Property wrapper that stores values in UserDefaults and works with SwiftUI and Combine.

UserDefault.swift

/**
 * I needed a property wrapper that fulfilled the following four requirements:
 *
 * 1. Values are stored in UserDefaults.
 * 2. Properties using the property wrapper can be used with SwiftUI.
 * 3. The property wrapper exposes a Publisher to be used with Combine.
 * 4. The publisher is only called when the value is updated and not
 *    when_any_ value stored in UserDefaults is updated.
 *
 * First I tried using SwiftUI's builtin @AppStorage property wrapper
 * but this doesn't provide a Publisher to be used with Combine.
 *
 * So I posted a tweet asking people how I can go about creating my own property wrapper:
 * https://twitter.com/simonbs/status/1387648636352348160
 *
 * A lot people replied but I didn't find a solution that was exactly what I wanted. Many suggestions came close
 * and based on those suggestions, I have implemented the property wrapper below.
 *
 * The main downside of this property wrapper is that it inherits from NSObject.
 * That's not very Swift-y but I can live wit that.
 */

// This is our property wrapper. Other types in this gist is just example usages of the property wrapper.
// The type inherits from NSObject to do old-fashined KVO without the KeyPath type.
//
// For simplicity sake the type in this gist only supports property list objects but can easily be combined
// with an approach similar to the one Jesse Squires takes in their Foil framework to support any type:
// https://github.com/jessesquires/Foil
@propertyWrapper
final class UserDefault<T>: NSObject {
    // This ensures requirement 1 is fulfilled. The wrapped value is stored in user defaults.
    var wrappedValue: T {
        get {
            return userDefaults.object(forKey: key) as! T
        }
        set {
            userDefaults.setValue(newValue, forKey: key)
        }
    }
    private(set) var projectedValue: CurrentValueSubject<T, Never>

    private let key: String
    private let userDefaults: UserDefaults
    private var observerContext = 0

    init(wrappedValue defaultValue: T, _ key: String, userDefaults: UserDefaults = .standard) {
        self.key = key
        self.userDefaults = userDefaults
        self.projectedValue = CurrentValueSubject(defaultValue)
        super.init()
        userDefaults.register(defaults: [key: defaultValue])
        // This fulfills requirement 4. Some implementations use NSUserDefaultsDidChangeNotification
        // but that is sent every time any value is updated in UserDefaults.
        userDefaults.addObserver(self, forKeyPath: key, options: .new, context: &observerContext)
        projectedValue.value = wrappedValue
    }

    override func observeValue(
        forKeyPath keyPath: String?,
        of object: Any?,
        change: [NSKeyValueChangeKey : Any]?,
        context: UnsafeMutableRawPointer?) {
        if context == &observerContext {
            projectedValue.value = wrappedValue
        } else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }

    deinit {
        userDefaults.removeObserver(self, forKeyPath: key, context: &observerContext)
    }
}

// Holds a reference to all the values we store in UserDefaults. This isn't necessary but once you start
// having a lot of preferences in your app, you'll probably want to have those in a single place.
struct Preferences {
    private enum Key {
        static let isLineWrappingEnabled = "isLineWrappingEnabled"
    }

    @UserDefault(Preferences.Key.isLineWrappingEnabled) var isLineWrappingEnabled = true
}

// This proves that requirement 3 is fulfilled. We can use properties with Combine.
final class PreferencesViewModel: ObservableObject {
    @Published var preferences = Preferences()

    private var lineWrappingCancellable: AnyCancellable?

    init() {
        lineWrappingCancellable = preferences.$isLineWrappingEnabled.sink { isEnabled in
            print(isEnabled)
        }
    }
}

// This proves that requirement 2 is fulfilled. We can use properties in SwiftUI.
struct PreferencesView: View {
    @ObservedObject private var viewModel: PreferencesViewModel

    var body: some View {
        Toggle("Enable Line Wrapping", isOn: $viewModel.preferences.isLineWrappingEnabled)
    }
}