Make it possible to map between sections even if they're empty (#660)

* Make section mapping work even for empty sections

* Unlock more cases and update changelog

* Fix complexity documentation

CHANGELOG.md

@@ -8,6 +8,7 @@
 - [Layout] Fixes a deadlock in layout. [#638](https://github.com/TextureGroup/Texture/pull/638) [Garrett Moon](https://github.com/garrettmoon)
 - Updated to be backwards compatible with Xcode 8. [Adlai Holler](https://github.com/Adlai-Holler)
 - [API CHANGES] `ASPerformMainThreadDeallocation` and `ASPerformBackgroundDeallocation` functions take `id *` instead of `id` and they're now more reliable. Also, in Swift, `ASDeallocQueue.sharedDeallocationQueue() -> ASDeallocQueue.sharedDeallocationQueue`. [Adlai Holler](https://github.com/Adlai-Holler) [#651](https://github.com/TextureGroup/Texture/pull/651)
+- [Collection/Table] Added direct support for mapping section indexes between data spaces. [Adlai Holler](https://github.com/Adlai-Holler) [#651](https://github.com/TextureGroup/Texture/pull/660)
 
 ## 2.6
 - [Xcode 9] Updated to require Xcode 9 (to fix warnings) [Garrett Moon](https://github.com/garrettmoon)

Source/ASCollectionView.mm

@@ -64,8 +64,6 @@
     return __val; \
   }
 
-#define ASIndexPathForSection(section) [NSIndexPath indexPathForItem:0 inSection:section]
-
 #define ASFlowLayoutDefault(layout, property, default)                                        \
 ({                                                                                            \
   UICollectionViewFlowLayout *flowLayout = ASDynamicCast(layout, UICollectionViewFlowLayout); \
@@ -680,18 +678,12 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
     return nil;
   }
 
-  // If this is a section index path, we don't currently have a method
-  // to do a mapping.
-  if (indexPath.item == NSNotFound) {
-    return indexPath;
-  } else {
   NSIndexPath *viewIndexPath = [_dataController.visibleMap convertIndexPath:indexPath fromMap:_dataController.pendingMap];
   if (viewIndexPath == nil && wait) {
     [self waitUntilAllUpdatesAreCommitted];
     return [self convertIndexPathFromCollectionNode:indexPath waitingIfNeeded:NO];
   }
   return viewIndexPath;
-  }
 }
 
 /**
@@ -725,13 +717,7 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
     return nil;
   }
 
-  // If this is a section index path, we don't currently have a method
-  // to do a mapping.
-  if (indexPath.item == NSNotFound) {
-    return indexPath;
-  } else {
   return [_dataController.pendingMap convertIndexPath:indexPath fromMap:_dataController.visibleMap];
-  }
 }
 
 - (NSArray<NSIndexPath *> *)convertIndexPathsToCollectionNode:(NSArray<NSIndexPath *> *)indexPaths
@@ -1050,7 +1036,7 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
   ASDisplayNodeAssertMainThread();
   ASElementMap *map = _dataController.visibleMap;
   ASCollectionElement *e = [map supplementaryElementOfKind:UICollectionElementKindSectionHeader
-                                               atIndexPath:ASIndexPathForSection(section)];
+                                               atIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]];
   return e ? [self sizeForElement:e] : ASFlowLayoutDefault(l, headerReferenceSize, CGSizeZero);
 }
 
@@ -1060,29 +1046,28 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
   ASDisplayNodeAssertMainThread();
   ASElementMap *map = _dataController.visibleMap;
   ASCollectionElement *e = [map supplementaryElementOfKind:UICollectionElementKindSectionFooter
-                                               atIndexPath:ASIndexPathForSection(section)];
+                                               atIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]];
   return e ? [self sizeForElement:e] : ASFlowLayoutDefault(l, footerReferenceSize, CGSizeZero);
 }
 
 // For the methods that call delegateIndexPathForSection:withSelector:, translate the section from
 // visibleMap to pendingMap. If the section no longer exists, or the delegate doesn't implement
-// the selector, we will return a nil indexPath (and then use the ASFlowLayoutDefault).
-- (NSIndexPath *)delegateIndexPathForSection:(NSInteger)section withSelector:(SEL)selector
+// the selector, we will return NSNotFound (and then use the ASFlowLayoutDefault).
+- (NSInteger)delegateIndexForSection:(NSInteger)section withSelector:(SEL)selector
 {
   if ([_asyncDelegate respondsToSelector:selector]) {
-    return [_dataController.pendingMap convertIndexPath:ASIndexPathForSection(section)
-                                                fromMap:_dataController.visibleMap];
+    return [_dataController.pendingMap convertSection:section fromMap:_dataController.visibleMap];
   } else {
-    return nil;
+    return NSNotFound;
   }
 }
 
