I'm working with counting table and have some problem. How to fix 'Fatal error: String index is out of bounds' - ios

Question

I'm working with counting label String. When I run my project all my text shows and at the end it crashes with:
'Fatal error: String index is out of bounds'
on the line:
startText += String(endText[index])
in the handleUpdate method.
I don't understand why.
class AboutViewController: BaseListController {
let countingLabel: UILabel = {
let label = UILabel()
label.textColor = .white
label.textAlignment = .center
label.font = UIFont.boldSystemFont(ofSize: 18)
return label
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(countingLabel)
countingLabel.frame = view.frame
let displayLink = CADisplayLink(target: self, selector: #selector(handleUpdate))
displayLink.add(to: .main, forMode: .default)
}
var startText = " "
let endText = "Hey! I need your help..."
var startValue = 0
#objc func handleUpdate(){
let endTextValue = endText.count - 1
let index = endText.index(endText.startIndex, offsetBy: startValue)
self.countingLabel.text = "\(startText)"
// Error on the following line:
startText += String(endText[index])
startValue += 1
if startValue > endTextValue{
startText = endText
}
print(endTextValue)
}
let animationStartDate = Date()
}

Answer 1

You are calling handleUpdate on a CADisplayLink, it will continue to be called after startValue has been incremented beyond the length of the string. You do have a check for startValue > endTextValue but this is after you have already tried to index using startValue, so you get a crash.
You need to remove the CADisplayLink handler once its job is done, which means you need to keep a reference to it.
var startText = ""
let endText = "Hey! I need your help..."
var startValue = 0
var displayLink: CADisplayLink?
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(countingLabel)
countingLabel.frame = view.frame
self.displayLink = CADisplayLink(target: self, selector: #selector(handleUpdate))
self.displayLink!.add(to: .main, forMode: .default)
}
#objc func handleUpdate(){
guard startValue < endText.count else {
self.displayLink?.invalidate()
self.displayLink = nil
return
}
let index = endText.index(endText.startIndex, offsetBy: startValue)
startText += String(endText[index])
self.countingLabel.text = "\(startText)"
startValue += 1
let animationStartDate = Date()
}