From 75eab1db07382d33598b082b27e65833dd65cc08 Mon Sep 17 00:00:00 2001
From: Max Gu <maxgu@pinterest.com>
Date: Thu, 18 Feb 2016 18:00:37 -0800
Subject: [PATCH] Adding scroll visibility

---
 AsyncDisplayKit/ASCellNode.h              |  5 +++++
 AsyncDisplayKit/ASCellNode.m              |  6 ++++++
 AsyncDisplayKit/ASCollectionView.mm       | 16 +++++++++++++++-
 AsyncDisplayKit/Details/ASDelegateProxy.m |  3 +++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/AsyncDisplayKit/ASCellNode.h b/AsyncDisplayKit/ASCellNode.h
index d1e575383e..669c6757f5 100644
--- a/AsyncDisplayKit/ASCellNode.h
+++ b/AsyncDisplayKit/ASCellNode.h
@@ -16,6 +16,7 @@ typedef NSUInteger ASCellNodeAnimation;
 
 @protocol ASCellNodeLayoutDelegate <NSObject>
 
+- (void)scrollViewDidScroll:(UIScrollView *)scrollView cellFrameInScrollView:(CGRect)cellFrame;
 /**
  * Notifies the delegate that the specified cell node has done a relayout.
  * The notification is done on main thread.
@@ -24,6 +25,8 @@ typedef NSUInteger ASCellNodeAnimation;
  * @param sizeChanged `YES` if the node's `calculatedSize` changed during the relayout, `NO` otherwise.
  */
 - (void)nodeDidRelayout:(ASCellNode *)node sizeChanged:(BOOL)sizeChanged;
+
+
 @end
 
 /**
@@ -75,6 +78,7 @@ typedef NSUInteger ASCellNodeAnimation;
  */
 @property (nonatomic, weak) id<ASCellNodeLayoutDelegate> layoutDelegate;
 
+@property (nonatomic, assign) BOOL shouldObserveVisibility;
 /*
  * ASCellNode must forward touch events in order for UITableView and UICollectionView tap handling to work. Overriding
  * these methods (e.g. for highlighting) requires the super method be called.
@@ -107,6 +111,7 @@ typedef NSUInteger ASCellNodeAnimation;
  */
 - (instancetype)initWithViewControllerBlock:(ASDisplayNodeViewControllerBlock)viewControllerBlock didLoadBlock:(ASDisplayNodeDidLoadBlock)didLoadBlock;
 
+- (void)updateScrollSituationWithScrollVIew:(UIScrollView *)scrollView;
 @end
 
 
diff --git a/AsyncDisplayKit/ASCellNode.m b/AsyncDisplayKit/ASCellNode.m
index df7040d92e..840c5edea3 100644
--- a/AsyncDisplayKit/ASCellNode.m
+++ b/AsyncDisplayKit/ASCellNode.m
@@ -134,6 +134,12 @@
   [(_ASDisplayView *)self.view __forwardTouchesCancelled:touches withEvent:event];
 }
 
+- (void)updateScrollSituationWithScrollVIew:(UIScrollView *)scrollView
+{
+    // TODO(Max): Fix the cellFrame here
+    [self.layoutDelegate scrollViewDidScroll:scrollView cellFrameInScrollView:CGRectZero];
+}
+
 @end
 
 
diff --git a/AsyncDisplayKit/ASCollectionView.mm b/AsyncDisplayKit/ASCollectionView.mm
index b9f58ee5ff..ccfaa73115 100644
--- a/AsyncDisplayKit/ASCollectionView.mm
+++ b/AsyncDisplayKit/ASCollectionView.mm
@@ -67,7 +67,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   ASRangeController *_rangeController;
   ASCollectionViewLayoutController *_layoutController;
   ASCollectionViewFlowLayoutInspector *_flowLayoutInspector;
-    
+  NSMutableArray *_cellsForVisibilityUpdates;
   id<ASCollectionViewLayoutFacilitatorProtocol> _layoutFacilitator;
   
   BOOL _performingBatchUpdates;
@@ -212,6 +212,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   
   _registeredSupplementaryKinds = [NSMutableSet set];
   
+  _cellsForVisibilityUpdates = [[NSMutableArray alloc] init];
   self.backgroundColor = [UIColor whiteColor];
   
   [self registerClass:[_ASCollectionViewCell class] forCellWithReuseIdentifier:kCellReuseIdentifier];
@@ -589,6 +590,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   if (cellNode.neverShowPlaceholders) {
     [cellNode recursivelyEnsureDisplaySynchronously:YES];
   }
+  [_cellsForVisibilityUpdates addObject:cell];
 }
 
 - (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
@@ -600,6 +602,7 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
     ASDisplayNodeAssertNotNil(node, @"Expected node associated with removed cell not to be nil.");
     [_asyncDelegate collectionView:self didEndDisplayingNode:node forItemAtIndexPath:indexPath];
   }
+  [_cellsForVisibilityUpdates removeObject:cell];
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
   if ([_asyncDelegate respondsToSelector:@selector(collectionView:didEndDisplayingNodeForItemAtIndexPath:)]) {
@@ -650,6 +653,17 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   }
 }
 
+- (void)scrollViewDidScroll:(UIScrollView *)scrollView{
+    for (ASCellNode *node in _cellsForVisibilityUpdates) {
+        if (node.shouldObserveVisibility) {
+            [node updateScrollSituationWithScrollVIew:scrollView];
+        }
+    }
+    if ([_asyncDelegate respondsToSelector:@selector(scrollViewDidScroll:)]) {
+        [_asyncDelegate scrollViewDidScroll:scrollView];
+    }
+}
+
 - (BOOL)shouldBatchFetch
 {
   // if the delegate does not respond to this method, there is no point in starting to fetch
diff --git a/AsyncDisplayKit/Details/ASDelegateProxy.m b/AsyncDisplayKit/Details/ASDelegateProxy.m
index 2af0e7c7d5..baa65a65c7 100644
--- a/AsyncDisplayKit/Details/ASDelegateProxy.m
+++ b/AsyncDisplayKit/Details/ASDelegateProxy.m
@@ -56,6 +56,9 @@
           // used for batch fetching API
           selector == @selector(scrollViewWillEndDragging:withVelocity:targetContentOffset:) ||
           
+          // used for ASCellNode visibility
+          selector == @selector(scrollViewDidScroll:) ||
+          
           // intercepted due to not being supported by ASCollectionView (prevent bugs caused by usage)
           selector == @selector(collectionView:canMoveItemAtIndexPath:) ||
           selector == @selector(collectionView:moveItemAtIndexPath:toIndexPath:) ||