graduate IGListExperimentBackgroundDiffing to a property

Summary:
`IGListExperimentBackgroundDiffing` has been running on Instagram for a couple months now, and we're seeing nice scroll performance improvements (less frame drops) without increasing crashes. Lets move the experiment to a real property!

Should it be enabled by default?
   * I'm leaning toward no, but I could be convinced otherwise. This optimization is good for large/complex applications with lots of diffing, but I'm not sure the more common apps would need it initially. It does make updates more complicated and the order of operation will be a bit different. For example, we've seen some places call `-performUpdatesAnimated` and almost immediately expect the `IGListAdapter` data to be updated, leading to missing views. This issue can still happen without background diffing, but less often.

Reviewed By: joetam

Differential Revision: D25884786

fbshipit-source-id: 3c8755a774f63868b7dfbc8e7a2e5c012a9e7e27

CHANGELOG.md

@@ -38,6 +38,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Removed `allowsBackgroundReloading` from `IGListAdapterUpdater` because it's causing performance issues and other bugs. [Maxime Ollivier](https://github.com/maxolls) (tbd)
 
+- Introducing `allowsBackgroundDiffing` on `IGListAdapterUpdater`! This property lets the updater perform the diffing on a background thread. Originally introduced by Ryan Nystrom a while back. [Maxime Ollivier](https://github.com/maxolls) (tbd)
+
 ### Enhancements
 
 - Added `shouldSelectItemAtIndex:` to `IGListSectionController` . [dirtmelon](https://github.com/dirtmelon)

Source/IGListDiffKit/IGListExperiments.h

@@ -16,14 +16,12 @@ NS_SWIFT_NAME(ListExperiment)
 typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     /// Specifies no experiments.
     IGListExperimentNone = 1 << 1,
-    /// Test updater diffing performed on a background queue.
-    IGListExperimentBackgroundDiffing = 1 << 2,
     /// Test invalidating layout when cell reloads/updates in IGListBindingSectionController.
-    IGListExperimentInvalidateLayoutForUpdates = 1 << 3,
+    IGListExperimentInvalidateLayoutForUpdates = 1 << 2,
     /// Test skipping performBatchUpdate if we don't have any updates.
-    IGListExperimentSkipPerformUpdateIfPossible = 1 << 4,
+    IGListExperimentSkipPerformUpdateIfPossible = 1 << 3,
     /// Test skipping creating {view : section controller} map, which has inconsistency issue.
-    IGListExperimentSkipViewSectionControllerMap = 1 << 5
+    IGListExperimentSkipViewSectionControllerMap = 1 << 4
 };
 
 /**

Source/IGListKit/IGListAdapterUpdater.h

@@ -69,6 +69,13 @@ NS_SWIFT_NAME(ListAdapterUpdater)
  */
 @property (nonatomic, assign) BOOL allowsReloadingOnTooManyUpdates;
 
+/**
+ Allow the diffing to be performed on a background thread.
+
+ Default is NO.
+ */
+@property (nonatomic, assign) BOOL allowsBackgroundDiffing;
+
 /**
  A bitmask of experiments to conduct on the updater.
  */

Source/IGListKit/IGListAdapterUpdater.m

@@ -90,6 +90,7 @@ - (void)update {
         .singleItemSectionUpdates = _singleItemSectionUpdates,
         .preferItemReloadsForSectionReloads = _preferItemReloadsForSectionReloads,
         .allowsReloadingOnTooManyUpdates = _allowsReloadingOnTooManyUpdates,
+        .allowsBackgroundDiffing = _allowsBackgroundDiffing,
         .experiments = _experiments,
     };
 

Source/IGListKit/Internal/IGListAdapterUpdater+DebugDescription.m

@@ -22,7 +22,8 @@ @implementation IGListAdapterUpdater (DebugDescription)
         [NSString stringWithFormat:@"sectionMovesAsDeletesInserts: %@", IGListDebugBOOL(self.sectionMovesAsDeletesInserts)],
         [NSString stringWithFormat:@"singleItemSectionUpdates: %@", IGListDebugBOOL(self.singleItemSectionUpdates)],
         [NSString stringWithFormat:@"preferItemReloadsForSectionReloads: %@", IGListDebugBOOL(self.preferItemReloadsForSectionReloads)],
-        [NSString stringWithFormat:@"allowsReloadingOnTooManyUpdates: %@", IGListDebugBOOL(self.allowsReloadingOnTooManyUpdates)]
+        [NSString stringWithFormat:@"allowsReloadingOnTooManyUpdates: %@", IGListDebugBOOL(self.allowsReloadingOnTooManyUpdates)],
+        [NSString stringWithFormat:@"allowsBackgroundDiffing: %@", IGListDebugBOOL(self.allowsBackgroundDiffing)]
     ];
     [debug addObjectsFromArray:IGListDebugIndentedLines(options)];
 

Source/IGListKit/Internal/IGListBatchUpdateTransaction.m

@@ -102,7 +102,7 @@ - (void)_diff {
     IGListTransitionData *data = self.sectionData;
     [self.delegate listAdapterUpdater:self.updater willDiffFromObjects:data.fromObjects toObjects:data.toObjects];
 
-    const BOOL onBackground = IGListExperimentEnabled(self.config.experiments, IGListExperimentBackgroundDiffing);
+    const BOOL onBackground = self.config.allowsBackgroundDiffing;
     if (onBackground) {
         __weak __typeof__(self) weakSelf = self;
         dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), ^{

Source/IGListKit/Internal/IGListUpdateTransactable.h

@@ -20,6 +20,7 @@ typedef struct {
     BOOL singleItemSectionUpdates;
     BOOL preferItemReloadsForSectionReloads;
     BOOL allowsReloadingOnTooManyUpdates;
+    BOOL allowsBackgroundDiffing;
     IGListExperiment experiments;
 } IGListUpdateTransactationConfig;
 

Tests/IGListAdapterE2ETests.m

@@ -2377,7 +2377,7 @@ - (void)test_whenSchedulingItemUpdate_thenChangeDataSource_thatLastestDataIsAppl
 
 - (void)test_whenSchedulingSectionUpdate_thenBeginDiffing_thenChangeCollectionView_thatLastestDataIsApplied {
     IGListAdapterUpdater *updater = (IGListAdapterUpdater *)self.updater;
-    updater.experiments |= IGListExperimentBackgroundDiffing;
+    updater.allowsBackgroundDiffing = YES;
 
     [self setupWithObjects:@[
         genTestObject(@0, @"Foo")
@@ -2437,7 +2437,7 @@ - (void)test_whenSchedulingSectionUpdate_thenBeginDiffing_thenChangeCollectionVi
 
 - (void)test_whenSchedulingSectionUpdate_thenBeginDiffing_thenChangeTheDataSource_thatLastestDataIsApplied {
     IGListAdapterUpdater *updater = (IGListAdapterUpdater *)self.updater;
-    updater.experiments |= IGListExperimentBackgroundDiffing;
+    updater.allowsBackgroundDiffing = YES;
 
     [self setupWithObjects:@[
         genTestObject(@0, @"Foo")

Tests/IGListAdapterUpdaterTests.m

@@ -868,7 +868,7 @@ - (void)test_whenPerformingUpdatesMultipleTimesInARow_thenUpdateWorks {
 }
 
 - (void)test_whenPerformingUpdate_thatCallsDiffingDelegate {
-    self.updater.experiments |= IGListExperimentBackgroundDiffing;
+    self.updater.allowsBackgroundDiffing = YES;
 
     NSArray *from = @[
         [IGSectionObject sectionWithObjects:@[] identifier:@"0"]