UITableView scrolls to top when reloading cells with changing cell heights - ios

Question

I have a table view which contains a placeholders while it loads in images. When the image is loaded, I call reloadRowsAtIndexPaths:withRowAnimation:. At this point, the cell changes height, based on the size of the image. When that happens, I want the table view's content offset to remain in place, and for the cells below to be pushed further down, as you might imagine.
The effect I'm getting instead is that the scroll view scrolls back to the top. I'm not sure why this is, and I can't seem to prevent it. Putting beginUpdates() before and endUpdates()after the reloadRows line has no effect.
I am using estimatedRowHeight, as is needed as my table view can potentially have hundreds of rows of different heights. I am also implementing tableView:heightForRowAtIndexPath:.
EDIT: I've set up a demo project to test this, and admittedly I can't get the demo project to reproduce this effect. I'll keep working at it.

Answer 1

It's an issue with the estimatedRowHeight.
The more the estimatedRowHeight differs from the actual height, the more the table may jump when it is reloaded, especially the further down it has been scrolled. This is because the table's estimated size radically differs from its actual size, forcing the table to adjust its content size and offset.
The easiest workaround is to use a really accurate estimate. If the height per row varies greatly, determine the median height for a row, and use that as the estimate.

Answer 2

Always update the UI on the main thread. So just place
[self.tableView reloadData];
inside a main thread:
dispatch_async(dispatch_get_main_queue(), ^{
//UI Updating code here.
[self.tableView reloadData];
});

Answer 3

I had the same problem and decide it by this way: save heights of cells when they loads and give exact value in tableView:estimatedHeightForRowAtIndexPath:
// declare cellHeightsDictionary
NSMutableDictionary *cellHeightsDictionary;
// 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;
}

Answer 4

I was seeing this, and the fix that worked for me was to choose an estimated row height that is the smallest of the possible rows. It had originally been set to the largest possible row height when the unintended scrolling was happening. I am just using the single tableView.estimatedRowHeight property, not the delegate method.