 - (UIEdgeInsets)collectionView:(UICollectionView *)cv layout:(UICollectionViewLayout *)l
                                       insetForSectionAtIndex:(NSInteger)section
 {
-  NSIndexPath *indexPath = [self delegateIndexPathForSection:section withSelector:_cmd];
-  if (indexPath) {
-    return [(id)_asyncDelegate collectionView:cv layout:l insetForSectionAtIndex:indexPath.section];
+  section = [self delegateIndexForSection:section withSelector:_cmd];
+  if (section != NSNotFound) {
+    return [(id)_asyncDelegate collectionView:cv layout:l insetForSectionAtIndex:section];
   }
   return ASFlowLayoutDefault(l, sectionInset, UIEdgeInsetsZero);
 }
@@ -1090,10 +1075,10 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
 - (CGFloat)collectionView:(UICollectionView *)cv layout:(UICollectionViewLayout *)l
                minimumInteritemSpacingForSectionAtIndex:(NSInteger)section
 {
-  NSIndexPath *indexPath = [self delegateIndexPathForSection:section withSelector:_cmd];
-  if (indexPath) {
+  section = [self delegateIndexForSection:section withSelector:_cmd];
+  if (section != NSNotFound) {
     return [(id)_asyncDelegate collectionView:cv layout:l
-               minimumInteritemSpacingForSectionAtIndex:indexPath.section];
+               minimumInteritemSpacingForSectionAtIndex:section];
   }
   return ASFlowLayoutDefault(l, minimumInteritemSpacing, 10.0); // Default is documented as 10.0
 }
@@ -1101,10 +1086,10 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
 - (CGFloat)collectionView:(UICollectionView *)cv layout:(UICollectionViewLayout *)l
                     minimumLineSpacingForSectionAtIndex:(NSInteger)section
 {
-  NSIndexPath *indexPath = [self delegateIndexPathForSection:section withSelector:_cmd];
-  if (indexPath) {
+  section = [self delegateIndexForSection:section withSelector:_cmd];
+  if (section != NSNotFound) {
     return [(id)_asyncDelegate collectionView:cv layout:l
-                    minimumLineSpacingForSectionAtIndex:indexPath.section];
+                    minimumLineSpacingForSectionAtIndex:section];
   }
   return ASFlowLayoutDefault(l, minimumLineSpacing, 10.0);      // Default is documented as 10.0
 }

Source/ASTableView.mm

@@ -588,18 +588,12 @@ static NSString * const kCellReuseIdentifier = @"_ASTableViewCell";
 
 - (NSIndexPath *)convertIndexPathFromTableNode:(NSIndexPath *)indexPath waitingIfNeeded:(BOOL)wait
 {
-  // If this is a section index path, we don't currently have a method
-  // to do a mapping.
-  if (indexPath == nil || indexPath.row == NSNotFound) {
-    return indexPath;
-  } else {
   NSIndexPath *viewIndexPath = [_dataController.visibleMap convertIndexPath:indexPath fromMap:_dataController.pendingMap];
   if (viewIndexPath == nil && wait) {
     [self waitUntilAllUpdatesAreCommitted];
     return [self convertIndexPathFromTableNode:indexPath waitingIfNeeded:NO];
   }
   return viewIndexPath;
-  }
 }
 
 - (NSIndexPath *)convertIndexPathToTableNode:(NSIndexPath *)indexPath
@@ -608,13 +602,7 @@ static NSString * const kCellReuseIdentifier = @"_ASTableViewCell";
     return nil;
   }
 
-  // If this is a section index path, we don't currently have a method
-  // to do a mapping.
-  if (indexPath.row == NSNotFound) {
-    return indexPath;
-  } else {
   return [_dataController.pendingMap convertIndexPath:indexPath fromMap:_dataController.visibleMap];
-  }
 }
 
 - (NSArray<NSIndexPath *> *)convertIndexPathsToTableNode:(NSArray<NSIndexPath *> *)indexPaths

