Issues with scroll to top when constantly updating cell size - ios

Question

I have a UITableView with automatic sizing cells. The contents of each cell revolve around the counting of time, so the size of the cell could change every second. I am currently using a timer scheduled every 1 second to tell the table view to update the cell sizes:
Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timeUpdateNotification), userInfo: nil, repeats: true)
#objc func timeUpdateNotification() {
// ... cells get notified to update contents here ...
tableView.beginUpdates()
tableView.endUpdates()
}
This works fairly well but has issues when the user taps to scroll to the top of the list. The animation is a bit janky and often times doesn't make it to the top. Is there a better way to handle this?

Answer 1

If you set a flag when the table is scrolling you can detect it in your timer function and not update while it is set. UITableView is a descendant of UIScrollView, so you can use some of the scroll view delegates to do this. If you override scrollViewShouldScrollToTop() and scrollViewDidScrollToTop() you will know when the scroll view is scrolling to the top and when it has finished.
override func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
NSLog("scrollViewShouldScrollToTop")
isScrolling = true
return true
}
override func scrollViewDidScrollToTop(_ scrollView: UIScrollView) {
NSLog("scrollViewDidScrollToTop")
isScrolling = false
}
You could also extend this to detect when the user is dragging/scrolling the view, to prevent the timer function from updating at these occasions too.
override func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
NSLog("scrollViewWillBeginDragging")
isScrolling = true
}
override func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
if (!decelerate) {
// Only catch if scrolling stopped
NSLog("scrollViewDidEndDragging")
isScrolling = false
}
}
override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
NSLog("scrollViewDidEndDecelerating")
isScrolling = false
}
I have added some logging to the functions, so that you can see what is going on. You can remove these, of course.

Answer 2

Invalidate the timer when they click to scroll to top, then as soon as it reaches top start it again.
Edit: That way, it won't update content which may move content higher without updating the "top" of the table view.

Answer 3

I had a similar problem.
I solved it with the help of a manual height calculation for each cell.
var cellHeights: [Int: CGFloat] = [:]
In the cellForRowAtIndexPath method, calculate the height:
cellHeights[byRow] = cell.getCellHeight()
In the cell itself
func getCellHeight() -> CGFloat {
let userInfoHeight: CGFloat = userInfoHeightConstraint.constant
let actionHeight: CGFloat = actionViewHeightConstraint.constant
let descriptionBottom: CGFloat = descriptionBottomConstraint.constant
let descriptionWidth = self.frame.width - leftAvatarConstraint.constant - descriptionRightConstraint.constant
let descriptionHeight: CGFloat = descriptionLabel.textHeight(width: descriptionWidth)
let height = userInfoHeight + actionHeight + descriptionBottom + descriptionHeight
return height
}
extension UILabel {
func textHeight(width: CGFloat) -> CGFloat {
var textHeight: CGFloat = 0
if let text = self.text {
let customLabel = UILabel(frame: CGRect(x: 0, y: 0, width: width, height: .greatestFiniteMagnitude))
customLabel.numberOfLines = self.numberOfLines
customLabel.text = text
customLabel.font = self.font
customLabel.sizeToFit()
textHeight = customLabel.frame.height
}
return textHeight
}}
I'm using UITableViewAutomaticDimension.
I additionally have the functionality to change the size of the cell at various clicked on it.
Therefore, in my task, this works fine:
func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
return presenter.cellHeights[indexPath.row] ?? 0
}
But I think that you can immediately set the height:
func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
return presenter.cellHeights[indexPath.row] ?? 0
}