ios - SwiftUI: What is @AppStorage property wrapper

Question

I used to save important App data like login credentials into UserDefaults using the following statement:

UserDefaults.standard.set("[email protected]", forKey: "emailAddress")

Now, I have come to know SwiftUI has introduced new property wrapper called:

@AppStorage

Could anyone please explain how the new feature works?

Answer 1

AppStorage

@AppStorage is a convenient way to save and read variables from UserDefaults and use them in the same way as @State properties. It can be seen as a @State property which is automatically saved to (and read from) UserDefaults.

You can think of the following:

@AppStorage("emailAddress") var emailAddress: String = "[email protected]"

as an equivalent of this (which is not allowed in SwiftUI and will not compile):

@State var emailAddress: String = "[email protected]" {
    get {
        UserDefaults.standard.string(forKey: "emailAddress")
    }
    set {
        UserDefaults.standard.set(newValue, forKey: "emailAddress")
    }
}

Note that @AppStorage behaves like a @State: a change to its value will invalidate and redraw a View.

By default @AppStorage will use UserDefaults.standard. However, you can specify your own UserDefaults store:

@AppStorage("emailAddress", store: UserDefaults(...)) ...

---

Unsupported types (e.g., Array):

As mentioned in iOSDevil's answer, AppStorage is currently of limited use:

types you can use in @AppStorage are (currently) limited to: Bool, Int, Double, String, URL, Data

If you want to use any other type (like Array), you can add conformance to RawRepresentable:

extension Array: RawRepresentable where Element: Codable {
    public init?(rawValue: String) {
        guard let data = rawValue.data(using: .utf8),
              let result = try? JSONDecoder().decode([Element].self, from: data)
        else {
            return nil
        }
        self = result
    }

    public var rawValue: String {
        guard let data = try? JSONEncoder().encode(self),
              let result = String(data: data, encoding: .utf8)
        else {
            return "[]"
        }
        return result
    }
}

Demo:

struct ContentView: View {
    @AppStorage("itemsInt") var itemsInt = [1, 2, 3]
    @AppStorage("itemsBool") var itemsBool = [true, false, true]

    var body: some View {
        VStack {
            Text("itemsInt: (String(describing: itemsInt))")
            Text("itemsBool: (String(describing: itemsBool))")
            Button("Add item") {
                itemsInt.append(Int.random(in: 1...10))
                itemsBool.append(Int.random(in: 1...10).isMultiple(of: 2))
            }
        }
    }
}

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Useful links:

• What is the @AppStorage property wrapper?
• AppStorage Property Wrapper SwiftUI