Merge pull request #1861 from facebook/AHCollectionViewLayerRetaining

[ASCollectionView] Add strong Layer Pointer Under iOS < 9 to Workaround Crashes

AsyncDisplayKit/ASCollectionView.mm

@@ -107,6 +107,15 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   
   ASCollectionViewInvalidationStyle _nextLayoutInvalidationStyle;
   
+  /**
+   * Our layer, retained. Under iOS < 9, when collection views are removed from the hierarchy,
+   * their layers may be deallocated and become dangling pointers. This puts the collection view
+   * into a very dangerous state where pretty much any call will crash it. So we manually retain our layer.
+   *
+   * You should never access this, and it will be nil under iOS >= 9.
+   */
+  CALayer *_retainedLayer;
+  
   /**
    * If YES, the `UICollectionView` will reload its data on next layout pass so we should not forward any updates to it.
    
@@ -248,6 +257,10 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   
   [self registerClass:[_ASCollectionViewCell class] forCellWithReuseIdentifier:kCellReuseIdentifier];
   
+  if (!AS_AT_LEAST_IOS9) {
+    _retainedLayer = self.layer;
+  }
+  
   return self;
 }