Crash when creating NSRange in String

Question

I am encountering a random crash in the NSRange creation. Because I added a check before, I do not understand how it can happen:

let textStr = "whatever you want"   
if let end = textStr.unicodeScalars.index(textStr.startIndex, offsetBy: slide.endIndex, limitedBy: textStr.endIndex){
    let realEnd = textStr.unicodeScalars.index(textStr.startIndex, offsetBy: slide.endIndex)
    nsRange = NSRange(start...realEnd, in: textStr)
}

• slide is an object generated by a parser and representing a portion of textStr.
• Parser uses textStr.unicodeScalars for performances reasons

Answer 1

This works in a Playground:

import Foundation

let textStr = "whatever you want"
let slide = "you"
let start = slide.startIndex // not sure what this was meant to be in your code
if textStr.index(textStr.unicodeScalars.startIndex, offsetBy: slide.unicodeScalars.distance(from: slide.startIndex, to: slide.endIndex), limitedBy: textStr.unicodeScalars.endIndex) != nil {
    let realEnd = textStr.index(textStr.unicodeScalars.startIndex, offsetBy: slide.unicodeScalars.distance(from: slide.unicodeScalars.startIndex, to: slide.unicodeScalars.endIndex))
    let nsRange = NSRange(start..<realEnd, in: textStr)
}

Seems like a weird way to go about this though. Perhaps rethink your approach.