObjects:to listIndexSetResult:[OCMArg checkWithBlock:^BOOL(IGListIndexSetResult *result) {
+        if (result.deletes.count != 0 || result.moves.count != 0) {
+            return NO;
+        }
+        // Make sure we note that index 1 is updated (id1 from @[@1] -> @[@2]), and "id2" was inserted at index 1
+        return result.updates.firstIndex == 0 && result.inserts.firstIndex == 1;
+    }]];
+    [[mockDelegate expect] listAdapterUpdater:self.updater didPerformBatchUpdates:OCMOCK_ANY collectionView:self.collectionView];
+    
+    XCTestExpectation *expectation = genExpectation;
+    
+    // Manually set the data source to be nil.
+    self->_collectionView.dataSource = nil;
+    
+    [self.updater performUpdateWithCollectionViewBlock:[self collectionViewBlock] fromObjects:from toObjectsBlock:genToBlock animated:NO objectTransitionBlock:^(NSArray * _Nonnull toObjects) {
+    }  completion:^(BOOL finished) {
+        [expectation fulfill];
+    }];
+    waitExpectation;
+    [mockDelegate verify];
+}
+
 # pragma mark - preferItemReloadsFroSectionReloads
 
 - (void)test_whenReloadIsCalledWithSameItemCount_andPreferItemReload_updateIndexPathsHappen {