Virtual Kale

Turns out this topic needs more than just one post. In part 1, I described how I was able to use AppStorage for an enum with associated values.

Here I will discuss a challenge I encountered when attempting to build the View to enable users to change their value for the languageChoice enum.

enum LanguageChoice: Equatable, Codable {
    case all
    case oneToOne(Language, Language)
    case oneToAll(Language)
    case allToOne(Language)
    case symmetricSubset(Set<Language>)

Step 1, create a version of LanguageChoice with no associated values:

    enum SimpleLanguageChoice: String, CaseIterable {
        case all
        case oneToOne
        case oneToAll
        case allToOne
        case symmetricSubset
    }

Step 2, create a state variable using the new enum type and bind it to a picker:

    @State var languageChoice: SimpleLanguageChoice = .all
    var body: some View {
        VStack {
            Picker("Languages Choice", selection: $languageChoice) {
                ForEach(SimpleLanguageChoice.allCases, id: \.self) {
                    Text($0.rawValue)
                        .tag($0)
                }
            }
        }
    }

Step 3, add UI below the picker to display the appropriate controls to go with the value of languageChoice.

    @ViewBuilder var bottomStuff: some View {
        switch languageChoice {
        case .all:
            EmptyView()
        case .oneToAll:
            Picker("Ask in", selection: $language) {
                ForEach(Language.allCases, id: \.self) {
                    Text($0.rawValue)
                        .tag($0)
                }
            }
     // et cetera

Step 4, add an AppStorage var to get the persisted LanguageChoice value and use it to configure the Picker UI.

    @AppStorage("languageChoice") var persistedLanguageChoice: LanguageChoice = .all
    @State var languageChoice: SimpleLanguageChoice

init() {
     languageChoice = SimpleLanguageChoice.simpleChoice(for: persistedLanguageChoice)
}

Insert record scratch sound here! This code created the following compiler error:
'self' used before all stored properties are initialized

I wanted to use the persisted languageChoice to set up the UI to allow users to choose a new languageChoice. But doing this required reading persisted languageChoice, which was not allowed. Alas. So to get things to work I needed to set a default value for languageChoice in the declaration and then update the value in init. Just do this, right?

    @State var languageChoice: SimpleLanguageChoice = .all
    init() {
        languageChoice = SimpleLanguageChoice.simpleChoice(for: persistedLanguageChoice)

Nope. If you set an initial value for a State variable in the declaration, updating it in init gets more complicated. You need to do this:

    init() {
        _languageChoice = State(initialValue: SimpleLanguageChoice.simpleChoice(for: persistedLanguageChoice)

The SwiftUI property wrapper AppStorage is a great way to cut down on boiler plate code needed to manage/access UserDefaults. At the risk of sounding like I’m looking a gift horse in the mouth, I feel compelled to describe a couple of challenges I encountered while using AppStorage in my current project, the language flash cards app.

AppStorage for enums with associated values

The type I wanted to persist using AppStorage was a fairly gnarly enum with a variety of associated values. It is the value that persists the users preferences for languages to use for asking and answering. See this blog post for more details on how this enum came to be.

enum LanguageChoice: Equatable, Codable {
    case all
    case oneToOne(Language, Language)
    case oneToAll(Language)
    case allToOne(Language)
    case symmetricSubset(Set<Language>)
}

Out of the box, AppStorage only works with the most basic of types. (Int, String, Bool, etc.) There are several great resources on how to extend AppStorage support to basic structs. If you google “AppStorage RawRepresentable” you’ll be away to the races in no time. The tl;dr version is: implement
public init?(rawValue: String)
and
public var rawValue: String

My first inclination was to use JSONDecoder() in init and JSONEncoder() in rawValue. Unfortunately JSONEncoder() was calling rawValue, which was calling JSONEncoder() etc.

So instead I defined a custom scheme to encode (and decode) the LanguageChoice enum. This required a fair bit of custom code (see below) and allowed LanguageChoice to work with @AppStorage. But I wasn’t out of the woods yet. There was still more work to be done, which I’ll describe in a future post.

Here is how I chose to conform to RawRepresentable.

    public init?(rawValue: String) {
        let components = rawValue.components(separatedBy: ":")
        guard let first = components.first else {
            return nil
        }
        switch first {
        case "all":
            self = .all
        case "oneToOne":
            guard components.indices.contains(2),
                  let lang1 = Language.init(rawValue: components[1]),
                  let lang2 = Language.init(rawValue: components[2]) else {
                return nil
            }
            self = .oneToOne(lang1, lang2)
        case "oneToAll":
            guard components.indices.contains(1),
                  let lang = Language.init(rawValue: components[1]) else {
                return nil
            }
            self = .oneToAll(lang)
        case "allToOne":
            guard components.indices.contains(1),
                  let lang = Language.init(rawValue: components[1]) else {
                return nil
            }
            self = .allToOne(lang)
        case "symmetricSubset":
            guard components.indices.contains(1) else {
                return nil
            }
            let languageComponents = components[1].components(separatedBy: ",")
            let languages: Set<Language> = Set(languageComponents.compactMap { Language(rawValue: $0) } )
            self = .symmetricSubset(languages)
        default:
            return nil
        }
    }

    public var rawValue: String {
        let data = try? JSONEncoder().encode(self)
        print("data: \(data)")
        let result: String
        switch self {
        case .all:
            result = "all"
        case .oneToOne(let language, let language2):
            result = "oneToOne:\(language.rawValue):\(language2.rawValue)"
        case .oneToAll(let language):
            result = "oneToAll:\(language.rawValue)"
        case .allToOne(let language):
            result = "allToOne:\(language.rawValue)"
        case .symmetricSubset(let set):
            let setRawValue = set.sorted().reduce("") {
                $0 + $1.rawValue + ","
            }
            
            result = "symmetricSubset:\(setRawValue)"
        }
        return result
    }

Unit tests for this code have been left as an exercise for the student.