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

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.