UICollectionView indexPathsForVisibleItems顺序

Question

我使用UICollectionView显示第2列中的内容。我需要跟踪可见单元格中的更改。

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    NSArray* visibleCellIndex = collectionViewCatalog.indexPathsForVisibleItems;

    NSIndexPath* newFirstVisibleCell = [visibleCellIndex firstObject];
    NSIndexPath* newLastVisibleCell = [visibleCellIndex lastObject];
    NSLog(@"Visible cells [ %i ; %i]",newFirstVisibleCell.row,newLastVisibleCell.row);
}

记录：Visible cells [ 0 ; 3]，但有时崩溃顺序Visible cells [ 5 ; 4]或Visible cells [ 12 ; 11] 记录单元格的indexPathsForVisibleItems [5; 4]：

(
    "<NSIndexPath: 0x1653bb90> {length = 2, path = 0 - 5}",
    "<NSIndexPath: 0x1655ab10> {length = 2, path = 0 - 2}",
    "<NSIndexPath: 0x1655aa80> {length = 2, path = 0 - 3}",
    "<NSIndexPath: 0x16539680> {length = 2, path = 0 - 4}"
)

此后（可见细胞[5; 4]）下一个日志信息：Visible cells [ 6 ; 9]没有从5到9个细胞的记录

我只有20个细胞。为什么在数组indexPathsForVisibleItems中崩溃顺序indexPath？

Answer 1

来自NSIndexPath标头：

// comparison support
- (NSComparisonResult)compare:(NSIndexPath *)otherObject; // sorting an array of indexPaths using this comparison results in an array representing nodes in depth-first traversal order

你可以这么做：

NSArray *sortedVisibleItems = [[collectionViewCatalog indexPathsForVisibleItems] sortedArrayUsingSelector:@selector(compare:)];

Answer 2

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