[ASDisplayNode+Layout] In layoutThatFits:, check and use _pending lay…

…out if valid. (TextureGroup#413)

I believe this check is supposed to be here. Unit tests pass with it in place.
Without it, calling layoutThatFits: (as ASDataController does) and then calling
it again later (as ASCollectionNode does when answering the sizeForItem: call)
will recompute the layout, potentially on main.

This seems to have more of an impact / benefit for Yoga layouts, but I don't think
its benefit is exclusive to them.

Source/ASDisplayNode+Layout.mm

@@ -73,31 +73,34 @@ - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 - (ASLayout *)layoutThatFits:(ASSizeRange)constrainedSize parentSize:(CGSize)parentSize
 {
   ASDN::MutexLocker l(__instanceLock__);
- 
+
   // If one or multiple layout transitions are in flight it still can happen that layout information is requested
   // on other threads. As the pending and calculated layout to be updated in the layout transition in here just a
   // layout calculation wil be performed without side effect
   if ([self _isLayoutTransitionInvalid]) {
     return [self calculateLayoutThatFits:constrainedSize restrictedToSize:self.style.size relativeToParentSize:parentSize];
   }
 
+  ASLayout *layout = nil;
   if (_calculatedDisplayNodeLayout->isValidForConstrainedSizeParentSize(constrainedSize, parentSize)) {
     ASDisplayNodeAssertNotNil(_calculatedDisplayNodeLayout->layout, @"-[ASDisplayNode layoutThatFits:parentSize:] _calculatedDisplayNodeLayout->layout should not be nil! %@", self);
     // Our calculated layout is suitable for this constrainedSize, so keep using it and
     // invalidate any pending layout that has been generated in the past.
     _pendingDisplayNodeLayout = nullptr;
-    return _calculatedDisplayNodeLayout->layout ?: [ASLayout layoutWithLayoutElement:self size:{0, 0}];
+    layout = _calculatedDisplayNodeLayout->layout;
+  } else if (_pendingDisplayNodeLayout != nullptr && _pendingDisplayNodeLayout->isValidForConstrainedSizeParentSize(constrainedSize, parentSize)) {
+    ASDisplayNodeAssertNotNil(_pendingDisplayNodeLayout->layout, @"-[ASDisplayNode layoutThatFits:parentSize:] _pendingDisplayNodeLayout->layout should not be nil! %@", self);
+    layout = _pendingDisplayNodeLayout->layout;
+  } else {
+    // Create a pending display node layout for the layout pass
+    layout = [self calculateLayoutThatFits:constrainedSize
+                          restrictedToSize:self.style.size
+                      relativeToParentSize:parentSize];
+    _pendingDisplayNodeLayout = std::make_shared<ASDisplayNodeLayout>(layout, constrainedSize, parentSize);
+    ASDisplayNodeAssertNotNil(layout, @"-[ASDisplayNode layoutThatFits:parentSize:] newly calculated layout should not be nil! %@", self);
   }
 
-  // Create a pending display node layout for the layout pass
-  _pendingDisplayNodeLayout = std::make_shared<ASDisplayNodeLayout>(
-    [self calculateLayoutThatFits:constrainedSize restrictedToSize:self.style.size relativeToParentSize:parentSize],
-    constrainedSize,
-    parentSize
-  );
-
-  ASDisplayNodeAssertNotNil(_pendingDisplayNodeLayout->layout, @"-[ASDisplayNode layoutThatFits:parentSize:] _pendingDisplayNodeLayout->layout should not be nil! %@", self);
-  return _pendingDisplayNodeLayout->layout ?: [ASLayout layoutWithLayoutElement:self size:{0, 0}];
+  return layout ?: [ASLayout layoutWithLayoutElement:self size:{0, 0}];
 }
 
 #pragma mark ASLayoutElementStyleExtensibility