Add image loading delegate method / cache instead of respondsToSelector

AsyncDisplayKit/ASNetworkImageNode.h

@@ -95,6 +95,15 @@ NS_ASSUME_NONNULL_BEGIN
 
 @optional
 
+/**
+ * Notification that the image node started to load
+ *
+ * @param imageNode The sender.
+ *
+ * @discussion Called on a background queue.
+ */
+- (void)imageNodeDidStartLoading:(ASNetworkImageNode *)imageNode;
+
 /**
  * Notification that the image node failed to download the image.
  *