reloadData() of UITableView with Dynamic cell heights causes jumpy scrolling

Question

I feel like this might be a common issue and was wondering if there was any common solution to it.

Basically, my UITableView has dynamic cell heights for every cell. If I am not at the top of the UITableView and I tableView.reloadData(), scrolling up becomes jumpy.

I believe this is due to the fact that because I reloaded data, as I'm scrolling up, the UITableView is recalculating the height for each cell coming into visibility. How do I mitigate that, or how do I only reloadData from a certain IndexPath to the end of the UITableView?

Further, when I do manage to scroll all the way to the top, I can scroll back down and then up, no problem with no jumping. This is most likely because the UITableViewCell heights were already calculated.

Answer 1

To prevent jumping you should save heights of cells when they loads and give exact value in tableView:estimatedHeightForRowAtIndexPath:

Swift:

var cellHeights = [IndexPath: CGFloat]()

func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    cellHeights[indexPath] = cell.frame.size.height
}

func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
    return cellHeights[indexPath] ?? UITableView.automaticDimension
}

Objective C:

// declare cellHeightsDictionary
NSMutableDictionary *cellHeightsDictionary = @{}.mutableCopy;

// declare table dynamic row height and create correct constraints in cells
tableView.rowHeight = UITableViewAutomaticDimension;

// save height
- (void)tableView:(UITableView *)tableView willDisplayCell:(UITableViewCell *)cell forRowAtIndexPath:(NSIndexPath *)indexPath {
    [cellHeightsDictionary setObject:@(cell.frame.size.height) forKey:indexPath];
}

// give exact height value
- (CGFloat)tableView:(UITableView *)tableView estimatedHeightForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber *height = [cellHeightsDictionary objectForKey:indexPath];
    if (height) return height.doubleValue;
    return UITableViewAutomaticDimension;
}