UITextFieldDelegate extension. Get content the user would see in UITextField. Swift 4

Question

I would like to pass the content of the text field into the checkInput method as the user sees the input in the text field. Currently, if a user deletes whitespace resulting in what should be a valid input, this is not working. Such an operation appears to send what is one input behind the visual representation of the UITextField content to checkInput.

    extension ViewController: UITextFieldDelegate {
        func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
            var validInput = false
            if let textFieldText = textField.text {
                // TODO: handle removal of whitespace within text view
                let input = newInput(string, in: textFieldText, at: range.location)
                validInput = checkInput(newInput)
            }
            return !validInput
        }
    }

I have added a helper function, but this does not handle all cases (such as deleting whitespace)

func newInput(_ string: String, in stringText: String?, at rangeLocation: Int) -> String {
        let firstHalfString = (stringText as NSString?)?.substring(to: rangeLocation) ?? ""
        let secondHalfString = (stringText as NSString?)?.substring(from: rangeLocation) ?? ""
        return "\(firstHalfString)\(string)\(secondHalfString)"
    }

Answer 1

The problem here is that you have not understood what shouldChangeCharactersIn does. It is not "one input behind"; it is called before anything happens at all. That is what "should" means! This method is telling you what might happen and asking you what should happen.

That isn't what you want to know. You want to know what did happen. So you've implemented totally the wrong method.

What's the right method? A UITextField is a UIControl. Implement its editingChanged action-target pair to tell you whenever the text changes, and just read the text directly from the text field every time your action method is called.