UITableView dynamic cell height scroll to top after calling reloadData - ios

Question

I have dynamic cell height in my tableView
tableView.rowHeight = UITableViewAutomaticDimension
tableView.estimatedRowHeight = 100
I want to do same as whatsapp app (pagination), i want to load 25 by 25 row so when user scroll at top, i want to load new data but i want to keep same position of scroll, here is what i did
func scrollViewDidScroll(_ scrollView: UIScrollView) {
if isMore && scrollView.contentOffset.y == 0 {
loadMore() // function that add new data in my model
self.tableView.reloadData()
}
}
The problem that i have is after i reach top of my tableview it call reloadData but the tableView scroll to UP
I tester some other solution like this:
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 {
guard let height = cellHeights[indexPath] else { return 70.0 }
return height
}
But still not working

Answer 1

If you are reloading the table to add more data , then after reload call below function to retain the scroll position.
tableview.scrollToRow(at: indexPath, at: .middle, animated: true)
here indexPath is the indexPath of the last cell before reload.

Answer 2

In tableView:cellForRowAt when you detect indexPath is getting close to the last element (when indexPath.row is getting close to yourArray.count), query for more items. When you have them, add them to your data source (the array, or whatever it is, where you have all the objects represented in the table) and reload the table view. The table view should display the new items and the user can continue scrolling without doing anything special.
You shouldn't wait until tableView:cellForRowAt is called for the last cell, as the user will have to wait if you need to call a server to fetch more items. Whatsapp's scroll is fast because they download the items well before you scroll to the last elements (some time ago it did stop at the end, but now they are probably fetching new items way earlier).
Keep in mind that 25 items might be too few, it the network is slow the user might scroll through the items you have before more items to display and will have to wait, which is not a good user experience. Simulate a slow network connection and see if 25 items are enough to scroll through while your app contacts the server (depends on network speed, request and response sizes, how fast the server answers your request, etc.).

Answer 3

I faced the same problem in a messaging App. Here's how I solved it:
Assigned an unique messageId to every message in datasource array.
In cellForRow:, I save that messageId of very first row in a variable called messageIdFirst.
if(indexPath.row == 0){
messageIdFirst = message.id!
}
In loadMore() method, I check which current indexpath row have the same messageId in now updated datasource array.
if(message.id == messageIdFirst){
previousTopIndexPath = i
}
,where i being that index in datasource array which have the same messageId.
Now, I scroll to that previousTopIndexPath like this:
tableview.scrollToRow(at: previousTopIndexPath, at: .middle, animated: true)
Note 1: As this was not working efficiently in UITableView, I end up using UICollectionView with the same solution.
Note 2: While this does work smoothly, I know there must exist a cleaner way to do this.