textField(_:shouldDeleteBackwardsCharactersIn:)

DeleteBackwardTextField.swift

protocol MyTextFieldDelegate: UITextFieldDelegate {

    /// Asks the delegate if the character before the cursor or currently selected text should be deleted.
    ///
    /// - Parameters:
    ///   - textField: The text field containing the text.
    ///   - range: Range of the text that is going to be deleted. If cursor is at the beginning, range's `location` and `length` are 0.
    /// - Returns: `true` if the specified text range should be deleted; otherwise, `false` to keep the old text.
    func textField(_ textField: MyTextField, shouldDeleteBackwardsCharactersIn range: NSRange) -> Bool
}


class MyTextField: UITextField {

    override func deleteBackward() {
        guard let delegate = delegate as? MyTextFieldDelegate, let range = selectedTextRange else {
            super.deleteBackward()
            return
        }

        let startIndex = offset(from: beginningOfDocument, to: range.start)
        let endIndex = offset(from: beginningOfDocument, to: range.end)

        let deleteRange: NSRange
        if startIndex == endIndex {
            if startIndex > 0 {
                deleteRange = NSRange(location: startIndex - 1, length: 1)
            } else {
                deleteRange = NSRange(location: 0, length: 0)
            }
        } else {
            deleteRange = NSRange(startIndex..<endIndex)
        }

        if delegate.textField(self, shouldDeleteBackwardsCharactersIn: deleteRange) {
            super.deleteBackward()
        }
    }
}