Adds trampoline for inserting and deletion of nodes

Currently measurement always needs to happen on the main thread if implicit hierarchy management is enabled as adding and removing from nodes needs to happen on the main thread. We now will trampoline to the main thread to do the insertion and deletion of nodes.

This also resolves the issue that can occur if a node is already loaded deep in the layout hierarchy in the layout that the node is transforming to. Before insertion or deletion is happening we need to crawl the layout hierarchy to check that though.

AsyncDisplayKitTests/ASDisplayNodeImplicitHierarchyTests.m

@@ -130,4 +130,88 @@
   XCTAssertEqual(node.subnodes[2], node2);
 }
 
+- (void)testMeasurementInBackgroundThreadWithLoadedNode
+{
+  ASDisplayNode *node1 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node2 = [[ASDisplayNode alloc] init];
+  
+  ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
+  node.layoutSpecBlock = ^(ASDisplayNode *weakNode, ASSizeRange constrainedSize) {
+    ASSpecTestDisplayNode *strongNode = (ASSpecTestDisplayNode *)weakNode;
+    if ([strongNode.layoutState isEqualToNumber:@1]) {
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1]];
+    } else {
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node2]];
+    }
+  };
+  
+  // Intentionally trigger view creation
+  [node2 view];
+  
+  XCTestExpectation *expectation = [self expectationWithDescription:@"Fix IHM layout also if one node is already loaded"];
+  
+  dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    
+    [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
+    XCTAssertEqual(node.subnodes[0], node1);
+    
+    node.layoutState = @2;
+    [node invalidateCalculatedLayout];
+    [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
+    
+    // Dispatch back to the main thread to let the insertion / deletion of subnodes happening
+    dispatch_async(dispatch_get_main_queue(), ^{
+      XCTAssertEqual(node.subnodes[0], node2);
+      [expectation fulfill];
+    });
+  });
+  
+  [self waitForExpectationsWithTimeout:5.0 handler:^(NSError *error) {
+    if (error) {
+      NSLog(@"Timeout Error: %@", error);
+    }
+  }];
+}
+
+- (void)testTransitionLayoutWithAnimationWithLoadedNodes
+{
+  ASDisplayNode *node1 = [[ASDisplayNode alloc] init];
+  ASDisplayNode *node2 = [[ASDisplayNode alloc] init];
+  
+  ASSpecTestDisplayNode *node = [[ASSpecTestDisplayNode alloc] init];
+  
+  node.layoutSpecBlock = ^(ASDisplayNode *weakNode, ASSizeRange constrainedSize) {
+    ASSpecTestDisplayNode *strongNode = (ASSpecTestDisplayNode *)weakNode;
+    if ([strongNode.layoutState isEqualToNumber:@1]) {
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node1]];
+    } else {
+      return [ASStaticLayoutSpec staticLayoutSpecWithChildren:@[node2]];
+    }
+  };
+ 
+  // Intentionally trigger view creation
+  [node2 view];
+  
+  XCTestExpectation *expectation = [self expectationWithDescription:@"Fix IHM layout transition also if one node is already loaded"];
+  
+  [node measureWithSizeRange:ASSizeRangeMake(CGSizeZero, CGSizeZero)];
+  XCTAssertEqual(node.subnodes[0], node1);
+  
+  node.layoutState = @2;
+  [node invalidateCalculatedLayout];
+  [node transitionLayoutWithAnimation:YES shouldMeasureAsync:YES measurementCompletion:^{
+    // Push this to the next runloop to let async insertion / removing of nodes finished before checking
+    dispatch_async(dispatch_get_main_queue(), ^{
+      XCTAssertEqual(node.subnodes[0], node2);
+      [expectation fulfill];
+    });
+  }];
+  
+  [self waitForExpectationsWithTimeout:5.0 handler:^(NSError *error) {
+    if (error) {
+      NSLog(@"Timeout Error: %@", error);
+    }
+  }];
+}
+
 @end