From a09242e2791a47769fb70fc31f5e18f0ea0b07ee Mon Sep 17 00:00:00 2001
From: Tim Johnsen <tjohnsen@instagram.com>
Date: Thu, 3 Jan 2019 09:24:23 -0800
Subject: [PATCH] Add experiment to perform IGListAdapterUpdater background
 updates on a serial queue.

Summary: This diff adds a new `IGListExperimentBackgroundDiffingSerial` experiment that behaves similarly to `IGListExperimentBackgroundDiffing` but executes changes in a per-`IGListAdapterUpdater` serial queue instead of a global queue. Global queues are concurrent, and concurrent queues by nature don't ensure the execution order of the blocks that are enqueued into them, so there's a chance that the former `IGListExperimentBackgroundDiffing` experiment would lead to changes being applied out of order.

Reviewed By: calimarkus

Differential Revision: D13567403

fbshipit-source-id: a2aebb20e5e7dc22601fad3c2dfc80112747c81b
---
 Source/Common/IGListExperiments.h              |  2 ++
 Source/IGListAdapterUpdater.m                  | 12 ++++++++++--
 Source/Internal/IGListAdapterUpdaterInternal.h |  2 ++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Source/Common/IGListExperiments.h b/Source/Common/IGListExperiments.h
index 30048f33..0f243327 100644
--- a/Source/Common/IGListExperiments.h
+++ b/Source/Common/IGListExperiments.h
@@ -26,6 +26,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentGetCollectionViewAtUpdate = 1 << 7,
     /// Test skipping layout when UICollectionView isn't visible
     IGListExperimentSkipLayout = 1 << 8,
+    /// Test updater diffing performed on a serial background queue.
+    IGListExperimentBackgroundDiffingSerial = 1 << 9,
 };
 
 /**
diff --git a/Source/IGListAdapterUpdater.m b/Source/IGListAdapterUpdater.m
index 778c0c5c..859c5993 100644
--- a/Source/IGListAdapterUpdater.m
+++ b/Source/IGListAdapterUpdater.m
@@ -267,9 +267,17 @@ willPerformBatchUpdatesWithCollectionView:collectionView
         }
     };
 
-    // temporary test to try out background diffing
+    dispatch_queue_t asyncQueue = nil;
     if (IGListExperimentEnabled(experiments, IGListExperimentBackgroundDiffing)) {
-        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+        asyncQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
+    } else if (IGListExperimentEnabled(experiments, IGListExperimentBackgroundDiffingSerial)) {
+        if (_backgroundUpdateQueue == nil) {
+            _backgroundUpdateQueue = dispatch_queue_create("io.github.instagram.IGListKit.backgroundupdatequeue", DISPATCH_QUEUE_SERIAL);
+        }
+        asyncQueue = _backgroundUpdateQueue;
+    }
+    if (asyncQueue) {
+        dispatch_async(asyncQueue, ^{
             IGListIndexSetResult *result = performDiff();
             dispatch_async(dispatch_get_main_queue(), ^{
                 performUpdate(result);
diff --git a/Source/Internal/IGListAdapterUpdaterInternal.h b/Source/Internal/IGListAdapterUpdaterInternal.h
index b794608b..40c8ef52 100644
--- a/Source/Internal/IGListAdapterUpdaterInternal.h
+++ b/Source/Internal/IGListAdapterUpdaterInternal.h
@@ -41,6 +41,8 @@ FOUNDATION_EXTERN void convertReloadToDeleteInsert(NSMutableIndexSet *reloads,
 @property (nonatomic, assign) IGListBatchUpdateState state;
 @property (nonatomic, strong, nullable) IGListBatchUpdateData *applyingUpdateData;
 
+@property (nonatomic, strong) dispatch_queue_t backgroundUpdateQueue;
+
 - (void)performReloadDataWithCollectionViewBlock:(IGListCollectionViewBlock)collectionViewBlock;
 - (void)performBatchUpdatesWithCollectionViewBlock:(IGListCollectionViewBlock)collectionViewBlock;
 - (void)cleanStateBeforeUpdates;