Detect the current top cell in a UITableView after scrolling

Question

a simple question but I don't seem to have the right terminology to search Stackoverflow by it.

I have a UITableView with no sections, the user can scroll up and down a long list of data (rows) presented inside this tableview.

Question: how can I detect the most top cell row number after the user scrolled. (for example, if the user is scrolling 30 cells down, the top cell after the scroll is = 30)

Answer 1

You could try using UITableView's -indexPathsForVisibleRows or -indexPathForRowAtPoint.

For example, let's say that you want to print the indexPath of the topmost visible cell, when you stop dragging your table. You could do something like this:

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{
    NSIndexPath *firstVisibleIndexPath = [[self.tableView indexPathsForVisibleRows] objectAtIndex:0];
    NSLog(@"first visible cell's section: %i, row: %i", firstVisibleIndexPath.section, firstVisibleIndexPath.row);
}

For Swift 3.0

let topVisibleIndexPath:IndexPath = self.tableView.indexPathsForVisibleRows![0]

Answer 2

You get the index paths for the visible rows

NSArray* indexPaths = [tableView indexPathsForVisibleRows];

Then sort using compare:

NSArray* sortedIndexPaths = [indexPaths sortedArrayUsingSelector:@selector(compare:)];

Then get the first element's row

NSInteger row = [(NSIndexPath*)[sortedIndexPaths objectAtIndex:0] row];

Answer 3

This is the Swift 3+ code:

override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    let firstVisibleIndexPath = self.tableView.indexPathsForVisibleRows?[0]
    print("First visible cell section=\(firstVisibleIndexPath?.section), and row=\(firstVisibleIndexPath?.row)")
}

Answer 4

In Swift 4:

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    let firstVisibleIndexPath = self.tableview.indexPathsForVisibleRows?[0]
    print("top visible cell section  is \([firstVisibleIndexPath!.section])")
}