Bronze Challenge: Disallow Alphabetic Characters (simple solution)

iOS Programming: The Big Nerd Ranch Guide (6th Edi 4. Text Input and Delegation
1

import UIKit

class ConversionViewController: UIViewController, UITextFieldDelegate {

// MARK: - IBOutlet

@IBOutlet var celsiusLabel: UILabel!

@IBOutlet var textField: UITextField!

var fahrenheitValue: Measurement<UnitTemperature>? {
    didSet {
        updateCelsiusLabel()
    }
}

var celsiusValue: Measurement<UnitTemperature>? {
    if let fahrenheitValue = fahrenheitValue {
        return fahrenheitValue.converted(to: .celsius)
    } else {
        return nil }
}

let numberFormatter: NumberFormatter = {
    let nf = NumberFormatter()
    nf.numberStyle = .decimal
    nf.minimumFractionDigits = 0
    nf.maximumFractionDigits = 1
    return nf
}()

// MARK: - IBAction

@IBAction func fahrenheitFieldEditingChanged(_ textField: UITextField) {
    
    if let text = textField.text, let value = Double(text) {
        fahrenheitValue = Measurement(value: value, unit: .fahrenheit)
    } else {
        fahrenheitValue = nil
    }
}

@IBAction func dismissKeyboard(_ sender: UITapGestureRecognizer) {
    textField.resignFirstResponder()
}

// MARK: - UIViewController Method

override func viewDidLoad() {
    super.viewDidLoad()
    updateCelsiusLabel()
}

// MARK: - Utility Method

func updateCelsiusLabel() {
    if let celsiusValue = celsiusValue {
        celsiusLabel.text = numberFormatter.string(from: NSNumber(value: celsiusValue.value))
    } else {
        celsiusLabel.text = "???"
    }
}

// MARK: - UITextFieldDelegate Method

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let existingTextHasDecimalSeparator = textField.text?.range(of: ".")
    let replacementTextHasDecimalSeparator = string.range(of: ".")
    let replacementTextHasMinusForBelowZero = string.range(of: "-")                 // my add for the Bronze Challenge
    
    // isSuperset(of:) won't work anymore properly with NSCharacterSet, therefore I replaced it with CharacterSet
    let numericOnly = CharacterSet.decimalDigits                                    // my add for the Bronze Challenge
    let myStringSet = CharacterSet(charactersIn: string)                            // my add for the Bronze Challenge
    
    if existingTextHasDecimalSeparator != nil && replacementTextHasDecimalSeparator != nil /* // my add for the Bronze Challenge */ || numericOnly.isSuperset(of: myStringSet) == false && replacementTextHasDecimalSeparator == nil && replacementTextHasMinusForBelowZero == nil {
        return false
    }
    else {
        return true
    }
}

}