[ASTableNode] expose ASTableView's -visibleNodes (#2449)

* expose ASTableView's -visibleNodes getter at the ASTableNode level

* add safegaurds

* make visibleNodes a read only property for ASTableNode

* make collectionNode .visibleNodes property

AsyncDisplayKit/ASCollectionNode.h

@@ -335,9 +335,9 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Similar to -visibleCells.
  *
- * @return an array containing the nodes being displayed on screen.
+ * @return an array containing the nodes being displayed on screen. This must be called on the main thread.
  */
-- (NSArray<__kindof ASCellNode *> *)visibleNodes AS_WARN_UNUSED_RESULT;
+@property(readonly, copy) NSArray<__kindof ASCellNode *> *visibleNodes;
 
 /**
  * Retrieves the node for the item at the given index path.

AsyncDisplayKit/ASCollectionNode.mm

@@ -349,7 +349,8 @@
 
 - (NSArray<__kindof ASCellNode *> *)visibleNodes
 {
-  return [self.view visibleNodes];
+  ASDisplayNodeAssertMainThread();
+  return self.isNodeLoaded ? [self.view visibleNodes] : @[];
 }
 
 - (NSIndexPath *)indexPathForNode:(ASCellNode *)cellNode

AsyncDisplayKit/ASTableNode.h

@@ -339,6 +339,13 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (NSInteger)tableNode:(ASTableNode *)tableNode numberOfRowsInSection:(NSInteger)section;
 
+/**
+ * Similar to -visibleCells.
+ *
+ * @return an array containing the nodes being displayed on screen. This must be called on the main thread.
+ */
+@property(readonly, copy) NSArray<__kindof ASCellNode *> *visibleNodes;
+
 /**
  * Asks the data source for a block to create a node to represent the row at the given index path.
  * The block will be run by the table node concurrently in the background before the row is inserted

AsyncDisplayKit/ASTableNode.mm

@@ -371,6 +371,12 @@ ASEnvironmentCollectionTableSetEnvironmentState(_environmentStateLock)
   return [self.dataController numberOfSections];
 }
 
+- (NSArray<__kindof ASCellNode *> *)visibleNodes
+{
+  ASDisplayNodeAssertMainThread();
+  return self.isNodeLoaded ? [self.view visibleNodes] : @[];
+}
+
 - (NSIndexPath *)indexPathForNode:(ASCellNode *)cellNode
 {
   return [self.dataController indexPathForNode:cellNode];