sectionHeader悬浮

设置collectionView sectionHeader 悬浮  （针对MVVM）

viewMode中设置多个sectionArray  并且为每个section 设置header

将sectionArray添加进 dataArray中 并返回dataArray

VC中设置layout

//section header 悬浮自定义layout

- (UICollectionViewLayout*)collectionViewLayout {

    UICollectionViewLayout * layout = self.collectionView.collectionViewLayout;

    if(!layout) {

        UICollectionViewFlowLayout * flowLayout = [[UICollectionViewFlowLayout alloc] init];

        flowLayout.scrollDirection = UICollectionViewScrollDirectionVertical;

        flowLayout.minimumLineSpacing=10;

        flowLayout.minimumInteritemSpacing = 0;

        flowLayout.sectionInset = UIEdgeInsetsZero;

        flowLayout.headerReferenceSize = CGSizeZero;

        flowLayout.footerReferenceSize = CGSizeZero;

        if(IOS_9_OR_LATER()){

            flowLayout.sectionHeadersPinToVisibleBounds = YES;

        }

        layout = flowLayout;

    }

    returnlayout;

}