Add all overriden methods in flow layout. In NSCollectionView set collectionView in layout. Minor code cleanup.

Headers/AppKit/NSCollectionViewLayout.h

@@ -94,6 +94,7 @@ NSCollectionViewSupplementaryElementKind NSCollectionElementKindInterItemGapIndi
 
 - (NSCollectionElementCategory) representedElementCategory;
 - (NSString *) representedElementKind;
+
 @end
 
 enum
@@ -116,15 +117,16 @@ typedef NSInteger NSCollectionUpdateAction;
 - (NSIndexPath *) indexPathBeforeUpdate;
 - (NSIndexPath *) indexPathAfterUpdate;
 - (NSCollectionUpdateAction) updateAction;
+
 @end
 
 
 @interface NSCollectionViewLayoutInvalidationContext : NSObject
 {
-  NSPoint _contentOffsetAdjustment; 
+  NSPoint _contentOffsetAdjustment;
   NSSize _contentSizeAdjustment;
   NSSet *_invalidatedItemIndexPaths;
-  NSDictionary *_invalidatedSupplementaryIndexPaths; 
+  NSDictionary *_invalidatedSupplementaryIndexPaths;
   NSDictionary *_invalidatedDecorationIndexPaths;
 }
 
@@ -149,6 +151,7 @@ typedef NSInteger NSCollectionUpdateAction;
 
 - (NSDictionary *) invalidatedSupplementaryIndexPaths;
 - (NSDictionary *) invalidatedDecorationIndexPaths;
+
 @end
 
 @interface NSCollectionViewLayout : NSObject <NSCoding>
@@ -163,7 +166,7 @@ typedef NSInteger NSCollectionUpdateAction;
 
 // Initializers
 - (void)invalidateLayout;
-- (void)invalidateLayoutWithContext:(NSCollectionViewLayoutInvalidationContext *)context;
+- (void)invalidateLayoutWithContext: (NSCollectionViewLayoutInvalidationContext *)context;
 
 - (void)registerClass: (Class)viewClass
         forDecorationViewOfKind: (NSCollectionViewDecorationElementKind)elementKind;
@@ -173,7 +176,7 @@ typedef NSInteger NSCollectionUpdateAction;
 // Properties
 - (NSCollectionView *) collectionView;
 - (void) setCollectionView: (NSCollectionView *)cv;
-
+  
 @end
 
 @interface NSCollectionViewLayout (NSSubclassingHooks)

Source/NSCollectionView.m

@@ -439,6 +439,8 @@ static NSString *placeholderItem = nil;
 - (void) setCollectionViewLayout: (NSCollectionViewLayout *)layout
 {
   ASSIGN(_collectionViewLayout, layout);
+  
+  [_collectionViewLayout setCollectionView: self]; // weak reference
   [self reloadData];
 }
 
@@ -1363,8 +1365,9 @@ static NSString *placeholderItem = nil;
 {
   NSInteger ns = [self numberOfSections];
   NSInteger cs = 0;
-  
+
   NSLog(@"reloading data... number of sections = %ld, %@", ns, _collectionViewLayout);
+  [_collectionViewLayout prepareLayout];
   for (cs = 0; cs < ns; cs++)
     {
       NSInteger ni = [self numberOfItemsInSection: cs];

Source/NSCollectionViewFlowLayout.m

@@ -174,5 +174,87 @@
 {
 }
 
+// Methods to override for specific layouts...
+- (void) prepareLayout
+{
+  [super prepareLayout];
+  _invalidateFlowLayoutAttributes = NO;
+  _invalidateFlowLayoutDelegateMetrics = NO;
+}
+
+- (NSArray *) layoutAttributesForElementsInRect: (NSRect)rect
+{
+  return nil;
+}
+
+- (NSCollectionViewLayoutAttributes *) layoutAttributesForItemAtIndexPath: (NSIndexPath *)indexPath
+{
+  return nil;
+}
+
+- (NSCollectionViewLayoutAttributes *)
+  layoutAttributesForSupplementaryViewOfKind: (NSCollectionViewSupplementaryElementKind)elementKind
+  atIndexPath: (NSIndexPath *)indexPath
+{
+  return nil;
+}
+
+- (NSCollectionViewLayoutAttributes *)
+  layoutAttributesForDecorationViewOfKind: (NSCollectionViewDecorationElementKind)elementKind
+                              atIndexPath: (NSIndexPath *)indexPath
+{
+  return nil;
+}
+
+- (NSCollectionViewLayoutAttributes *)layoutAttributesForDropTargetAtPoint: (NSPoint)pointInCollectionView
+{
+  return nil;
+}
+
+- (NSCollectionViewLayoutAttributes *)layoutAttributesForInterItemGapBeforeIndexPath: (NSIndexPath *)indexPath
+{
+  return nil;
+}
+
+- (BOOL)shouldInvalidateLayoutForBoundsChange: (NSRect)newBounds
+{
+  return NO;
+}
+
+- (NSCollectionViewLayoutInvalidationContext *)invalidationContextForBoundsChange: (NSRect)newBounds
+{
+  return nil;
+}
+
+- (BOOL)shouldInvalidateLayoutForPreferredLayoutAttributes: (NSCollectionViewLayoutAttributes *)preferredAttributes
+                                    withOriginalAttributes: (NSCollectionViewLayoutAttributes *)originalAttributes
+{
+  return NO;
+}
+
+- (NSCollectionViewLayoutInvalidationContext *)
+  invalidationContextForPreferredLayoutAttributes: (NSCollectionViewLayoutAttributes *)preferredAttributes
+                           withOriginalAttributes: (NSCollectionViewLayoutAttributes *)originalAttributes
+{
+  return nil;
+}
+
+- (NSPoint) targetContentOffsetForProposedContentOffset: (NSPoint)proposedContentOffset
+                                  withScrollingVelocity: (NSPoint)velocity
+{
+  return NSZeroPoint;
+}
+
+- (NSPoint) targetContentOffsetForProposedContentOffset: (NSPoint)proposedContentOffset
+{
+  return NSZeroPoint;  
+}
+
+- (NSSize) collectionViewContentSize
+{
+  return [_collectionView frame].size;
+}
+// end subclassing hooks...
+
 @end
 

Source/NSCollectionViewLayout.m

@@ -242,6 +242,14 @@
 
 - (instancetype) initWithCoder: (NSCoder *)coder
 {
+  self = [super initWithCoder: coder];
+  if (self != nil)
+    {
+      if ([coder containsValueForKey: @"NSItemSize"])
+        {
+          [coder decodeSizeForKey: @"NSItemSize"];
+        }
+    }
   return self;
 }