When does UITableView content size update with row insert/delete animation

Question

I am curious when the UITableView's content size is updated after doing an insert/delete animation call. I figured it would be like most [UIView animation...] blocks in that the frame size/content size would be updated immediately even though the animation hasnt completed, but it does not seem to be that way. Any ideas?

Answer 1

Unfortunately, I haven't been able to find a good way to update the contentSize when adding/deleting rows from a UITableView either, but I did find a way to calculate what it will be if you know the index paths of the cells you are adding/removing.

Using the index path of the modified rows, you can calculate the heights of the respective cells with the tableView:heightForRowAtIndexPath: method and add it to the current content size height of the table view. Here's an example if you're adding just one row:

[self.tableView insertRowsAtIndexPaths:@[indexPathOfNewRow] withRowAnimation:UITableViewRowAnimationAutomatic];
CGFloat newCellHeight = [self tableView:self.tableView heightForRowAtIndexPath:indexPathOfNewRow];
CGFloat newContentSizeHeight = self.tableView.contentSize.height + newCellHeight

Answer 2

It looks like sizeThatFits() can expose the height of a new contentSize that's pending. You can then assign that pending size to have it resolve early for scroll animations.

With something like this:

extension UIScrollView {

    var pendingContentSize: CGSize {
        var tallSize = contentSize
        tallSize.height = .greatestFiniteMagnitude
        return sizeThatFits(tallSize)
    }

    func scrollToBottom(animated: Bool) {
        contentSize = pendingContentSize
        let contentRect = CGRect(origin: .zero, size: contentSize)
        let (bottomSlice, _) = contentRect.divided(atDistance: 1, from: .maxYEdge)
        guard !bottomSlice.isEmpty else { return }
        scrollRectToVisible(bottomSlice, animated: animated)
    }

}

I'm able to write view controller code like this:

tableView.insertRows(at: [newIndexPath], with: .none)
tableView.scrollToBottom(animated: true)

and have the table scroll all the way to the bottom (using the new content size) instead of it scrolling down to the 2nd-to-last row (using the old content size).