Only clear ASCollectionView's data during deallocation (facebookarchi…

…ve#1154)

This is a follow up on facebookarchive#1136. Our experiment results show that clearing data frequently is the cause of our facebookarchive#1 crash. @maicki and I believe that this is because if the collection view is being used, silently clearing its data without notifying the backing UICollectionView can put it out-of-sync and causes mayhem next time the collection view processes a batch update. If you look at the stack trace closely, you'll notice that the crash doesn't occur on the same run loop that clearData is called. This made it extremely tricky to investigate and identify the root cause.

Another interesting question would be whether or not we want to clear the data during deallocation at all, since the data will be cleared out soon anyway.

CHANGELOG.md

@@ -51,6 +51,7 @@
 - Optimize layout process by removing `ASRectMap`.  [Adlai Holler](https://github.com/Adlai-Holler)
 - Remove necessity to use view to access rangeController in ASTableNode, ASCollectionNode. [Michael Schneider](https://github.com/maicki)
 - Remove display node's reliance on shared_ptr. [Adlai Holler](https://github.com/Adlai-Holler)
+- [ASCollectionView] Fix a crash that is caused by clearing a collection view's data while it's still being used. [Huy Nguyen](https://github.com/nguyenhuy) [#1154](https://github.com/TextureGroup/Texture/pull/1154)
 - Clean up timing of layout tree flattening/ copying of unflattened tree for Weaver. [Michael Zuccarino](https://github.com/mikezucc) [#1157](https://github.com/TextureGroup/Texture/pull/1157)
 
 ## 2.7

Schemas/configuration.json

@@ -21,7 +21,8 @@
                     "exp_network_image_queue",
                     "exp_dealloc_queue_v2",
                     "exp_collection_teardown",
-                    "exp_framesetter_cache"
+                    "exp_framesetter_cache",
+                    "exp_clear_data_during_deallocation"
                 ]
     		}
 		}

Source/ASCollectionView.mm

@@ -577,7 +577,7 @@ - (void)_asyncDelegateOrDataSourceDidChange
 {
   ASDisplayNodeAssertMainThread();
 
-  if (_asyncDataSource == nil && _asyncDelegate == nil && !ASActivateExperimentalFeature(ASExperimentalSkipClearData)) {
+  if (_asyncDataSource == nil && _asyncDelegate == nil && _isDeallocating && ASActivateExperimentalFeature(ASExperimentalClearDataDuringDeallocation)) {
     [_dataController clearData];
   }
 }

Source/ASExperimentalFeatures.h

@@ -23,7 +23,7 @@ typedef NS_OPTIONS(NSUInteger, ASExperimentalFeatures) {
   ASExperimentalNetworkImageQueue = 1 << 5,                 // exp_network_image_queue
   ASExperimentalCollectionTeardown = 1 << 6,                // exp_collection_teardown
   ASExperimentalFramesetterCache = 1 << 7,                  // exp_framesetter_cache
-  ASExperimentalSkipClearData = 1 << 8,                     // exp_skip_clear_data
+  ASExperimentalClearDataDuringDeallocation = 1 << 8,       // exp_clear_data_during_deallocation
   ASExperimentalFeatureAll = 0xFFFFFFFF
 };
 

Source/ASExperimentalFeatures.m

@@ -21,7 +21,7 @@
                                       @"exp_dealloc_queue_v2",
                                       @"exp_collection_teardown",
                                       @"exp_framesetter_cache",
-                                      @"exp_skip_clear_data"]));
+                                      @"exp_clear_data_during_deallocation"]));
 
   if (flags == ASExperimentalFeatureAll) {
     return allNames;