[Batch Fetching] Add ASBatchFetchingDelegate (#281)

* Add ASBatchFetchingDelegate
- In addition to checking remaining leading screens, ASBatchFetching now also calculates a remaining time and consults its delegate if needed.
- The delegate can override the decision of ASBatchFetching, for example based on remaining time and average time of past batch requests.

* Fix up tests

* Update CHANGELOG

Source/Private/ASBatchFetching.h

@@ -21,13 +21,17 @@
 
 ASDISPLAYNODE_EXTERN_C_BEGIN
 
+NS_ASSUME_NONNULL_BEGIN
+
 @class ASBatchContext;
+@protocol ASBatchFetchingDelegate;
 
 @protocol ASBatchFetchingScrollView <NSObject>
 
 - (BOOL)canBatchFetch;
 - (ASBatchContext *)batchContext;
 - (CGFloat)leadingScreensForBatching;
+- (nullable id<ASBatchFetchingDelegate>)batchFetchingDelegate;
 
 @end
 
@@ -39,9 +43,14 @@ ASDISPLAYNODE_EXTERN_C_BEGIN
  @param scrollDirection The current scrolling direction of the scroll view.
  @param scrollableDirections The possible scrolling directions of the scroll view.
  @param contentOffset The offset that the scrollview will scroll to.
+ @param velocity The velocity of the scroll view (in points) at the moment the touch was released.
  @return Whether or not the current state should proceed with batch fetching.
  */
-BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollView> *scrollView, ASScrollDirection scrollDirection, ASScrollDirection scrollableDirections, CGPoint contentOffset);
+BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollView> *scrollView,
+                                            ASScrollDirection scrollDirection,
+                                            ASScrollDirection scrollableDirections,
+                                            CGPoint contentOffset,
+                                            CGPoint velocity);
 
 
 /**
@@ -54,6 +63,8 @@ BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollVi
  @param targetOffset The offset that the scrollview will scroll to.
  @param leadingScreens How many screens in the remaining distance will trigger batch fetching.
  @param visible Whether the view is visible or not.
+ @param velocity The velocity of the scroll view (in points) at the moment the touch was released.
+ @param delegate The delegate to be consulted if needed.
  @return Whether or not the current state should proceed with batch fetching.
  @discussion This method is broken into a category for unit testing purposes and should be used with the ASTableView and
  * ASCollectionView batch fetching API.
@@ -65,6 +76,9 @@ extern BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
                                                 CGSize contentSize,
                                                 CGPoint targetOffset,
                                                 CGFloat leadingScreens,
-                                                BOOL visible);
+                                                BOOL visible,
+                                                CGPoint velocity,
+                                                _Nullable id<ASBatchFetchingDelegate> delegate);
 
+NS_ASSUME_NONNULL_END
 ASDISPLAYNODE_EXTERN_C_END