[ASLayoutTransition] Remove taking a snapshot in the default layout transition code (facebookarchive#2940)

* Remove taking a snapshot in the default default layout transition code

* Set groupOpacity for root node of animation to true before and restore after animation finished

Author: maicki

AsyncDisplayKit/ASDisplayNode.mm

@@ -1600,7 +1600,6 @@ - (void)animateLayoutTransition:(id<ASContextTransitioning>)context
   NSAssert(node.isNodeLoaded == YES, @"Invalid node state");
 
   NSArray<ASDisplayNode *> *removedSubnodes = [context removedSubnodes];
-  NSMutableArray<UIView *> *removedViews = [NSMutableArray array];
   NSMutableArray<ASDisplayNode *> *insertedSubnodes = [[context insertedSubnodes] mutableCopy];
   NSMutableArray<ASDisplayNode *> *movedSubnodes = [NSMutableArray array];
 
@@ -1613,15 +1612,6 @@ - (void)animateLayoutTransition:(id<ASContextTransitioning>)context
       CGRect fromFrame = [context initialFrameForNode:subnode];
       CGRect toFrame = [context finalFrameForNode:subnode];
       if (CGSizeEqualToSize(fromFrame.size, toFrame.size) == NO) {
-        // To crossfade resized subnodes, show a snapshot of it on top.
-        // The node itself can then be treated as a newly-inserted one.
-        UIView *snapshotView = [subnode.view snapshotViewAfterScreenUpdates:YES];
-        snapshotView.frame = [context initialFrameForNode:subnode];
-        snapshotView.alpha = 1;
-        
-        [node.view insertSubview:snapshotView aboveSubview:subnode.view];
-        [removedViews addObject:snapshotView];
-        
         [insertedSubnodes addObject:subnode];
       }
       if (CGPointEqualToPoint(fromFrame.origin, toFrame.origin) == NO) {
@@ -1643,15 +1633,16 @@ - (void)animateLayoutTransition:(id<ASContextTransitioning>)context
     insertedSubnode.frame = [context finalFrameForNode:insertedSubnode];
     insertedSubnode.alpha = 0;
   }
+  
+  // Adjust groupOpacity for animation
+  BOOL originAllowsGroupOpacity = node.allowsGroupOpacity;
+  node.allowsGroupOpacity = YES;
 
   [UIView animateWithDuration:self.defaultLayoutTransitionDuration delay:self.defaultLayoutTransitionDelay options:self.defaultLayoutTransitionOptions animations:^{
     // Fade removed subnodes and views out
     for (ASDisplayNode *removedSubnode in removedSubnodes) {
       removedSubnode.alpha = 0;
     }
-    for (UIView *removedView in removedViews) {
-      removedView.alpha = 0;
-    }
 
     // Fade inserted subnodes in
     for (_ASAnimatedTransitionContext *insertedSubnodeContext in insertedSubnodeContexts) {
@@ -1674,9 +1665,10 @@ - (void)animateLayoutTransition:(id<ASContextTransitioning>)context
     for (_ASAnimatedTransitionContext *removedSubnodeContext in removedSubnodeContexts) {
       removedSubnodeContext.node.alpha = removedSubnodeContext.alpha;
     }
-    for (UIView *removedView in removedViews) {
-      [removedView removeFromSuperview];
-    }
+    
+    // Restore group opacity
+    node.allowsGroupOpacity = originAllowsGroupOpacity;
+    
     // Subnode removals are automatically performed
     [context completeTransition:finished];
   }];