Merge pull request #11 from foursquare/csmulhern-section-pinning

Add ability to have non-pinned headers.

Also adds contentFrameForSection: and other documentation cleanup.

Refactors and resolves conflicts in PR #9

Author: Brian Dorfman

FSQCollectionViewAlignedLayout.h

@@ -80,6 +80,22 @@
  */
 @property (nonatomic) UIEdgeInsets contentInsets;
 
+/**
+ Determines if the current section's header is pinned to the top of the collection view.
+ 
+ Defaults to YES.
+ 
+ @note This is similar to the behavior observed in UITableView's with the style UITableViewStylePlain.
+ */
+@property (nonatomic) BOOL shouldPinSectionHeadersToTop;
+
+/**
+ @param section The section of the collection view.
+ 
+ @return The frame for the section, or CGRectZero if the section is out of range.
+ */
+- (CGRect)contentFrameForSection:(NSInteger)section;
+
 @end
 
 
@@ -106,14 +122,14 @@
       remainingLineSpace:(CGFloat)remainingLineSpace;
 
 /**
- *  Asks the delegate for the height of the header view in the specified section.
- *  If you do not implement this method, or the height returned is 0, no header is added.
- *
- *  @param collectionView       The collection view object displaying the layout.
- *  @param collectionViewLayout The layout object requesting the information.
- *  @param section              The index of the section whose header size is being requested.
- *
- *  @return The height of the header. If you return a value of 0, no header is added.
+ Asks the delegate for the height of the header view in the specified section.
+ If you do not implement this method, or the height returned is 0, no header is added.
+ 
+ @param collectionView       The collection view object displaying the layout.
+ @param collectionViewLayout The layout object requesting the information.
+ @param section              The index of the section whose header size is being requested.
+ 
+ @return The height of the header. If you return a value of 0, no header is added.
  */
 - (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceHeightForHeaderInSection:(NSInteger)section;
 

FSQCollectionViewAlignedLayout.m

@@ -122,6 +122,7 @@ - (void)setupDefaults {
     self.totalContentSize = CGSizeZero;
     self.sectionSpacing = 10.f;
     self.contentInsets = UIEdgeInsetsMake(5., 5., 5., 5.);
+    self.shouldPinSectionHeadersToTop = YES;
 }
 
 #pragma mark - Layout calculation -
@@ -568,22 +569,24 @@ - (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind
         if (sectionData.headerAttributes) {
             attributes = [sectionData.headerAttributes copy];
 
-            NSInteger section = 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];
-            
-            if (firstCellAttributes && lastCellAttributes) {
-                CGFloat headerHeight = attributes.frame.size.height;
-                CGFloat minY = CGRectGetMinY(sectionData.sectionRect);
-                CGFloat maxY = CGRectGetMaxY(sectionData.sectionRect) - headerHeight;
-                CGFloat yOffset = MIN(MAX(self.collectionView.contentOffset.y + self.collectionView.contentInset.top, minY), maxY);
-                attributes.frame = CGRectMake(0.0f, yOffset, self.collectionViewContentSize.width, headerHeight);
-                attributes.zIndex = NSIntegerMax;
+            if (self.shouldPinSectionHeadersToTop) {
+                NSInteger section = 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];
+                
+                if (firstCellAttributes && lastCellAttributes) {
+                    CGFloat headerHeight = attributes.frame.size.height;
+                    CGFloat minY = CGRectGetMinY(sectionData.sectionRect);
+                    CGFloat maxY = CGRectGetMaxY(sectionData.sectionRect) - headerHeight;
+                    CGFloat yOffset = MIN(MAX(self.collectionView.contentOffset.y + self.collectionView.contentInset.top, minY), maxY);
+                    attributes.frame = CGRectMake(0.0f, yOffset, self.collectionViewContentSize.width, headerHeight);
+                    attributes.zIndex = NSIntegerMax;
+                }
             }
         }
     }
@@ -673,6 +676,12 @@ - (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state object
     return [self.sectionsData countByEnumeratingWithState:state objects:buffer count:len];
 }
 
+#pragma mark - Layout information helpers - 
+
+- (CGRect)contentFrameForSection:(NSInteger)section {
+    return (section < self.sectionsData.count) ? [self.sectionsData[section] sectionRect] : CGRectZero;
+}
+
 @end
 
 @implementation FSQCollectionViewAlignedLayoutSectionAttributes