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

AsyncDisplayKit/ASDisplayNode+Subclasses.h

@@ -143,6 +143,8 @@ NS_ASSUME_NONNULL_BEGIN
  * encouraged.
  *
  * @note This method should not be called directly outside of ASDisplayNode; use -measure: or -calculatedLayout instead.
+ *
+ * @warning Overwriting layoutSpecThatFits: in a subclass and providing a layoutSpecBlock block is currently not supported
  */
 - (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize;
 

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.
  *

AsyncDisplayKit/ASDisplayNode.mm

@@ -167,6 +167,14 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   return overrides;
 }
 
+     // At most a layoutSpecBlock or one of the three layout methods is overridden
+#define __ASDisplayNodeCheckForLayoutMethodOverrides \
+    ASDisplayNodeAssert(_layoutSpecBlock != nil || \
+    (ASDisplayNodeSubclassOverridesSelector(self.class, @selector(calculateSizeThatFits:)) ? 1 : 0) \
+    + (ASDisplayNodeSubclassOverridesSelector(self.class, @selector(layoutSpecThatFits:)) ? 1 : 0) \
+    + (ASDisplayNodeSubclassOverridesSelector(self.class, @selector(calculateLayoutThatFits:)) ? 1 : 0) <= 1, \
+    @"Subclass %@ must at least provide a layoutSpecBlock or override at most one of the three layout methods: calculateLayoutThatFits, layoutSpecThatFits or calculateSizeThatFits", NSStringFromClass(self.class))
+
 + (void)initialize
 {
   if (self != [ASDisplayNode class]) {
@@ -178,12 +186,6 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
     ASDisplayNodeAssert(!ASDisplayNodeSubclassOverridesSelector(self, @selector(measureWithSizeRange:)), @"Subclass %@ must not override measureWithSizeRange method", NSStringFromClass(self));
     ASDisplayNodeAssert(!ASDisplayNodeSubclassOverridesSelector(self, @selector(recursivelyClearContents)), @"Subclass %@ must not override recursivelyClearContents method", NSStringFromClass(self));
     ASDisplayNodeAssert(!ASDisplayNodeSubclassOverridesSelector(self, @selector(recursivelyClearFetchedData)), @"Subclass %@ must not override recursivelyClearFetchedData method", NSStringFromClass(self));
-
-    // At most one of the three layout methods is overridden
-    ASDisplayNodeAssert((ASDisplayNodeSubclassOverridesSelector(self, @selector(calculateSizeThatFits:)) ? 1 : 0)
-                        + (ASDisplayNodeSubclassOverridesSelector(self, @selector(layoutSpecThatFits:)) ? 1 : 0)
-                        + (ASDisplayNodeSubclassOverridesSelector(self, @selector(calculateLayoutThatFits:)) ? 1 : 0) <= 1,
-                        @"Subclass %@ must override at most one of the three layout methods: calculateLayoutThatFits, layoutSpecThatFits or calculateSizeThatFits", NSStringFromClass(self));
   }
 
   // Below we are pre-calculating values per-class and dynamically adding a method (_staticInitialize) to populate these values
@@ -1848,8 +1850,10 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
 
 - (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize
 {
+  __ASDisplayNodeCheckForLayoutMethodOverrides;
+
   ASDN::MutexLocker l(_propertyLock);
-  if (_methodOverrides & ASDisplayNodeMethodOverrideLayoutSpecThatFits) {
+  if ((_methodOverrides & ASDisplayNodeMethodOverrideLayoutSpecThatFits) || _layoutSpecBlock != nil) {
     ASLayoutSpec *layoutSpec = [self layoutSpecThatFits:constrainedSize];
     layoutSpec.parent = self; // This causes upward propogation of any non-default layoutable values.
     layoutSpec.isMutable = NO;
@@ -1876,13 +1880,22 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
 
 - (CGSize)calculateSizeThatFits:(CGSize)constrainedSize
 {
+  __ASDisplayNodeCheckForLayoutMethodOverrides;
+    
   ASDN::MutexLocker l(_propertyLock);
   return _preferredFrameSize;
 }
 
 - (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize
 {
+  __ASDisplayNodeCheckForLayoutMethodOverrides;
+
   ASDN::MutexLocker l(_propertyLock);
+  
+  if (_layoutSpecBlock != nil) {
+    return _layoutSpecBlock(constrainedSize);
+  }
+  
   return nil;
 }
 
@@ -1904,6 +1917,14 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
   return _constrainedSize;
 }
 
+- (void)setLayoutSpecThatFitsBlock:(ASLayoutSpecBlock)layoutSpecBlock
+{
+  // For now there should never be a overwrite of layoutSpecThatFits: and a layoutSpecThatFitsBlock: be provided
+  ASDisplayNodeAssert(!(_methodOverrides & ASDisplayNodeMethodOverrideLayoutSpecThatFits), @"Overwriting layoutSpecThatFits: and providing a layoutSpecBlock block is currently not supported");
+  
+  _layoutSpecBlock = layoutSpecBlock;
+}
+
 - (void)setPendingTransitionID:(int32_t)pendingTransitionID
 {
   ASDN::MutexLocker l(_propertyLock);

AsyncDisplayKitTests/ASDisplayNodeImplicitHierarchyTests.m

@@ -17,8 +17,6 @@
 
 @interface ASSpecTestDisplayNode : ASDisplayNode
 
-@property (copy, nonatomic) ASLayoutSpec * (^layoutSpecBlock)(ASSizeRange constrainedSize, NSNumber *layoutState);
-
 /**
  Simple state identifier to allow control of current spec inside of the layoutSpecBlock
  */
@@ -37,11 +35,6 @@
   return self;
 }
 
-- (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize
-{
-  return self.layoutSpecBlock(constrainedSize, _layoutState);
-}
-
 @end
 
 @interface ASDisplayNodeImplicitHierarchyTests : XCTestCase
@@ -83,7 +76,7 @@
   ASDisplayNode *node5 = [[ASDisplayNode alloc] init];
 
   ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
-  node.layoutSpecBlock = ^(ASSizeRange constrainedSize, NSNumber *layoutState) {
+  node.layoutSpecBlock = ^(ASSizeRange constrainedSize) {
     ASStaticLayoutSpec *staticLayout = [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node4]];
     
     ASStackLayoutSpec *stack1 = [[ASStackLayoutSpec alloc] init];
@@ -109,8 +102,9 @@
   ASDisplayNode *node3 = [[ASDisplayNode alloc] init];
   
   ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
-  node.layoutSpecBlock = ^(ASSizeRange constrainedSize, NSNumber *layoutState){
+  __weak ASSpecTestDisplayNode *weakNode = node;
-    if ([layoutState isEqualToNumber:@1]) {
+  node.layoutSpecBlock = ^(ASSizeRange constrainedSize){
+    if ([weakNode.layoutState isEqualToNumber:@1]) {
       return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1, node2]];
     } else {
       ASStackLayoutSpec *stackLayout = [[ASStackLayoutSpec alloc] init];