randomsequence · October 26, 2016 14:57 · Jun 13, 2013 · Jun 13, 2013
diff --git a/StickyHeadersCollectionViewFlowLayout.m b/StickyHeadersCollectionViewFlowLayout.m
@@ -1,3 +1,15 @@
+// StickyHeadersCollectionViewFlowLayout  
+// 
+// A subclass of UICollectionViewFlowLayout which has UITableView style sticky headers. 
+// 
+// This code is based on Evadne Wu's code^1, with the following changes:
+// 
+// * Fixes a crash for sections with zero items
+// * Adds support for UIScrollView's contentInset
+// * Adds support for UICollectionViewFlowLayout's sectionInset
+// 
+// [1]: http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using
+
 @implementation StickyHeadersCollectionViewFlowLayout
 
 - (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect

diff --git a/StickyHeadersCollectionViewFlowLayout.m b/StickyHeadersCollectionViewFlowLayout.m
@@ -0,0 +1,74 @@
+@implementation StickyHeadersCollectionViewFlowLayout
+
+- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
+{
+    NSMutableArray * attributesArray = [[super layoutAttributesForElementsInRect: rect] mutableCopy];
+
+    UICollectionView * const cv = self.collectionView;
+    CGPoint const contentOffset = cv.contentOffset;
+    UIEdgeInsets contentInset = cv.contentInset;
+    UIEdgeInsets sectionInset = self.sectionInset;
+
+    NSMutableIndexSet *missingSections = [NSMutableIndexSet indexSet];
+    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
+        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell) {
+            [missingSections addIndex:layoutAttributes.indexPath.section];
+        }
+    }
+    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
+        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
+            [missingSections removeIndex:layoutAttributes.indexPath.section];
+        }
+    }
+
+    [missingSections enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
+        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];
+        UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
+        [attributesArray addObject:layoutAttributes];
+    }];
+
+    for (UICollectionViewLayoutAttributes *layoutAttributes in attributesArray) {
+
+        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
+
+            NSInteger section = layoutAttributes.indexPath.section;
+            NSInteger numberOfItemsInSection = [cv numberOfItemsInSection:section];
+
+            if (numberOfItemsInSection > 0) {
+                NSIndexPath *firstCellIndexPath = [NSIndexPath indexPathForItem:0 inSection:section];
+                NSIndexPath *lastCellIndexPath = [NSIndexPath indexPathForItem:MAX(0, (numberOfItemsInSection - 1)) inSection:section];
+
+                UICollectionViewLayoutAttributes *firstCellAttrs = [self layoutAttributesForItemAtIndexPath:firstCellIndexPath];
+                UICollectionViewLayoutAttributes *lastCellAttrs = [self layoutAttributesForItemAtIndexPath:lastCellIndexPath];
+
+
+
+                CGFloat headerHeight = CGRectGetHeight(layoutAttributes.frame);
+                CGPoint origin = layoutAttributes.frame.origin;
+                origin.y = MIN(
+                               MAX(
+                                   contentOffset.y + contentInset.top,
+                                   (CGRectGetMinY(firstCellAttrs.frame) - headerHeight - sectionInset.top)
+                                   ),
+                               (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight)
+                               );
+
+                layoutAttributes.zIndex = 1024;
+                layoutAttributes.frame = (CGRect){
+                    .origin = origin,
+                    .size = layoutAttributes.frame.size
+                };
+            }
+        }
+
+    }
+
+    return attributesArray;
+
+}
+
+- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
+    return YES;
+}
+
+@end