Add indexesOfVisibleObjects

Summary:
It's inefficient to get the unordered array of visibleObjects to then try to find the order of those objects after.  Instead, introduce a new `indexesOfVisibleObjects` API to `IGListAdapter` which will inherently preserve order.

Will use this to help find the previous and next "item" to scroll to with Main Feed.

Differential Revision: D70835725

fbshipit-source-id: a5f66681e0184299d92f220be194b5554639a7da

Author: Nolan O'Brien

Source/IGListKit/IGListAdapter.h

@@ -239,6 +239,13 @@ NS_SWIFT_NAME(ListAdapter)
  */
 - (NSArray *)visibleObjects;
 
+/**
+ Less performant that `visibleObjects` but gives you an ordering to the list of visible objects in `self.objects`.
+
+ @return An index set for objects in `self.objects`.   Result's `.count` will be `0` if no visible objects.
+ */
+- (NSIndexSet *)indexesOfVisibleObjects;
+
 /**
  An unordered array of the currently visible cells for a given object.
 

Source/IGListKit/IGListAdapter.m

@@ -583,7 +583,7 @@ - (NSArray *)objects {
     return [[self.displayHandler visibleListSections] allObjects];
 }
 
-- (NSArray *)visibleObjects {
+- (NSSet *)_visibleObjectsSet FB_OBJC_DIRECT {
     IGAssertMainThread();
 
     NSArray<UICollectionViewCell *> *visibleCells = [self.collectionView visibleCells];
@@ -602,7 +602,32 @@ - (NSArray *)visibleObjects {
             }
         }
     }
-    return [visibleObjects allObjects];
+    return visibleObjects;
+}
+
+- (NSArray *)visibleObjects {
+    return [[self _visibleObjectsSet] allObjects];
+}
+
+- (NSIndexSet *)indexesOfVisibleObjects {
+    /*
+        This is a naive implementation, going through all objects and checking if they are visible.
+        It is not optimized for performance, but it is correct.
+
+        In the future, this could potentially be optimized by getting the index paths of visible cells,
+        and converting those index paths into a range of object indexes within `self.objects`.
+    */
+
+    NSSet *visibleObjects = [self _visibleObjectsSet];
+    NSMutableIndexSet *indexSet = [NSMutableIndexSet indexSet];
+    NSUInteger idx = 0;
+    for (id object in self.objects) {
+        if ([visibleObjects containsObject:object]) {
+            [indexSet addIndex:idx];
+        }
+        idx++;
+    }
+    return [indexSet copy];
 }
 
 - (NSArray<UICollectionViewCell *> *)visibleCellsForObject:(id)object {
@@ -624,7 +649,6 @@ - (NSArray *)visibleObjects {
     return [visibleCells filteredArrayUsingPredicate:controllerPredicate];
 }
 
-
 #pragma mark - Layout
 
 - (CGSize)sizeForItemAtIndexPath:(NSIndexPath *)indexPath {