Error when unwrapping a optional
In the following code I get an error from xcode saying: "Cannot use mutating member on immutable value: 'fahrenheitValue' is a 'let' constant."
This code sample is from The Big Nerd Ranch Guide for iOS 6th edition.
A part from not really understanding why the book would be wrong, I understand the meaning of the error but I don't get how I could work around this...
Could somebody tell me what I'm doing wrong here?
import UIKit
class ConversionViewController: UIViewController {
@IBOutlet var celsiusLabel: UILabel!
var fahrenheitValue: Measurement<UnitTemperature>? {
didSet {
updateCelsiusLabel()
}
}
var celsiusValue: Measurement<UnitTemperature>? {
if let fahrenheitValue = fahrenheitValue {
return fahrenheitValue.convert(to: .celsius)
}else{
return nil
}
}
}
The problem is these two lines:
if let fahrenheitValue = fahrenheitValue {
return fahrenheitValue.convert(to: .celsius)
You can't call convert(to:) on fahrenheitValue because fahrenheitValue is a constant and convert(to:) is trying to modify that constant, hence the error.
The solution is to replace convert(to:) to converted(to:). The former doesn't return anything and tries to modify the receiver. The latter creates a new measurement and returns the new value. That is what you want.
return fahrenheitValue.converted(to: .celsius)