UITableView with custom cells lags while scrolling - ios

Question

My problem is that UITableView lags quite a lot while scrolling.
This is what I am trying to achieve
Starting from the top I have a simple section header with only one checkbox and one UILabel. Under this header, you can see a custom cell with only one UILabel aligned to the center. This custom cell works like another header for the data that would be shown below (Basically a 3D array). Under these "headers" are custom cells that contain one multiline UILabel and under this label is a container for a variable amount of lines containing a checkbox and an UILabel. On the right side of the cell is also a button (blue/white arrow).
So this means the content is shown like this:
Section header (containing day and date)
Custom UITableViewCell = header (containing some header information)
Custom UITableViewCell (containing data to be shown)
Here is my code:
cellForRowAt:
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let (isHeader, headerNumber, semiResult) = checkIfIsHeader(section: indexPath.section, row: indexPath.row)
let row = indexPath.row
if isHeader {
let chod = objednavkaDny[indexPath.section].chody[headerNumber+1]
let cell = tableView.dequeueReusableCell(withIdentifier: cellHeaderReuseIdentifier, for: indexPath) as! ObjednavkyHeaderTableViewCell
cell.titleLabel.text = chod.popisPoradiJidla
cell.selectionStyle = .none
return cell
}else{
let chod = objednavkaDny[indexPath.section].chody[headerNumber]
let cell = tableView.dequeueReusableCell(withIdentifier: reuseIdentifier, for: indexPath) as! ObjednavkyTableViewCell
cell.updateData(objednavka: chod.objednavky[row-semiResult], canSetAmount: self.typDialogu == 3)
return cell
}
}
checkIfIsHeader:
func checkIfIsHeader(section: Int, row: Int) -> (Bool, Int, Int){
if let cachedResult = checkIfHeaderCache[section]?[row] {
return (cachedResult[0] == 1, cachedResult[1], cachedResult[2])
}
var isHeader = false
var semiResult = 0
var headerNumber = -1
for (index, chod) in objednavkaDny[section].chody.enumerated() {
let sum = chod.objednavky.count
if row == semiResult {
isHeader = true
break
}else if row < semiResult {
semiResult -= objednavkaDny[section].chody[index-1].objednavky.count
break
}else {
headerNumber += 1
semiResult += 1
if index != objednavkaDny[section].chody.count - 1 {
semiResult += sum
}
}
}
checkIfHeaderCache[section] = [Int:[Int]]()
checkIfHeaderCache[section]![row] = [isHeader ? 1 : 0, headerNumber, semiResult]
return (isHeader, headerNumber, semiResult)
}
and the main cell that shows the data:
class ObjednavkyTableViewCell: UITableViewCell {
lazy var numberTextField: ObjednavkyTextField = {
let textField = ObjednavkyTextField()
textField.translatesAutoresizingMaskIntoConstraints = false
return textField
}()
let mealLabel: UILabel = {
let label = UILabel()
label.translatesAutoresizingMaskIntoConstraints = false
label.textColor = .black
label.textAlignment = .left
label.font = UIFont(name: ".SFUIText", size: 15)
label.numberOfLines = 0
label.backgroundColor = .white
label.isOpaque = true
return label
}()
lazy var detailsButton: UIButton = {
let button = UIButton(type: .custom)
button.translatesAutoresizingMaskIntoConstraints = false
button.setImage(UIImage(named: "arrow-right")?.withRenderingMode(.alwaysTemplate), for: .normal)
button.imageView?.tintColor = UIColor.custom.blue.classicBlue
button.imageView?.contentMode = .scaleAspectFit
button.contentHorizontalAlignment = .right
button.imageEdgeInsets = UIEdgeInsetsMake(10, 0, 10, 0)
button.addTarget(self, action: #selector(detailsButtonPressed), for: .touchUpInside)
button.backgroundColor = .white
button.isOpaque = true
return button
}()
let pricesContainerView: UIView = {
let view = UIView()
view.translatesAutoresizingMaskIntoConstraints = false
view.backgroundColor = .white
view.isOpaque = true
return view
}()
var canSetAmount = false {
didSet {
canSetAmount ? showNumberTextField() : hideNumberTextField()
}
}
var shouldShowPrices = false {
didSet {
shouldShowPrices ? showPricesContainerView() : hidePricesContainerView()
}
}
var pricesContainerHeight: CGFloat = 0
private let priceViewHeight: CGFloat = 30
var mealLabelLeadingConstraint: NSLayoutConstraint?
var mealLabelBottomConstraint: NSLayoutConstraint?
var pricesContainerViewHeightConstraint: NSLayoutConstraint?
var pricesContainerViewBottomConstraint: NSLayoutConstraint?
