[Sample] Resolve mutation during enumeration issue

- Mutating a dictionary using -enumerateKeysAndObjectsUsingBlock:
  can have unintended consequences.
- Using a copy of the keys to iterate over the values inside the
  dictionary instead.

AsyncDisplayKit/Details/ASCollectionDataController.mm

@@ -51,7 +51,9 @@
 
 - (void)willReloadData
 {
-  [_pendingContexts enumerateKeysAndObjectsUsingBlock:^(NSString *kind, NSMutableArray<ASIndexedNodeContext *> *contexts, BOOL *stop) {
+  NSArray *keys = _pendingContexts.allKeys;
+  for (NSString *kind in keys) {
+    NSMutableArray<ASIndexedNodeContext *> *contexts = _pendingContexts[kind];
     // Remove everything that existed before the reload, now that we're ready to insert replacements
     NSArray *indexPaths = [self indexPathsForEditingNodesOfKind:kind];
     [self deleteNodesOfKind:kind atIndexPaths:indexPaths completion:nil];
@@ -72,7 +74,7 @@
       [self insertNodes:nodes ofKind:kind atIndexPaths:indexPaths completion:nil];
     }];
     [_pendingContexts removeObjectForKey:kind];
-  }];
+  }
 }
 
 - (void)prepareForInsertSections:(NSIndexSet *)sections