How to use KVO to update tableViewCells based on underlying array element changes? - ios

Question

I have a table view representing an underlying array. The cells have a label and a slider which should show the value of the percentage property of the array.
I want to use key-value observing to update the label whenever the percentage property changes. (I know KVO is overkill in this example but eventually sliding one slider will affect the other cells including the position of the slider and the underlying array will be set from multiple places in the app and at any time so KVO is the way to go.)
I've had a bunch of help from this answer, but I can't get it to fire and update the label. I'm including all my code here. Not sure where I'm going wrong.
import UIKit
class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, CustomCellDelegate {
#IBOutlet weak var tableView: UITableView!
override func viewDidLoad() {
super.viewDidLoad()
tableView.dataSource = self
tableView.delegate = self
for i in 0...4 {
items.append(Items(ID: i, percentage: 50))
}
}
func numberOfSections(in tableView: UITableView) -> Int {
return 1
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return items.count
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
if let cell = tableView.dequeueReusableCell(withIdentifier: myTableViewCell.ID) as? myTableViewCell {
cell.object = items[indexPath.row]
cell.mySlider.tag = indexPath.row
return cell
} else {
return UITableViewCell()
}
}
func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
return 100
}
#IBAction func sliderValueChanged(_ sender: UISlider) {
items[sender.tag].percentage = Double(sender.value)
print("percentage at \(items[sender.tag].ID) is \(items[sender.tag].percentage)")
}
func didUpdateObject(for cell: UITableViewCell) {
if let indexPath = tableView.indexPath(for: cell) {
tableView.reloadRows(at: [indexPath], with: .automatic)
print("hello")
}
}
}
class myTableViewCell: UITableViewCell {
static let ID = "myCell"
weak var delegate: CustomCellDelegate?
private var token: NSKeyValueObservation?
var object: Items? {
willSet {
token?.invalidate()
}
didSet {
myLabel.text = "\(object?.percentage ?? 0)"
token = object?.observe(\.percentage) { [weak self] object, change in
if let cell = self {
cell.delegate?.didUpdateObject(for: cell)
}
}
}
}
override func awakeFromNib() {
super.awakeFromNib()
}
#IBOutlet weak var myLabel: UILabel!
#IBOutlet weak var mySlider: UISlider!
}
class Items: NSObject {
let ID: Int
#objc dynamic var percentage: Double
init(ID: Int, percentage: Double){
self.ID = ID
self.percentage = percentage
super.init()
}
}
var items: [Items] = []
protocol CustomCellDelegate: class {
func didUpdateObject(for cell: UITableViewCell)
}

Answer 1

To do the KVO in Swift 4, you have to declare the property as dynamic and call observe(_:options:changeHandler:) on that object, saving the resulting NSKeyValueObservation token. When that token falls out of scope (or replaced with another token), the original observer will automatically be removed.
In your case, you have your observer calling the delegate, which then reloads the cell. But you never appear to set that delegate property, so I suspect that method isn't getting called.
But this all seems a bit fragile. I'd be inclined to just update the label directly in the observer's changeHandler. I also think you can do a more direct updating of the cell (I'd put the "value changed" IBAction in the cell, not the table view), and eliminate that rather awkward use of the tag to identify which row in the model array had its slider updated (which can be problematic if you insert or delete rows).
So consider this object:
class CustomObject: NSObject {
let name: String
#objc dynamic var value: Float // this is the property that the custom cell will observe
init(name: String, value: Float) {
self.name = name
self.value = value
super.init()
}
}
You could then have a table view controller that populates an array of objects with instances of this model type. The details here are largely unrelated to the observation (which we'll cover below), but I include this just to provide a complete example:
class ViewController: UITableViewController {
var objects: [CustomObject]!
override func viewDidLoad() {
super.viewDidLoad()
// self sizing cells
tableView.estimatedRowHeight = 60
tableView.rowHeight = UITableViewAutomaticDimension
// populate model with random data
let formatter = NumberFormatter()
formatter.numberStyle = .spellOut
objects = (0 ..< 1000).map {
CustomObject(name: formatter.string(for: $0)!, value: 0.5)
}
}
}
// MARK: - UITableViewDataSource
extension ViewController {
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return objects?.count ?? 0
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "CustomCell", for: indexPath) as! CustomCell
cell.object = objects[indexPath.row]
return cell
}
}
Having done that, you can now have the base class for your cell (a) update the model object if the slider changes; and (b) observe changes to that dynamic property, in this example updating the label when the value changes are observed in the model object:
class CustomCell: UITableViewCell {
#IBOutlet weak var nameLabel: UILabel!
#IBOutlet weak var valueLabel: UILabel!
#IBOutlet weak var valueSlider: UISlider!
static private let formatter: NumberFormatter = {
let _formatter = NumberFormatter()
_formatter.maximumFractionDigits = 2
_formatter.minimumFractionDigits = 2
_formatter.minimumIntegerDigits = 1
return _formatter
}()
private var token: NSKeyValueObservation?
weak var object: CustomObject? {
didSet {
let value = object?.value ?? 0
nameLabel.text = object?.name
valueLabel.text = CustomCell.formatter.string(for: value)
valueSlider.value = value
token = object?.observe(\.value) { [weak self] object, change in
self?.valueLabel.text = CustomCell.formatter.string(for: object.value)
}
}
}
#IBAction func didChangeSlider(_ slider: UISlider) {
object?.value = slider.value
}
}
That yields:
For more information, see the "Key-Value Observing" section of the Using Swift with Cocoa and Objective-C: Adopting Cocoa Patterns.

Answer 2

hi #sean problem is in UITableview cell class you have already make diSet Method , so you dont need to pass value for cell.lable and slider Just try below code
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
if let cell = tableView.dequeueReusableCell(withIdentifier: myTableViewCell.ID) as? myTableViewCell {
//pass the object to which you wanna add observer to cell
cell.object = items[indexPath.row]
return cell
} else {
return UITableViewCell()
}
}