[feature] enable sticky header

CHTCollectionViewWaterfallLayout.h

@@ -297,6 +297,16 @@ extern NSString *const CHTCollectionElementKindSectionFooter;
  */
 @property (nonatomic, assign) UIEdgeInsets sectionInset;
 
+
+/**
+ * @brief control when user scroll the collecitonView , sectionHeader stay at top the screen
+ *
+ *  
+ */
+@property (nonatomic, assign) BOOL enableStickyHeader;
+
+
+
 /**
  *  @brief The direction in which items will be rendered in subsequent rows.
  *  @discussion

CHTCollectionViewWaterfallLayout.m

@@ -465,18 +465,42 @@ - (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];
   result = [result arrayByAddingObjectsFromArray:decorAttrDict.allValues];
   return result;
 }
 
 - (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
-  CGRect oldBounds = self.collectionView.bounds;
-  if (CGRectGetWidth(newBounds) != CGRectGetWidth(oldBounds)) {
-    return YES;
-  }
-  return NO;
+    if (self.enableStickyHeader) {
+        return YES;
+    }
+    CGRect oldBounds = self.collectionView.bounds;
+    if (CGRectGetWidth(newBounds) != CGRectGetWidth(oldBounds)) {
+        return YES;
+    }
+    return NO;
 }
 
 #pragma mark - Private Methods

Demo/Objective-C/Demo/ViewController.m

@@ -28,7 +28,7 @@ @implementation ViewController
 - (UICollectionView *)collectionView {
   if (!_collectionView) {
     CHTCollectionViewWaterfallLayout *layout = [[CHTCollectionViewWaterfallLayout alloc] init];
-
+      layout.enableStickyHeader = YES;
     layout.sectionInset = UIEdgeInsetsMake(10, 10, 10, 10);
     layout.headerHeight = 15;
     layout.footerHeight = 10;

Demo/Objective-C/Demo/en.lproj/MainStoryboard_iPhone.storyboard

@@ -1,28 +1,27 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="4514" systemVersion="13A603" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" initialViewController="2">
+<?xml version="1.0" encoding="UTF-8"?>
+<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="14460.31" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" colorMatched="YES" initialViewController="2">
+    <device id="retina4_7" orientation="portrait">
+        <adaptation id="fullscreen"/>
+    </device>
     <dependencies>
-        <deployment defaultVersion="1536" identifier="iOS"/>
-        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="3746"/>
+        <deployment version="1792" identifier="iOS"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="14460.20"/>
+        <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
     </dependencies>
     <scenes>
         <!--View Controller-->
         <scene sceneID="5">
             <objects>
                 <viewController id="2" customClass="ViewController" sceneMemberID="viewController">
                     <view key="view" contentMode="scaleToFill" id="3">
-                        <rect key="frame" x="0.0" y="0.0" width="320" height="568"/>
+                        <rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMaxY="YES"/>
-                        <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite"/>
+                        <color key="backgroundColor" red="1" green="1" blue="1" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                     </view>
                     <extendedEdge key="edgesForExtendedLayout" bottom="YES"/>
                 </viewController>
                 <placeholder placeholderIdentifier="IBFirstResponder" id="4" sceneMemberID="firstResponder"/>
             </objects>
         </scene>
     </scenes>
-    <simulatedMetricsContainer key="defaultSimulatedMetrics">
-        <simulatedStatusBarMetrics key="statusBar"/>
-        <simulatedOrientationMetrics key="orientation"/>
-        <simulatedScreenMetrics key="destination" type="retina4"/>
-    </simulatedMetricsContainer>
-</document>
+</document>