enable sticky header

chiahsien#188

Author: kjisoo

CHTCollectionViewWaterfallLayout.h

@@ -297,6 +297,7 @@ extern NSString *const CHTCollectionElementKindSectionFooter;
  */
 @property (nonatomic, assign) UIEdgeInsets sectionInset;
 
+@property (nonatomic, assign) BOOL enableStickyHeader;
 /**
  *  @brief The direction in which items will be rendered in subsequent rows.
  *  @discussion

CHTCollectionViewWaterfallLayout.m

@@ -464,6 +464,27 @@ - (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
       }
     }
   }
+  if (self.enableStickyHeader) {
+    for (int i = 0; i < self.allItemAttributes.count; i++) {
+      UICollectionViewLayoutAttributes *attr = self.allItemAttributes[i];
+      if ([attr.representedElementKind isEqualToString:CHTCollectionElementKindSectionHeader]) {
+        NSInteger section =  attr.indexPath.section;
+        NSInteger numberOfItemsInSection = [self.collectionView numberOfItemsInSection:section];
+        NSIndexPath *firstCellIndexPath =  [NSIndexPath indexPathForItem:0 inSection:section];
+        NSIndexPath *lastCellIndexPath = [NSIndexPath indexPathForItem:MAX(0, numberOfItemsInSection - 1) inSection:section];
+        UICollectionViewLayoutAttributes *firstCellAttributes = [self layoutAttributesForItemAtIndexPath:firstCellIndexPath];
+        UICollectionViewLayoutAttributes *lastCellAttributes = [self layoutAttributesForItemAtIndexPath:lastCellIndexPath];
+        CGFloat headerHeight = CGRectGetHeight(attr.frame);
+        CGPoint origin = attr.frame.origin;
+        CGFloat y1 =  MAX(self.collectionView.contentOffset.y, (CGRectGetMinY(firstCellAttributes.frame) - headerHeight));
+        CGFloat y2 = CGRectGetMaxY(lastCellAttributes.frame) - headerHeight;
+        origin.y = MIN(y1, y2);
+        attr.zIndex = 1024;
+        attr.frame = CGRectMake(origin.x, origin.y, CGRectGetWidth(attr.frame), CGRectGetHeight(attr.frame));
+        supplHeaderAttrDict[attr.indexPath] = attr;
+      }
+    }
+  }
 
   NSArray *result = [cellAttrDict.allValues arrayByAddingObjectsFromArray:supplHeaderAttrDict.allValues];
   result = [result arrayByAddingObjectsFromArray:supplFooterAttrDict.allValues];
@@ -472,6 +493,9 @@ - (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
 }
 
 - (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
+  if (self.enableStickyHeader) {
+    return YES;
+  }
   CGRect oldBounds = self.collectionView.bounds;
   if (CGRectGetWidth(newBounds) != CGRectGetWidth(oldBounds)) {
     return YES;