With the basics of the interface wired up, let’s implement the conversion from Fahrenheit to Celsius. You are going to store the current Fahrenheit value and compute the Celsius value whenever the text field changes.

In ConversionViewController.swift, add a property for the Fahrenheit value. This will be an optional measurement for temperature, a Measurement<UnitTemperature>?. (Do not worry about the strange syntax of the measurement type; it is a generic type, and you will be learning more about them later.)

@IBOutlet var celsiusLabel: UILabel!
@IBOutlet var textField: UITextField!

var fahrenheitValue: Measurement<UnitTemperature>?

This property is optional because the user might not have typed in a number. (Earlier, you used optional binding to manage the same issue for the Celsius label.)

Now, add a computed property for the Celsius value.

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

If there is a Fahrenheit value, it will be converted to the equivalent value in Celsius. Otherwise, nil is returned.

Any time the Fahrenheit value changes, the Celsius label needs to be updated. Take care of that next.

Add a method to ConversionViewController that updates the celsiusLabel.

func updateCelsiusLabel() {
    if let celsiusValue = celsiusValue {
        celsiusLabel.text = "(celsiusValue.value)"
    } else {
        celsiusLabel.text = "???"
    }
}

This method should be called whenever the Fahrenheit value changes. To do this, you will use a property observer, which is a chunk of code that is called whenever a property’s value changes.

A property observer is declared using curly braces immediately after the property declaration. Inside the braces, you declare your observer using either willSet or didSet, depending on whether you want it to run immediately before or immediately after the property value changes, respectively.

(Note that property observers are not triggered when the property value is changed from within an initializer.)

Add a property observer to fahrenheitValue that will be called after the property value changes.

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

With that logic in place, you can now update the Fahrenheit value when the text field changes (which, in turn, will trigger an update of the Celsius label).

In fahrenheitFieldEditingChanged(_:), delete your earlier non-converting implementation and instead update the Fahrenheit value.

@IBAction func fahrenheitFieldEditingChanged(_ textField: UITextField) {

    if let text = textField.text, !text.isEmpty {
        celsiusLabel.text = text
    } else {
        celsiusLabel.text = "???"
    }

    if let text = textField.text, let value = Double(text) {
        fahrenheitValue = Measurement(value: value, unit: .fahrenheit)
    } else {
        fahrenheitValue = nil
    }
}

If there is text in the text field and that text can be represented by a Double, then the Fahrenheit value is set to a Measurement initialized with that Double value. For example, if the text field contains 3.14, then fahrenheitValue is set to a value of 3.14. But if the text field contains something like three or 1.2.3, then the initial checks fail. In this case, the Fahrenheit value is set to nil.

Build and run the application. The conversion between Fahrenheit and Celsius works great – so long as you enter a valid number. (It also shows more digits than you probably want it to, which you will address in a moment.)

It would be nice if the celsiusLabel updated when the application first launches, instead of showing the value 100. Override viewDidLoad() to set the initial value, similar to what you did in Chapter 1.

override func viewDidLoad() {
    super.viewDidLoad()

    print("ConversionViewController loaded its view.")

    updateCelsiusLabel()
}

Build and run again to see the effect of this change.

In the remainder of this chapter, you will update WorldTrotter to address two issues: You will format the Celsius value to show a precision up to one fractional digit, and you will not allow the user to type in more than one decimal separator. There are a couple of other issues with your app, but you will focus on these two for now. One of the other issues will be presented as a challenge at the end of this chapter.

Let’s start with updating the precision of the Celsius value.

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

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