Add a block API to provide an ASLayoutSpec without having to subclass ASDisplayNode

AsyncDisplayKit/ASDisplayNode.h

@@ -42,6 +42,11 @@ typedef void (^ASDisplayNodeDidLoadBlock)(ASDisplayNode * _Nonnull node);
  */
 typedef void (^ASDisplayNodeContextModifier)(_Nonnull CGContextRef context);
 
+/**
+ * ASDisplayNode layout spec block. This block can be used instead of implementing layoutSpecThatFits: in subclass
+ */
+typedef ASLayoutSpec * _Nonnull(^ASLayoutSpecBlock)(ASSizeRange constrainedSize);
+
 /**
  Interface state is available on ASDisplayNode and ASViewController, and
  allows checking whether a node is in an interface situation where it is prudent to trigger certain
@@ -252,6 +257,17 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize;
 
+
+/**
+ * @abstract Provides a way to declare a block to provide an ASLayoutSpec without having to subclass ASDisplayNode and
+ * implement layoutSpecThatFits:
+ *
+ * @return The block used to provide a ASLayoutSpec
+ *
+ * @warning Overwriting layoutSpecThatFits: in a subclass and providing a layoutSpecBlock block is currently not supported
+ */
+@property (nonatomic, readwrite, copy, nullable) ASLayoutSpecBlock layoutSpecBlock;
+
 /** 
  * @abstract Return the calculated size.
  *