expose didModifySection on IGListCollectionViewLayout

Summary: * IGListCollectionViewLayout can now keep track of the minimumInvalidatedSection given individual section updates.

Reviewed By: rnystrom

Differential Revision: D6545333

fbshipit-source-id: 64c9b5ff32339299b52059a4d990a42d9e6a1af5

Source/IGListCollectionViewLayout.h

@@ -88,6 +88,17 @@ NS_SWIFT_NAME(ListCollectionViewLayout)
  */
 @property (nonatomic, assign) CGFloat stickyHeaderYOffset;
 
+/**
+ Notify the layout that a specific section was modified before invalidation. Used to optimize layout re-calculation.
+
+ @note When updating a collection view (ex: calling `-insertSections`), `-invalidateLayoutWithContext` gets called on
+ the layout object. However, the invalidation context doesn't provide details on which index paths are being modified,
+ which typically forces a full layout re-calculation. We can use this method to keep track of which section actually
+ needs to be updated on the following `-invalidateLayoutWithContext`. See `IGListCollectionView`.
+
+ @param modifiedSection The section that was modified.
+ */
+- (void)didModifySection:(NSInteger)modifiedSection;
 
 /**
  Create and return a new collection view layout.

Source/IGListCollectionViewLayout.mm

@@ -67,6 +67,18 @@ static NSIndexPath *indexPathForSection(NSInteger section) {
     return [NSIndexPath indexPathForItem:0 inSection:section];
 }
 
+static NSInteger IGListMergeMinimumInvalidatedSection(NSInteger section, NSInteger otherSection) {
+    if (section == NSNotFound && otherSection == NSNotFound) {
+        return NSNotFound;
+    } else if (section == NSNotFound) {
+        return otherSection;
+    } else if (otherSection == NSNotFound) {
+        return section;
+    }
+
+    return MIN(section, otherSection);
+}
+
 struct IGListSectionEntry {
     /**
      Represents the minimum-bounding box of every element in the section. This includes all item frames as well as the
@@ -346,9 +358,12 @@ static void adjustZIndexForAttributes(UICollectionViewLayoutAttributes *attribut
 
     if (hasInvalidatedItemIndexPaths
         || [context invalidateEverything]
-        || [context invalidateDataSourceCounts]
         || context.ig_invalidateAllAttributes) {
-        _minimumInvalidatedSection = 0; // invalidates all
+        // invalidates all
+        _minimumInvalidatedSection = 0;
+    } else if ([context invalidateDataSourceCounts] && _minimumInvalidatedSection == NSNotFound) {
+        // invalidate all if count changed and we don't have information on the minimum invalidated section
+        _minimumInvalidatedSection = 0;
     }
 
     if (context.ig_invalidateSupplementaryAttributes) {
@@ -610,4 +625,10 @@ static void adjustZIndexForAttributes(UICollectionViewLayoutAttributes *attribut
     }];
 }
 
+#pragma mark - Minimum Invalidated Section
+
+- (void)didModifySection:(NSInteger)modifiedSection {
+    _minimumInvalidatedSection = IGListMergeMinimumInvalidatedSection(_minimumInvalidatedSection, modifiedSection);
+}
+
 @end