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
self.selectionStyle = .none
setupView()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
#objc func detailsButtonPressed() {
}
func updateData(objednavka: Objednavka, canSetAmount: Bool) {
self.canSetAmount = canSetAmount
if let popisJidla = objednavka.popisJidla, popisJidla != "", popisJidla != " " {
self.mealLabel.text = popisJidla
}else{
self.mealLabel.text = objednavka.nazevJidelnicku
}
if objednavka.objects.count > 1 {
shouldShowPrices = true
setPricesStackView(with: objednavka.objects)
checkIfSelected(objects: objednavka.objects)
}else{
shouldShowPrices = false
self.numberTextField.text = String(objednavka.objects[0].pocet)
//setSelected(objednavka.objects[0].pocet > 0, animated: false)
objednavka.objects[0].pocet > 0 ? setSelectedStyle() : setDeselectedStyle()
}
}
//---------------
func checkIfSelected(objects: [ObjednavkaObject]) {
var didChangeSelection = false
for object in objects { // Checks wether cell should be selected or not
if object.pocet > 0 {
setSelected(true, animated: false)
setSelectedStyle()
didChangeSelection = true
break
}
}
if !didChangeSelection {
setSelected(false, animated: false)
setDeselectedStyle()
}
}
//--------------
func showNumberTextField() {
numberTextField.isHidden = false
mealLabelLeadingConstraint?.isActive = false
mealLabelLeadingConstraint = mealLabel.leadingAnchor.constraint(equalTo: numberTextField.trailingAnchor, constant: 10)
mealLabelLeadingConstraint?.isActive = true
}
func hideNumberTextField() {
numberTextField.isHidden = true
mealLabelLeadingConstraint?.isActive = false
mealLabelLeadingConstraint = mealLabel.leadingAnchor.constraint(equalTo: readableContentGuide.leadingAnchor, constant: 0)
mealLabelLeadingConstraint?.isActive = true
}
func showPricesContainerView() {
hideNumberTextField()
pricesContainerView.isHidden = false
mealLabelBottomConstraint?.isActive = false
pricesContainerViewBottomConstraint?.isActive = true
}
func hidePricesContainerView() {
pricesContainerView.isHidden = true
pricesContainerViewBottomConstraint?.isActive = false
mealLabelBottomConstraint?.isActive = true
}
//--------------
func setSelectedStyle() {
self.backgroundColor = UIColor.custom.blue.classicBlue
mealLabel.textColor = .white
mealLabel.backgroundColor = UIColor.custom.blue.classicBlue
for subview in pricesContainerView.subviews where subview is ObjednavkyPriceView {
let priceView = (subview as! ObjednavkyPriceView)
priceView.titleLabel.textColor = .white
priceView.checkBox.backgroundColor = UIColor.custom.blue.classicBlue
priceView.titleLabel.backgroundColor = UIColor.custom.blue.classicBlue
priceView.backgroundColor = UIColor.custom.blue.classicBlue
}
pricesContainerView.backgroundColor = UIColor.custom.blue.classicBlue
detailsButton.imageView?.tintColor = .white
detailsButton.backgroundColor = UIColor.custom.blue.classicBlue
}
func setDeselectedStyle() {
self.backgroundColor = .white
mealLabel.textColor = .black
mealLabel.backgroundColor = .white
for subview in pricesContainerView.subviews where subview is ObjednavkyPriceView {
let priceView = (subview as! ObjednavkyPriceView)
priceView.titleLabel.textColor = .black
priceView.checkBox.backgroundColor = .white
priceView.titleLabel.backgroundColor = .white
priceView.backgroundColor = .white
}
pricesContainerView.backgroundColor = .white
detailsButton.imageView?.tintColor = UIColor.custom.blue.classicBlue
detailsButton.backgroundColor = .white
}
//-----------------
func setPricesStackView(with objects: [ObjednavkaObject]) {
let subviews = pricesContainerView.subviews
var subviewsToDelete = subviews.count
for (index, object) in objects.enumerated() {
subviewsToDelete -= 1
if subviews.count - 1 >= index {
let priceView = subviews[index] as! ObjednavkyPriceView
priceView.titleLabel.text = object.popisProduktu // + " " + NSNumber(value: object.cena).getFormattedString(currencySymbol: "Kč") // TODO: currencySymbol
priceView.canSetAmount = canSetAmount
priceView.count = object.pocet
priceView.canOrder = (object.nelzeObj == nil || object.nelzeObj == "")
}else {
let priceView = ObjednavkyPriceView(frame: CGRect(x: 0, y: CGFloat(index) * priceViewHeight + CGFloat(index * 5), width: pricesContainerView.frame.width, height: priceViewHeight))
