[ASDataController] Call All Completion Handlers after Nested Batch Updates (#2274)

* Add tests for batch update completion handler calling

* Ensure we call all completion handlers after collection view updates

* Tweak it

* Fix the doc

* Minor improvements

* Document addCompletionHandler better

AsyncDisplayKit/Details/ASChangeSetDataController.mm

@@ -20,6 +20,11 @@
   _ASHierarchyChangeSet *_changeSet;
 }
 
+- (void)dealloc
+{
+  ASDisplayNodeCAssert(_changeSetBatchUpdateCounter == 0, @"ASChangeSetDataController deallocated in the middle of a batch update.");
+}
+
 #pragma mark - Batching (External API)
 
 - (void)beginUpdates
@@ -40,18 +45,20 @@
   // Prevent calling endUpdatesAnimated:completion: in an unbalanced way
   NSAssert(_changeSetBatchUpdateCounter >= 0, @"endUpdatesAnimated:completion: called without having a balanced beginUpdates call");
   
+  [_changeSet addCompletionHandler:completion];
   if (_changeSetBatchUpdateCounter == 0) {
+    [self invalidateDataSourceItemCounts];
+    [_changeSet markCompletedWithNewItemCounts:[self itemCountsFromDataSource]];
+    void (^batchCompletion)(BOOL finished) = _changeSet.completionHandler;
+
     if (!self.initialReloadDataHasBeenCalled) {
-      if (completion) {
-        completion(YES);
+      if (batchCompletion != nil) {
+        batchCompletion(YES);
       }
       _changeSet = nil;
       return;
     }
     
-    [self invalidateDataSourceItemCounts];
-    [_changeSet markCompletedWithNewItemCounts:[self itemCountsFromDataSource]];
-    
     [super beginUpdates];
     
     for (_ASHierarchyItemChange *change in [_changeSet itemChangesOfType:_ASHierarchyChangeTypeDelete]) {
@@ -70,7 +77,7 @@
       [super insertRowsAtIndexPaths:change.indexPaths withAnimationOptions:change.animationOptions];
     }
 
-    [super endUpdatesAnimated:animated completion:completion];
+    [super endUpdatesAnimated:animated completion:batchCompletion];
     
     _changeSet = nil;
   }