michael's comments

AsyncDisplayKit/Layout/ASLayoutSpec.h

@@ -40,10 +40,9 @@ NS_ASSUME_NONNULL_BEGIN
  * only require a single child.
  *
  * For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
- * a subclass should use this method to set the "primary" child. It can then use setChild:forIdentifier:
- * to set any other required children. Ideally a subclass would hide this from the user, and use the
- * setChild:forIdentifier: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
- * property that behind the scenes is calling setChild:forIdentifier:.
+ * a subclass should use this method to set the "primary" child. This is actually the same as calling 
+ * setChild:forIdentifier:0. All other children should be set by defining convenience methods
+ * that call setChild:forIdentifier behind the scenes.
  */
 - (void)setChild:(id<ASLayoutable>)child;
 

AsyncDisplayKit/Layout/ASLayoutSpec.mm

@@ -121,7 +121,7 @@ typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASCh
       _children[0] = finalLayoutable;
       [self propagateUpLayoutable:finalLayoutable];
     }
-  } else if (_children.size() > 0) {
+  } else {
     _children.erase(0);
   }
 }