Add experiment for reloadData on large changesets

Summary:
Preparing another perf test. I noticed in some surfaces that have **massive** updates (8k+ inserts/deletes) that perf can be pretty awful. This is all due to internals of `UICollectionView` trying to create cells and animate a huge blob of changes. Instead, I'm picking a sensible (is it?) default to just skip batch updates and do `reloadData` instead.

Unit tests caught that the `layoutIfNeeded` is required to immediately configure cells.

Depends on D5392713

Reviewed By: jeremycohen

Differential Revision: D5392882

fbshipit-source-id: e429ddb7bca7400908898ebc6f097a489211b03d

Source/IGListAdapterUpdater.m

@@ -15,6 +15,7 @@
 #import <IGListKit/IGListDiff.h>
 
 #import "UICollectionView+IGListBatchUpdateData.h"
+#import "IGListIndexSetResultInternal.h"
 #import "IGListMoveIndexPathInternal.h"
 #import "IGListReloadIndexPath.h"
 
@@ -167,16 +168,21 @@ static NSArray *objectsWithDuplicateIdentifiersRemoved(NSArray<id<IGListDiffable
         }
     };
 
+    void (^reloadDataFallback)() = ^{
+        executeUpdateBlocks();
+        [self cleanStateAfterUpdates];
+        [self performBatchUpdatesItemBlockApplied];
+        [collectionView reloadData];
+        [collectionView layoutIfNeeded];
+        executeCompletionBlocks(YES);
+    };
+
     // if the collection view isn't in a visible window, skip diffing and batch updating. execute all transition blocks,
     // reload data, execute completion blocks, and get outta here
     const BOOL iOS83OrLater = (NSFoundationVersionNumber >= NSFoundationVersionNumber_iOS_8_3);
     if (iOS83OrLater && self.allowsBackgroundReloading && collectionView.window == nil) {
         [self beginPerformBatchUpdatesToObjects:toObjects];
-        executeUpdateBlocks();
-        [self cleanStateAfterUpdates];
-        [self performBatchUpdatesItemBlockApplied];
-        [collectionView reloadData];
-        executeCompletionBlocks(YES);
+        reloadDataFallback();
         return;
     }
 
@@ -217,7 +223,9 @@ static NSArray *objectsWithDuplicateIdentifiersRemoved(NSArray<id<IGListDiffable
     void (^performUpdate)(IGListIndexSetResult *) = ^(IGListIndexSetResult *result){
         @try {
             [delegate listAdapterUpdater:self willPerformBatchUpdatesWithCollectionView:collectionView];
-            if (animated) {
+            if (result.changeCount > 100 && IGListExperimentEnabled(experiments, IGListExperimentReloadDataFallback)) {
+                reloadDataFallback();
+            } else if (animated) {
                 [collectionView performBatchUpdates:^{
                     batchUpdatesBlock(result);
                 } completion:batchUpdatesCompletionBlock];

Tests/IGListAdapterE2ETests.m

@@ -1521,4 +1521,25 @@
     [self waitForExpectationsWithTimeout:30 handler:nil];
 }
 
+- (void)test_whenMassiveUpdate_thatUpdateApplied {
+    // init empty
+    [self setupWithObjects:@[]];
+
+    ((IGListAdapterUpdater *)self.updater).experiments = IGListExperimentReloadDataFallback;
+
+    NSMutableArray *objects = [NSMutableArray new];
+    for (NSInteger i = 0; i < 3000; i++) {
+        [objects addObject:genTestObject(@(i + 1), @4)];
+    }
+    self.dataSource.objects = objects;
+
+    XCTestExpectation *expectation = genExpectation;
+    [self.adapter performUpdatesAnimated:YES completion:^(BOOL finished) {
+        XCTAssertEqual([self.collectionView numberOfSections], 3000);
+        [expectation fulfill];
+    }];
+
+    [self waitForExpectationsWithTimeout:30 handler:nil];
+}
+
 @end