Format UITextField text without having cursor move to the end

Question

I am trying to apply NSAttributedString styles to a UITextField after processing a new text entry, keystroke by keystroke. The problem is t

Answer 1

To piggy back off an answer to a different question here: https://stackoverflow.com/a/51814368/431271, you shouldn't be modifying the text in shouldChangeCharactersInRange since that delegate method is intended only to let the field know whether or not to allow a change and isn't supposed to mutate.

Instead, I like to handle the text change by subscribing to the value change, like this:

textField.addTarget(self, action: #selector(handleTextChanged), for: .editingChanged)

// elsewhere in the file
func handleTextChanged(_ textField: UITextField) {
    textField.sanitizeText(map: formatText)
}

where the implementation of sanitizeText looks like this:

extension UITextField {

    // Use this to filter out or change the text value without 
    // losing the current selection
    func sanitizeText(map: ((String) -> String)) {
        guard let text = self.text,
            let selection = selectedTextRange else {
                return
        }

        let newText = map(text)

        // only execute below if text is different
        guard newText != text else { return }

        // determine where new cursor position should start
        // so the cursor doesnt get sent to the end
        let diff = text.count - newText.count
        let cursorPosition = offset(from: beginningOfDocument, to: selection.start) - diff

        self.text = newText

        // notify the value changed (to ensure delegate methods get triggered)
        sendActions(for: .valueChanged)

        // update selection afterwards
        if let newPosition = position(from: beginningOfDocument, offset: cursorPosition) {
            selectedTextRange = textRange(from: newPosition, to: newPosition)
        }
    }
}