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.

AsyncDisplayKit/ASDisplayNode.mm

@@ -15,6 +15,7 @@
 
 #import <objc/runtime.h>
 #import <deque>
+#import <queue>
 
 #import "_ASAsyncTransaction.h"
 #import "_ASAsyncTransactionContainer+Private.h"
@@ -626,7 +627,7 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   if (! [self shouldMeasureWithSizeRange:constrainedSize]) {
     return _layout;
   }
-
+  
   [self cancelLayoutTransitionsInProgress];
 
   ASLayout *previousLayout = _layout;
@@ -637,13 +638,14 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
                                                           pendingLayout:newLayout
                                                          previousLayout:previousLayout];
   } else {
-    ASLayoutTransition *layoutContext;
+    ASLayoutTransition *layoutTransition = nil;
     if (self.usesImplicitHierarchyManagement) {
-      layoutContext = [[ASLayoutTransition alloc] initWithNode:self
-                                                 pendingLayout:newLayout
-                                                previousLayout:previousLayout];
+      layoutTransition = [[ASLayoutTransition alloc] initWithNode:self
+                                                    pendingLayout:newLayout
+                                                   previousLayout:previousLayout];
     }
-    [self applyLayout:newLayout layoutContext:layoutContext];
+    
+    [self _applyLayout:newLayout layoutTransition:layoutTransition];
     [self _completeLayoutCalculation];
   }
 
@@ -680,6 +682,11 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   return ASLayoutableTypeDisplayNode;
 }
 
+- (BOOL)canLayoutAsynchronous
+{
+  return !self.isNodeLoaded;
+}
+
 #pragma mark - Layout Transition
 
 - (void)transitionLayoutWithAnimation:(BOOL)animated
@@ -750,10 +757,10 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
       }
       
       ASLayout *previousLayout = _layout;
-      [self applyLayout:newLayout layoutContext:nil];
+      [self _applyLayout:newLayout layoutTransition:nil];
       
       ASDisplayNodePerformBlockOnEverySubnode(self, ^(ASDisplayNode * _Nonnull node) {
-        [node applyPendingLayoutContext];
+        [node _applyPendingLayoutContext];
         [node _completeLayoutCalculation];
         node.hierarchyState &= (~ASHierarchyStateLayoutPending);
       });
@@ -776,6 +783,7 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
     });
   };
 
+  // TODO ihm: Can we always push the measure to the background thread and remove the parameter from the API?
   if (shouldMeasureAsync) {
     ASPerformBlockOnBackgroundThread(transitionBlock);
   } else {
@@ -838,8 +846,8 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
 
 - (BOOL)_hasTransitionInProgress
 {
-    ASDN::MutexLocker l(_propertyLock);
-    return _transitionInProgress;
+  ASDN::MutexLocker l(_propertyLock);
+  return _transitionInProgress;
 }
 
 /// Starts a new transition and returns the transition id
@@ -2414,16 +2422,16 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
   });
 }
 
-- (void)applyPendingLayoutContext
+- (void)_applyPendingLayoutContext
 {
   ASDN::MutexLocker l(_propertyLock);
   if (_pendingLayoutTransition) {
-    [self applyLayout:_pendingLayoutTransition.pendingLayout layoutContext:_pendingLayoutTransition];
+    [self _applyLayout:_pendingLayoutTransition.pendingLayout layoutTransition:_pendingLayoutTransition];
     _pendingLayoutTransition = nil;
   }
 }
 
-- (void)applyLayout:(ASLayout *)layout layoutContext:(ASLayoutTransition *)layoutContext
+- (void)_applyLayout:(ASLayout *)layout layoutTransition:(ASLayoutTransition *)layoutTransition
 {
   ASDN::MutexLocker l(_propertyLock);
   _layout = layout;
@@ -2432,10 +2440,22 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
   ASDisplayNodeAssertTrue(layout.size.width >= 0.0);
   ASDisplayNodeAssertTrue(layout.size.height >= 0.0);
   
-  if (self.usesImplicitHierarchyManagement && layoutContext != nil) {
-    [layoutContext applySubnodeInsertions];
-    [layoutContext applySubnodeRemovals];
+  if (layoutTransition == nil || self.usesImplicitHierarchyManagement == NO) {
+    return;
   }
+
+  // Trampoline to the main thread if necessary
+  if (ASDisplayNodeThreadIsMain() == NO && layoutTransition.canTransitionAsynchronous == NO) {
+
+    // Subnode insertions and removals need to happen always on the main thread if at least one subnode is already loaded
+    ASPerformBlockOnMainThread(^{
+      [layoutTransition applySubnodeTransition];
+    });
+    
+    return;
+  }
+  
+  [layoutTransition applySubnodeTransition];
 }
 
 - (void)layout