@property (nonatomic,strong) NSArray<NSNumber *> *stickySections;

- (NSArray *) layoutAttributesForElementsInRect:(CGRect)rect {      if (self.disableStickyFlow) {          return [super layoutAttributesForElementsInRect:rect];      }      NSMutableArray<UICollectionViewLayoutAttributes *> *allItems ;      //collectionView中的item（包括cell和header、footer这些）的《结构信息》.关键!!!!cell,header,footer都是利用这个数组的,在这个中,原来创建的section等,要按顺序存放到数组中来!      NSArray *originalAttributes = [super layoutAttributesForElementsInRect:rect];      //allItems = (NSMutableArray *)originalAttributes ;      allItems = [originalAttributes mutableCopy];//实际上layoutAttributesForElementsInRect返回的是NSMutableArray,所以,可以直接强转        NSMutableDictionary *headers = [[NSMutableDictionary alloc] init];//存放每个section的header      NSMutableDictionary<NSNumber *,UICollectionViewLayoutAttributes *> *lastCells = [[NSMutableDictionary alloc] init];//存放的是每个section的最后一个cell      [allItems enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {            NSIndexPath *indexPath = [obj indexPath];          BOOL isHeader = [[obj representedElementKind] isEqualToString:UICollectionElementKindSectionHeader];          BOOL isFooter = [[obj representedElementKind] isEqualToString:UICollectionElementKindSectionFooter];            if (isHeader) {              headers[@(indexPath.section)] = obj;          } else if (isFooter) {              // 不处理          } else {              //其实用这两句也是可以的              //NSNumber *sectionObj = @(indexPath.section);              //lastCells[sectionObj] = obj;                UICollectionViewLayoutAttributes *currentAttribute = lastCells[@(indexPath.section)];              // 确保取到的是section中最后一个cell              if ( ! currentAttribute || indexPath.row > currentAttribute.indexPath.row) {                  [lastCells setObject:obj forKey:@(indexPath.section)];              }          }            //如果按照正常情况下,header离开屏幕被系统回收，而header的层次关系又与cell相等，如果不去理会，会出现cell在header上面的情况          //通过打印可以知道cell的层次关系zIndex数值为0，我们可以将header的zIndex设置成1，如果不放心，也可以将它设置成非常大，这里随便填了个1024          if (isHeader) {              obj.zIndex = 1024;          } else {              // For iOS 7.0, the cell zIndex should be above sticky section header              obj.zIndex = 1;          }        }];          [lastCells enumerateKeysAndObjectsUsingBlock:^(NSNumber * _Nonnull key, UICollectionViewLayoutAttributes * _Nonnull obj, BOOL * _Nonnull stop) {          NSIndexPath *indexPath = obj.indexPath;          NSNumber *indexPathKey = @(indexPath.section);            UICollectionViewLayoutAttributes *header = headers[indexPathKey];            if ( ! header) {              // CollectionView自动将不再bounds内的headers移除了,所以,Headers可能为nil.这种情况下我们需要重新将其加回来 automatically removes headers not in bounds              header = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader                                                            atIndexPath:[NSIndexPath indexPathForItem:0 inSection:indexPath.section]];                if (!CGSizeEqualToSize(CGSizeZero, header.frame.size)) {                  [allItems addObject:header];              }          }          if (!CGSizeEqualToSize(CGSizeZero, header.frame.size)) {              [self updateHeaderAttributes:header lastCellAttributes:lastCells[indexPathKey]];          }      }];          return allItems;    }