Source/Details/ASDataController.mm

@@ -540,7 +540,7 @@ typedef dispatch_block_t ASDataControllerCompletionBlock;
       ASMutableElementMap *mutableMap = [previousMap mutableCopy];
 
       // Step 1.1: Update the mutable copies to match the data source's state
-      [self _updateSectionContextsInMap:mutableMap changeSet:changeSet];
+      [self _updateSectionsInMap:mutableMap changeSet:changeSet];
       ASPrimitiveTraitCollection existingTraitCollection = [self.node primitiveTraitCollection];
       [self _updateElementsInMap:mutableMap changeSet:changeSet traitCollection:existingTraitCollection shouldFetchSizeRanges:(! canDelegate) previousMap:previousMap];
 
@@ -599,43 +599,38 @@ typedef dispatch_block_t ASDataControllerCompletionBlock;
 /**
  * Update sections based on the given change set.
  */
-- (void)_updateSectionContextsInMap:(ASMutableElementMap *)map changeSet:(_ASHierarchyChangeSet *)changeSet
+- (void)_updateSectionsInMap:(ASMutableElementMap *)map changeSet:(_ASHierarchyChangeSet *)changeSet
 {
   ASDisplayNodeAssertMainThread();
   
-  if (!_dataSourceFlags.contextForSection) {
-    return;
-  }
-  
   if (changeSet.includesReloadData) {
-    [map removeAllSectionContexts];
+    [map removeAllSections];
     
     NSUInteger sectionCount = [self itemCountsFromDataSource].size();
     NSIndexSet *sectionIndexes = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, sectionCount)];
-    [self _insertSectionContextsIntoMap:map indexes:sectionIndexes];
+    [self _insertSectionsIntoMap:map indexes:sectionIndexes];
     // Return immediately because reloadData can't be used in conjuntion with other updates.
     return;
   }
   
   for (_ASHierarchySectionChange *change in [changeSet sectionChangesOfType:_ASHierarchyChangeTypeDelete]) {
-    [map removeSectionContextsAtIndexes:change.indexSet];
+    [map removeSectionsAtIndexes:change.indexSet];
   }
   
   for (_ASHierarchySectionChange *change in [changeSet sectionChangesOfType:_ASHierarchyChangeTypeInsert]) {
-    [self _insertSectionContextsIntoMap:map indexes:change.indexSet];
+    [self _insertSectionsIntoMap:map indexes:change.indexSet];
   }
 }
 
-- (void)_insertSectionContextsIntoMap:(ASMutableElementMap *)map indexes:(NSIndexSet *)sectionIndexes
+- (void)_insertSectionsIntoMap:(ASMutableElementMap *)map indexes:(NSIndexSet *)sectionIndexes
 {
   ASDisplayNodeAssertMainThread();
 
-  if (!_dataSourceFlags.contextForSection) {
-    return;
-  }
-  
   [sectionIndexes enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL * _Nonnull stop) {
-    id<ASSectionContext> context = [_dataSource dataController:self contextForSection:idx];
+    id<ASSectionContext> context;
+    if (_dataSourceFlags.contextForSection) {
+      context = [_dataSource dataController:self contextForSection:idx];
+    }
     ASSection *section = [[ASSection alloc] initWithSectionID:_nextSectionID context:context];
     [map insertSection:section atIndex:idx];
     _nextSectionID++;

Source/Details/ASElementMap.h

@@ -64,10 +64,20 @@ AS_SUBCLASSING_RESTRICTED
 @property (copy, readonly) NSArray<ASCollectionElement *> *itemElements;
 
 /**
- * Returns the index path that corresponds to the same element in @c map at the given @c indexPath. O(1)
+ * Returns the index path that corresponds to the same element in @c map at the given @c indexPath.
+ * O(1) for items, fast O(N) for sections.
+ *
+ * Note you can pass "section index paths" of length 1 and get a corresponding section index path.
  */
 - (nullable NSIndexPath *)convertIndexPath:(NSIndexPath *)indexPath fromMap:(ASElementMap *)map;
 
+/**
+ * Returns the section index into the receiver that corresponds to the same element in @c map at @c sectionIndex. Fast O(N).
+ *
+ * Returns @c NSNotFound if the section does not exist in the receiver.
+ */
+- (NSInteger)convertSection:(NSInteger)sectionIndex fromMap:(ASElementMap *)map;
+
 /**
  * Returns the index path for the given element. O(1)
  */

