如果需要用到UICollectionView已经显示的item做处理时，直接拿indexPathsForVisibleItems排序是有问题的，需要重新排序，visibleCells也是。
NSArray *visibleCellIndex = collectionView.indexPathsForVisibleItems;
NSArray *sortedIndexPaths = [visibleCellIndex sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
return [obj1 compare:obj2];
}];

NSArray *visibleCellIndex = collectionView.visibleCells;
NSArray *sortedIndexPaths = [visibleCellIndex sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
    NSIndexPath *path1 = (NSIndexPath *)[collectionView indexPathForCell:obj1];
    NSIndexPath *path2 = (NSIndexPath *)[collectionView indexPathForCell:obj2];
    return [path1 compare:path2];
}];