在iOS9.0后UICollectionView的頭部視圖也能像tableView的header一樣出現懸浮掛住的效果。

 UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
    //header
    flowLayout.sectionHeadersPinToVisibleBounds = YES;
    //footer
    flowLayout.sectionFootersPinToVisibleBounds = YES;