Source/Details/ASElementMap.m

@@ -47,6 +47,8 @@
 
 - (instancetype)initWithSections:(NSArray<ASSection *> *)sections items:(ASCollectionElementTwoDimensionalArray *)items supplementaryElements:(ASSupplementaryElementDictionary *)supplementaryElements
 {
+  NSCParameterAssert(items.count == sections.count);
+
   if (self = [super init]) {
     _sections = [sections copy];
     _sectionsOfItems = [[NSArray alloc] initWithArray:items copyItems:YES];
@@ -157,8 +159,25 @@
 
 - (NSIndexPath *)convertIndexPath:(NSIndexPath *)indexPath fromMap:(ASElementMap *)map
 {
-  id element = [map elementForItemAtIndexPath:indexPath];
+  if (indexPath.item == NSNotFound) {
+    // Section index path
+    NSInteger result = [self convertSection:indexPath.section fromMap:map];
+    return (result != NSNotFound ? [NSIndexPath indexPathWithIndex:result] : nil);
+  } else {
+    // Item index path
+    ASCollectionElement *element = [map elementForItemAtIndexPath:indexPath];
     return [self indexPathForElement:element];
+  }
+}
+
+- (NSInteger)convertSection:(NSInteger)sectionIndex fromMap:(ASElementMap *)map
+{
+  if (![map sectionIndexIsValid:sectionIndex assert:YES]) {
+    return NSNotFound;
+  }
+
+  ASSection *section = map.sections[sectionIndex];
+  return [_sections indexOfObjectIdenticalTo:section];
 }
 
 #pragma mark - NSCopying

Source/Private/ASMutableElementMap.h

@@ -37,10 +37,10 @@ AS_SUBCLASSING_RESTRICTED
 
 - (void)insertSection:(ASSection *)section atIndex:(NSInteger)index;
 
-- (void)removeAllSectionContexts;
+- (void)removeAllSections;
 
 /// Only modifies the array of ASSection * objects
-- (void)removeSectionContextsAtIndexes:(NSIndexSet *)indexes;
+- (void)removeSectionsAtIndexes:(NSIndexSet *)indexes;
 
 - (void)removeAllElements;
 

Source/Private/ASMutableElementMap.m

@@ -48,7 +48,7 @@ typedef NSMutableDictionary<NSString *, NSMutableDictionary<NSIndexPath *, ASCol
   return [[ASElementMap alloc] initWithSections:_sections items:_sectionsOfItems supplementaryElements:_supplementaryElements];
 }
 
-- (void)removeAllSectionContexts
+- (void)removeAllSections
 {
   [_sections removeAllObjects];
 }
@@ -63,7 +63,7 @@ typedef NSMutableDictionary<NSString *, NSMutableDictionary<NSIndexPath *, ASCol
   ASDeleteElementsInTwoDimensionalArrayAtIndexPaths(_sectionsOfItems, indexPaths);
 }
 
-- (void)removeSectionContextsAtIndexes:(NSIndexSet *)indexes
+- (void)removeSectionsAtIndexes:(NSIndexSet *)indexes
 {
   [_sections removeObjectsAtIndexes:indexes];
 }

Source/Private/ASSection.h

@@ -16,15 +16,29 @@
 //
 
 #import <Foundation/Foundation.h>
+#import <AsyncDisplayKit/ASBaseDefines.h>
 
 @protocol ASSectionContext;
 
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ * An object representing the metadata for a section of elements in a collection.
+ *
+ * Its sectionID is namespaced to the data controller that created the section.
+ *
+ * These are useful for tracking the movement & lifetime of sections, independent of
+ * their contents.
+ */
+AS_SUBCLASSING_RESTRICTED
 @interface ASSection : NSObject
 
-@property (nonatomic, assign, readonly) NSInteger sectionID;
-@property (nonatomic, strong, nullable, readonly) id<ASSectionContext> context;
+@property (assign, readonly) NSInteger sectionID;
+@property (strong, nullable, readonly) id<ASSectionContext> context;
 
-- (nullable instancetype)init __unavailable;
-- (nullable instancetype)initWithSectionID:(NSInteger)sectionID context:(nullable id<ASSectionContext>)context NS_DESIGNATED_INITIALIZER;
+- (instancetype)init NS_UNAVAILABLE;
+- (instancetype)initWithSectionID:(NSInteger)sectionID context:(nullable id<ASSectionContext>)context NS_DESIGNATED_INITIALIZER;
 
 @end
+
+NS_ASSUME_NONNULL_END