pricesContainerView.addSubview(priceView)
priceView.titleLabel.text = object.popisProduktu // + " " + NSNumber(value: object.cena).getFormattedString(currencySymbol: "Kč") // TODO: currencySymbol
priceView.numberTextField.delegate = self
priceView.canSetAmount = canSetAmount
priceView.canOrder = (object.nelzeObj == nil || object.nelzeObj == "")
priceView.count = object.pocet
pricesContainerHeight += ((index == 0) ? 30 : 35)
}
}
if subviewsToDelete > 0 { // Deletes unwanted subviews
for _ in 0..<subviewsToDelete {
pricesContainerView.subviews.last?.removeFromSuperview()
pricesContainerHeight -= pricesContainerHeight + 5
}
}
if pricesContainerHeight < 0 {
pricesContainerHeight = 0
}
pricesContainerViewHeightConstraint?.constant = pricesContainerHeight
}
func setupView() {
self.layer.shouldRasterize = true
self.layer.rasterizationScale = UIScreen.main.scale
self.backgroundColor = .white
contentView.addSubview(numberTextField)
contentView.addSubview(mealLabel)
contentView.addSubview(detailsButton)
contentView.addSubview(pricesContainerView)
setupConstraints()
}
func setupConstraints() {
numberTextField.anchor(leading: readableContentGuide.leadingAnchor, size: CGSize(width: 30, height: 30))
numberTextField.centerYAnchor.constraint(equalTo: mealLabel.centerYAnchor).isActive = true
detailsButton.anchor(trailing: readableContentGuide.trailingAnchor, size: CGSize(width: 30, height: 30))
detailsButton.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
mealLabel.anchor(top: contentView.topAnchor, trailing: detailsButton.leadingAnchor, padding: .init(top: 10, left: 0, bottom: 0, right: -10))
mealLabelBottomConstraint = mealLabel.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -10)
mealLabelBottomConstraint?.priority = UILayoutPriority(rawValue: 999)
pricesContainerView.anchor(top: mealLabel.bottomAnchor, leading: readableContentGuide.leadingAnchor, trailing: detailsButton.leadingAnchor, padding: .init(top: 10, left: 0, bottom: 0, right: -10))
pricesContainerViewBottomConstraint = pricesContainerView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -10)
pricesContainerViewBottomConstraint?.priority = UILayoutPriority(rawValue: 999)
pricesContainerViewHeightConstraint = pricesContainerView.heightAnchor.constraint(equalToConstant: 0)
pricesContainerViewHeightConstraint?.priority = UILayoutPriority(rawValue: 999)
pricesContainerViewHeightConstraint?.isActive = true
}
}
To conclude how it is done:
tableView.rowHeight is set to UITableViewAutomaticDymension
inside cellForRowAt I get the data from an array and give it to the
cell
all the cells are set up in code using constraints
all the views have set isOpaque = true
heights of the cells are cached
cells are set to rasterize
I also noticed that it lags at certain scroll levels and that sometimes it works just fine and sometimes it lags a lot.
Despite all of the optimization I have done, the tableView still lags while scrolling.
Here is a screenshot from Instruments
Any tip how to improve the scrolling performance is highly appreciated!
(I might have forgotten to include some code/information so feel free to ask me in the comments.)

Answer 1

I can't tell you where the lag happens exactly but when we are talking about lagging during scrolling, it's related to your cellForRowAt delegate method. What happends is that too many things are going on within this method and it's called for every cells that are displaying & going to display. I see that your are trying to cache the result by checkIfHeaderCache but still, there is a for loop at the very beginning to determine header cell.
Suggestions:
I don't know where you get data (objednavkaDny) from but right after you get the data, do a full loop through and determin cell type one by one, and save the result some where base on your design. During this loading time, you can show some loading message on the screen. Then, within the cellForRow method, you should be just simply using things like
if (isHeader) {
render header cell
} else {
render other cell
}
Bottom line:
cellForRow method is not designed to handle heavy calculations, and it will slow down the scrolling if you do so. This method is for assigning values to the cached table view cell only and that's the only thing it is good at.