Implement simple, in-order add/remove subnode support when changing layout specs

AsyncDisplayKitTests/ASDisplayNodeImplicitHierarchyTests.m

@@ -11,19 +11,35 @@
 #import "ASDisplayNode.h"
 #import "ASDisplayNode+Beta.h"
 #import "ASDisplayNode+Subclasses.h"
+
 #import "ASStaticLayoutSpec.h"
+#import "ASStackLayoutSpec.h"
 
 @interface ASSpecTestDisplayNode : ASDisplayNode
 
-@property (copy, nonatomic) ASLayoutSpec * (^layoutSpecBlock)(ASSizeRange constrainedSize);
+@property (copy, nonatomic) ASLayoutSpec * (^layoutSpecBlock)(ASSizeRange constrainedSize, NSNumber *layoutState);
+
+/**
+ Simple state identifier to allow control of current spec inside of the layoutSpecBlock
+ */
+@property (strong, nonatomic) NSNumber *layoutState;
 
 @end
 
 @implementation ASSpecTestDisplayNode
 
+- (instancetype)init
+{
+  self = [super init];
+  if (self) {
+    _layoutState = @1;
+  }
+  return self;
+}
+
 - (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize
 {
-  return self.layoutSpecBlock(constrainedSize);
+  return self.layoutSpecBlock(constrainedSize, _layoutState);
 }
 
 @end
@@ -49,18 +65,65 @@
   XCTAssert([ASDisplayNode usesImplicitHierarchyManagement]);
 }
 
-- (void)testInitialNodeInsertion
+- (void)testInitialNodeInsertionWithOrdering
 {
   ASDisplayNode *node1 = [[ASDisplayNode alloc] init];
   ASDisplayNode *node2 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node3 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node4 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node5 = [[ASDisplayNode alloc] init];
+
   ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
-  node.layoutSpecBlock = ^(ASSizeRange constrainedSize){
-    return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1, node2]];
+  node.layoutSpecBlock = ^(ASSizeRange constrainedSize, NSNumber *layoutState) {
+    ASStaticLayoutSpec *staticLayout = [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node4]];
+    
+    ASStackLayoutSpec *stack1 = [[ASStackLayoutSpec alloc] init];
+    [stack1 setChildren:@[node1, node2]];
+
+    ASStackLayoutSpec *stack2 = [[ASStackLayoutSpec alloc] init];
+    [stack2 setChildren:@[node3, staticLayout]];
+    
+    return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[stack1, stack2, node5]];
   };
+  [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
+  [node layout]; // Layout immediately
+  XCTAssertEqual(node.subnodes[0], node5);
+  XCTAssertEqual(node.subnodes[1], node1);
+  XCTAssertEqual(node.subnodes[2], node2);
+  XCTAssertEqual(node.subnodes[3], node3);
+  XCTAssertEqual(node.subnodes[4], node4);
+}
+
+- (void)testCalculatedLayoutHierarchyTransitions
+{
+  ASDisplayNode *node1 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node2 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node3 = [[ASDisplayNode alloc] init];
+  
+  ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
+  node.layoutSpecBlock = ^(ASSizeRange constrainedSize, NSNumber *layoutState){
+    if ([layoutState isEqualToNumber:@1]) {
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1, node2]];
+    } else {
+      ASStackLayoutSpec *stackLayout = [[ASStackLayoutSpec alloc] init];
+      [stackLayout setChildren:@[node3, node2]];
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1, stackLayout]];
+    }
+  };
+  
   [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
   [node layout]; // Layout immediately
   XCTAssertEqual(node.subnodes[0], node1);
   XCTAssertEqual(node.subnodes[1], node2);
+  
+  node.layoutState = @2;
+  [node invalidateCalculatedLayout]; // TODO(levi): Look into a way where measureWithSizeRange resizes when a new hierarchy is introduced but the size has not changed
+  [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
+  [node layout]; // Layout immediately
+
+  XCTAssertEqual(node.subnodes[0], node1);
+  XCTAssertEqual(node.subnodes[1], node3);
+  XCTAssertEqual(node.subnodes[2], node2);
 }
 
 @end