Make variables not reset when switching to a different view controller, and keep the variable like that when app is quit

Question

I'm making a weather app, and currently I'm adding support to switch from Imperial measurement to Metric, but when you switch from the settings view controller to the main menu to the weather viewer, it resets the unit variable to Imperial.

I have looked at many questions and articles, and either:

1. I don't understand how to do it
2. It doesn't work or needs an older version of Swift
3. It causes the whole app to break

========================================================================

Note: I am using segues to switch VCs

@IBAction func unitSwitcherMetric(_ sender: UIButton) {

    weatherDataModel.unit = "metric"
    weatherDataModel.windUnit = ""
    print("The unit is now \(weatherDataModel.unit)")

}

After you select that button, it prints The unit is now metric as expected, but when you switch to the main menu view controller, where it's supposed to print the unit variable, it prints imperial and the API supplies imperial measurements as well.

Answer 1

You could implemente a Singleton, that's a class that has only 1 instance and is meant to be kept in memory for different uses. You can implement it like this:

class MetricManager {

    static let sharedInstance = MetricManager()

    var currentUnit: String = "imperial"

    private init() {

    }
}

After that you can simply assign a value on Settings VC like this

@IBAction func unitSwitcherMetric(_ sender: UIButton) {
    MetricManager.sharedInstance.currentUnit = "metric"
}

And then on Menu or wherever you want to get the current unit value, you just consult it on:

MetricManager.sharedInstance.currentUnit

Answer 2

A couple of thoughts:

1. I would not suggest using string values. If I needed to support iOS versions prior to 10, I’d define an enumeration:

   enum CustomUnitTemperature: String {
       case celsius
       case fahrenheit
   }
   

2. In terms of saving this “preferred temperature units” so that it’s available for the next time you run the app, you can save the user’s preferences in UserDefaults, e.g.:

   struct Preferences {
       private let temperatureUnitKey = "customTemperatureUnitKey"
   
       var unitTemperature: CustomUnitTemperature {
           didSet {
               UserDefaults.standard.set(unitTemperature.rawValue, forKey: temperatureUnitKey)
           }
       }
   
       init() {
           unitTemperature = UserDefaults.standard.string(forKey: temperatureUnitKey)
               .flatMap { CustomUnitTemperature(rawValue: $0) } ?? .celsius
       }
   }
   

   Then you could have a preferences object:

   var preferences = Preferences()
   

   You can then read and write the unitTemperature to the preferences (which will persist between runs of the app) like so. E.g. to read it:

   let unit = preferences.unitTemperature
   
   switch unit {
   case .celsius:    ...
   case .fahrenheit: ...
   }
   

   Or, to change/save the value:

   preferences.unitTemperature = .fahrenheit
   

   Bottom line, it’s a property which is initialized by reading from UserDefaults and updated in UserDefaults when you set the property.

3. If you don’t need to support iOS versions before 10, rather than defining my own temperature unit of measure, I’d use the existing UnitTemperature type. See the Dimension documentation or the WWDC 2016 video Measurements and Units for information about how you might use this UnitTemperature type.

   Anyway, in iOS 10 and later, I might define my Preferences type to access and save using this UnitTemperature type, but saving the symbol to the UserDefaults:

   struct Preferences {
       private let temperatureUnitKey = "temperatureUnitKey"
   
       var unitTemperature: UnitTemperature {
           didSet {
               UserDefaults.standard.set(unitTemperature.symbol, forKey: temperatureUnitKey)
           }
       }
   
       init() {
           unitTemperature = UserDefaults.standard.string(forKey: temperatureUnitKey)
               .map { UnitTemperature(symbol: $0) } ?? .celsius
       }
   }
   

   Then you can access it the same way (though our switch would now need a default clause, since celsius and fahrenheit aren’t the only acceptable values:

   let unit = preferences.unitTemperature
   
   switch unit {
   case .celsius:    ...
   case .fahrenheit: ...
   default:          ...
   }
   

   And updating the preferences are the same:

   preferences.unitTemperature = .fahrenheit
   

4. In terms of passing data from a presenting view controller to a presented view controller, and back, see Passing Data between View Controllers.