How to convert character index from layoutManager to String scale in swift

Question

How to convert character index from layoutManager to String scale in swift? this is the code I'm using:

let touchPoint: CGPoint = gesture.locationOfTouch(0, inView: self.definitionLabel)
let index = layoutManager.characterIndexForPoint(touchPoint, inTextContainer: textContainer, fractionOfDistanceBetweenInsertionPoints: nil)

please don't tell me to use advanceBy() function on the first index of the string characterset since characters like ò count two in the scale of layoutManager but swift string counts theme once.

Answer 1

The index returned from the NSLayoutManager is " NSString based", ie it is the number of UTF-16 code units from the start of the string to the character at the given point. (So ò actually counts as one, but Emojis 😀 count two and flags 🇧🇪 even count four.)

To convert that index to a valid Swift String index, you can use the same approach as in https://stackoverflow.com/a/30404532/1187415 :

let text = ... // the stored text
let i16 = text.utf16.startIndex.advancedBy(index, limit: text.utf16.endIndex)
// i16 is a String.UTF16View.Index
if let strIndex = String.Index(i16, within: text) {
    // strIndex is a String.CharacterView.Index
    let char = text[strIndex]
    print(char)
} 

Answer 2

Updated for Swift 5

Martin R's answer gave me the rough outline; here's working Swift 5 version. Mine is for a UITextView with Attributed Text but should work just as well with regular String and for UITextField and UILabel.

    func handleTap(_ sender: UIGestureRecognizer) {
        guard let textView = sender.view as? UITextView else { return }

        guard let plaintext = textView.attributedText?.string else { return }
        //guard let plaintext = textView.text else { return }

        let location = sender.location(in: textView)
        let charIndex = textView.layoutManager.characterIndex(for: location, in: textView.textContainer,
                                                          fractionOfDistanceBetweenInsertionPoints: nil)

        if let strIndex = plaintext.utf16.index(plaintext.utf16.startIndex, offsetBy: charIndex, limitedBy: plaintext.utf16.endIndex) {
            let char = plaintext[strIndex]
            print("Character tapped was \(char)")
        }
    }

    let textTap = UITapGestureRecognizer(target: self, action: #selector(handleTap))
    textView.addGestureRecognizer(textTap)