Simplify Layout Transition State #trivial

Source/ASDisplayNode+Layout.mm

@@ -454,12 +454,6 @@ - (void)setAutomaticallyManagesSubnodes:(BOOL)automaticallyManagesSubnodes
 
 @implementation ASDisplayNode (ASLayoutTransition)
 
-- (BOOL)_isTransitionInProgress
-{
-  ASDN::MutexLocker l(__instanceLock__);
-  return _transitionInProgress;
-}
-
 - (BOOL)_isLayoutTransitionInvalid
 {
   ASDN::MutexLocker l(__instanceLock__);
@@ -475,40 +469,17 @@ - (BOOL)_isLayoutTransitionInvalid
 /// Starts a new transition and returns the transition id
 - (int32_t)_startNewTransition
 {
-  ASDN::MutexLocker l(__instanceLock__);
-  _transitionInProgress = YES;
-  _transitionID = OSAtomicAdd32(1, &_transitionID);
-  return _transitionID;
-}
-
-- (void)_finishOrCancelTransition
-{
-  ASDN::MutexLocker l(__instanceLock__);
-  _transitionInProgress = NO;
+  static std::atomic<int32_t> gNextTransitionID;
+  int32_t newTransitionID = gNextTransitionID.fetch_add(1) + 1;
+  _transitionID = newTransitionID;
+  return newTransitionID;
 }
 
-- (void)setPendingTransitionID:(int32_t)pendingTransitionID
-{
-  ASDN::MutexLocker l(__instanceLock__);
-  ASDisplayNodeAssertTrue(_pendingTransitionID < pendingTransitionID);
-  _pendingTransitionID = pendingTransitionID;
-}
-
-- (int32_t)pendingTransitionID
+/// Returns NO if there was no transition to cancel/finish.
+- (BOOL)_finishOrCancelTransition
 {
-  ASDN::MutexLocker l(__instanceLock__);
-  return _pendingTransitionID;
-}
-
-- (BOOL)_shouldAbortTransitionWithID:(int32_t)transitionID
-{
-  ASDN::MutexLocker l(__instanceLock__);
-  return [self _locked_shouldAbortTransitionWithID:transitionID];
-}
-
-- (BOOL)_locked_shouldAbortTransitionWithID:(int32_t)transitionID
-{
-  return (!_transitionInProgress || _transitionID != transitionID);
+  int32_t oldValue = _transitionID.exchange(ASLayoutElementContextInvalidTransitionID);
+  return oldValue != ASLayoutElementContextInvalidTransitionID;
 }
 
 #pragma mark Layout Transition
@@ -554,17 +525,21 @@ - (void)transitionLayoutWithSizeRange:(ASSizeRange)constrainedSize
 
   // Every new layout transition has a transition id associated to check in subsequent transitions for cancelling
   int32_t transitionID = [self _startNewTransition];
-
+  // NOTE: This block captures self. It's cheaper than hitting the weak table.
+  asdisplaynode_iscancelled_block_t isCancelled = ^{
+    return (BOOL)(_transitionID != transitionID);
+  };
+
   // Move all subnodes in layout pending state for this transition
   ASDisplayNodePerformBlockOnEverySubnode(self, NO, ^(ASDisplayNode * _Nonnull node) {
-    ASDisplayNodeAssert([node _isTransitionInProgress] == NO, @"Can't start a transition when one of the subnodes is performing one.");
+    ASDisplayNodeAssert(node->_transitionID == ASLayoutElementContextInvalidTransitionID, @"Can't start a transition when one of the subnodes is performing one.");
     node.hierarchyState |= ASHierarchyStateLayoutPending;
-    node.pendingTransitionID = transitionID;
+    node->_pendingTransitionID = transitionID;
   });
 
   // Transition block that executes the layout transition
   void (^transitionBlock)(void) = ^{
-    if ([self _shouldAbortTransitionWithID:transitionID]) {
+    if (isCancelled()) {
       return;
     }
 
@@ -587,22 +562,21 @@ - (void)transitionLayoutWithSizeRange:(ASSizeRange)constrainedSize
       ASLayoutElementClearCurrentContext();
     }
 
-    if ([self _shouldAbortTransitionWithID:transitionID]) {
+    if (isCancelled()) {
       return;
     }
 
