Initial pass with diffing fixtures

Source/ASDiffingSectionController.h

@@ -0,0 +1,75 @@
+//
+//  ASDiffingSectionController.h
+//  AsyncDisplayKit
+//
+//  Created by Adlai Holler on 6/20/17.
+//  Copyright © 2017 Facebook. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+#import <AsyncDisplayKit/ASAvailability.h>
+#import <AsyncDisplayKit/ASSectionController.h>
+
+#if AS_DIFFING
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ASDiffingSectionController : NSObject <ASSectionController>
+
+/**
+ * This property is automatically set by the infrastructure when a diff is performed
+ * and the section view model is updated.
+ *
+ * Subclasses may override the setter for this property, but not the getter.
+ */
+@property (atomic, nullable) id viewModel;
+
+/**
+ * The array of view models currently displayed by this section controller as items.
+ *
+ * This property is set by the infrastructure after each item-level diff.
+ *
+ * Subclasses should not override this property.
+ */
+@property (atomic, nullable, readonly) NSArray *itemViewModels;
+
+#pragma mark - Public API
+
+/**
+ * Invalidate the items for this section controller. Performs a diff against the current set of items.
+ *
+ * Akin to a series of [UICollectionView insert/delete/reload/moveItemsAtIndexPaths:] calls.
+ *
+ * This method is automatically called when the view model is updated.
+ *
+ * Calls to this method will join the current `performBatchUpdates:` if any.
+ *
+ * Subclasses should not override this method.
+ */
+- (void)invalidateItems;
+
+/**
+ * Invalidates the section including all items and supplementaries. Akin to [UICollectionView reloadSections:].
+ *
+ * Calls to this method will join the current `performBatchUpdates:` if any.
+ *
+ * Subclasses should not override this method.
+ */
+- (void)invalidateSection;
+
+#pragma mark - API for the Infra
+
+/**
+ * Asks the section controller to generate and return an array of item view models for the section.
+ *
+ * You should not call this method directly.
+ *
+ * Subclasses must override this method.
+ */
+- (NSArray<id<IGListDiffable>> *)generateItemViewModels;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+#endif