clean up IGListExperimentSkipPerformUpdateIfPossible

Summary: This is probably not super safe, so lets kill it. If we want to know all the section and cell updates outside the update block, we need to make cells diffable, which is something we're considering.

Reviewed By: candance

Differential Revision: D52743451

fbshipit-source-id: fd8004876c452552931bf84ee73cd3ac15f3ba40

Source/IGListDiffKit/IGListExperiments.h

@@ -18,10 +18,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentNone = 1 << 1,
     /// Test invalidating layout when cell reloads/updates in IGListBindingSectionController.
     IGListExperimentInvalidateLayoutForUpdates = 1 << 2,
-    /// Test skipping performBatchUpdate if we don't have any updates.
-    IGListExperimentSkipPerformUpdateIfPossible = 1 << 3,
     /// Throw NSInternalInconsistencyException during an update
-    IGListExperimentThrowOnInconsistencyException = 1 << 4
+    IGListExperimentThrowOnInconsistencyException = 1 << 3
 };
 
 /**

Source/IGListKit/Internal/IGListBatchUpdateTransaction.m

@@ -169,37 +169,8 @@ - (void)_applyDiff:(IGListIndexSetResult *)diffResult {
                             toObjects:self.sectionData.toObjects
                    listIndexSetResult:diffResult
                              animated:self.animated];
-
-    // Experiment to skip calling `[UICollectionView performBatchUpdates ...]` if we don't have changes. It does
-    // require us to call `_applyDataUpdates` outside the update block, but that should be ok as long as we call
-    // `performBatchUpdates` right after.
-    const BOOL skipPerformUpdateIfPossible = IGListExperimentEnabled(self.config.experiments, IGListExperimentSkipPerformUpdateIfPossible);
-    if (skipPerformUpdateIfPossible) {
-        // From Apple docs: If the collection view's layout is not up to date before you call performBatchUpdates, a reload may
-        // occur. To avoid problems, you should update your data model inside the updates block or ensure the layout is
-        // updated before you call performBatchUpdates(_:completion:).
-        [self.collectionView layoutIfNeeded];
-
-        [self _applyDataUpdates];
-
-        if (!diffResult.hasChanges && !self.inUpdateItemCollector.hasChanges) {
-            // If we don't have any section or item changes, take a shortcut.
-            [self _finishWithoutUpdate];
-            return;
-        }
-    }
-
-    // **************************
-    // **************************
-    // IMPORTANT: The very next thing we call must be `[UICollectionView performBatchUpdates ...]`, because
-    // we're in a state where the adapter is synced, but not the `UICollectionView`.
-    // **************************
-    // **************************
-
     void (^updates)(void) = ^ {
-        if (!skipPerformUpdateIfPossible) {
-            [self _applyDataUpdates];
-        }
+        [self _applyDataUpdates];
         [self _applyCollectioViewUpdates:diffResult];
     };
 

Tests/IGListAdapterUpdaterTests.m

@@ -985,48 +985,6 @@ - (void)_test_whenCollectionViewSectionCountIsIncorrect_thatDoesNotCrash {
     [self waitForExpectationsWithTimeout:30 handler:nil];
 }
 
-- (void)test_whenNoChanges_thatPerformUpdateExitsEarly {
-    self.updater.experiments |= IGListExperimentSkipPerformUpdateIfPossible;
-
-    NSArray *from = @[
-        [IGSectionObject sectionWithObjects:@[] identifier:@"Foo"]
-    ];
-    NSArray *to = @[
-        [IGSectionObject sectionWithObjects:@[] identifier:@"Foo"]
-    ];
-
-    self.dataSource.sections = from;
-    [self.updater reloadDataWithCollectionViewBlock:[self collectionViewBlock] reloadUpdateBlock:^{} completion:nil];
-    [self.updater update];
-
-    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:OCMOCK_ANY
-                                     animated:NO];
-
-    [[mockDelegate expect] listAdapterUpdater:self.updater didFinishWithoutUpdatesWithCollectionView:self.collectionView];
-
-    XCTestExpectation *expectation = genExpectation;
-    [self.updater performUpdateWithCollectionViewBlock:[self collectionViewBlock]
-                                              animated:NO
-                                      sectionDataBlock:[self dataBlockFromObjects:from toObjects:to]
-                                 applySectionDataBlock:self.applySectionDataBlock
-                                            completion:^(BOOL finished) {
-        XCTAssertTrue(finished);
-        XCTAssertEqual([self.collectionView numberOfSections], 1);
-        [expectation fulfill];
-    }];
-
-    [self waitForExpectationsWithTimeout:30 handler:nil];
-    [mockDelegate verify];
-}
-
 # pragma mark - preferItemReloadsFroSectionReloads
 
 - (void)test_whenReloadIsCalledWithSameItemCount_andPreferItemReload_updateIndexPathsHappen {