     ASPerformBlockOnMainThread(^{
+      if (isCancelled()) {
+        return;
+      }
       ASLayoutTransition *pendingLayoutTransition;
       _ASTransitionContext *pendingLayoutTransitionContext;
       {
         // Grab __instanceLock__ here to make sure this transition isn't invalidated
         // right after it passed the validation test and before it proceeds
         ASDN::MutexLocker l(__instanceLock__);
 
-        if ([self _locked_shouldAbortTransitionWithID:transitionID]) {
-          return;
-        }
-
         // Update calculated layout
         auto previousLayout = _calculatedDisplayNodeLayout;
         auto pendingLayout = std::make_shared<ASDisplayNodeLayout>(newLayout,
@@ -654,14 +628,7 @@ - (void)transitionLayoutWithSizeRange:(ASSizeRange)constrainedSize
 
 - (void)cancelLayoutTransition
 {
-  __instanceLock__.lock();
-  BOOL transitionInProgress = _transitionInProgress;
-  __instanceLock__.unlock();
-
-  if (transitionInProgress) {
-    // Cancel transition in progress
-    [self _finishOrCancelTransition];
-
+  if ([self _finishOrCancelTransition]) {
     // Tell subnodes to exit layout pending state and clear related properties
     ASDisplayNodePerformBlockOnEverySubnode(self, NO, ^(ASDisplayNode * _Nonnull node) {
       node.hierarchyState &= (~ASHierarchyStateLayoutPending);

Source/ASDisplayNode.mm

@@ -916,7 +916,7 @@ - (void)__layout
 
     // If a current layout transition is in progress there is no need to do a measurement and layout pass in here as
     // this is supposed to happen within the layout transition process
-    if (_transitionInProgress) {
+    if (_transitionID != ASLayoutElementContextInvalidTransitionID) {
       return;
     }
 
@@ -1807,15 +1807,14 @@ - (void)_setSupernode:(ASDisplayNode *)newSupernode
       // This is especially important as with automatic subnode management, adding subnodes can happen while a transition
       // is in fly
       if (ASHierarchyStateIncludesLayoutPending(stateToEnterOrExit)) {
-        int32_t pendingTransitionId = newSupernode.pendingTransitionID;
+        int32_t pendingTransitionId = newSupernode->_pendingTransitionID;
         if (pendingTransitionId != ASLayoutElementContextInvalidTransitionID) {
           {
-            ASDN::MutexLocker l(__instanceLock__);
             _pendingTransitionID = pendingTransitionId;
 
             // Propagate down the new pending transition id
             ASDisplayNodePerformBlockOnEverySubnode(self, NO, ^(ASDisplayNode * _Nonnull node) {
-              node.pendingTransitionID = pendingTransitionId;
+              node->_pendingTransitionID = pendingTransitionId;
             });
           }
         }

Source/Private/ASDisplayNode+FrameworkPrivate.h

@@ -229,7 +229,7 @@ __unused static NSString * _Nonnull NSStringFromASHierarchyState(ASHierarchyStat
 @end
 
 
-@interface ASDisplayNode (ASsLayoutInternal)
+@interface ASDisplayNode (ASLayoutInternal)
 
 /**
  * @abstract Informs the root node that the intrinsic size of the receiver is no longer valid.
@@ -260,11 +260,6 @@ __unused static NSString * _Nonnull NSStringFromASHierarchyState(ASHierarchyStat
 
 @interface ASDisplayNode (ASLayoutTransitionInternal)
 
-/**
- * Sentinel of the current layout transition
- */
-@property (atomic, assign) int32_t pendingTransitionID;
-
 /**
  * If one or multiple layout transitions are in flight this methods returns if the current layout transition that
  * happens in in this particular thread was invalidated through another thread is starting a transition for this node

Source/Private/ASDisplayNodeInternal.h

@@ -150,10 +150,9 @@ FOUNDATION_EXPORT NSString * const ASRenderingEngineDidDisplayNodesScheduledBefo
 
   ASLayoutSpecBlock _layoutSpecBlock;
 
-  int32_t _transitionID;
-  BOOL _transitionInProgress;
+  std::atomic<int32_t> _transitionID;
 
-  int32_t _pendingTransitionID;
+  std::atomic<int32_t> _pendingTransitionID;
   ASLayoutTransition *_pendingLayoutTransition;
   std::shared_ptr<ASDisplayNodeLayout> _calculatedDisplayNodeLayout;
   std::shared_ptr<ASDisplayNodeLayout> _pendingDisplayNodeLayout;