Creating a property wrapper to persist Codable types
In this post, we’ll create a property wrapper that can be used with Codable to automatically persists it in UserDefaults and update SwiftUI when its value changes.
Background
SwiftUI provides us with two great property wrappers for persisting data for the entire app or for the current scene - @AppStorage and @SceneStorage:
struct MyView: View {
@AppStorage("com.danielsaidi.demo.myInt")
private var myInt: Int = 1
@SceneStorage("com.danielsaidi.demo.myDouble")
private var myDouble: Double = 1.0
var body: some View {
Button("\(myInt) \(myDouble)") {
myInt += 1
myDouble += 1
}.buttonStyle(.bordered)
}
}
If you run this code, tapping the button updates the values, which automatically updates the view. Restarting the app restores myInt, which is persisted for the app, but will reset myDouble, since it’s only persisted for the current scene.
These wrappers are easy to use, support optionals and automatically update your views. However, they don’t (yet) support Codable, which may limit you.
We can fix this by defining a new property wrapper that supports Codable, persists data in UserDefaults and updates SwiftUI whenever its value changes:
@propertyWrapper
public struct Persisted<Value: Codable>: DynamicProperty {
public init(
key: String,
store: UserDefaults = .standard,
defaultValue: Value) {
self.key = key
self.store = store
let initialValue: Value? = Self.initialValue(for: key, in: store)
self._value = State(initialValue: initialValue ?? defaultValue)
}
@State
private var value: Value
private let key: String
private let store: UserDefaults
public var wrappedValue: Value {
get {
value
}
nonmutating set {
let data = try? JSONEncoder().encode(newValue)
store.set(data, forKey: key)
value = newValue
}
}
}
private extension Persisted {
static func initialValue<Value: Codable>(
for key: String,
in store: UserDefaults
) -> Value? {
guard let data = store.object(forKey: key) as? Data else { return nil }
return try? JSONDecoder().decode(Value.self, from: data)
}
}
This property wrapper implements DynamicProperty and takes a custom persistency key, a custom UserDefaults instance and a default value to apply when no value is persisted.
The wrapper has a private @State property that trigger updates whenever it changes. It’s initialized with either a previously persisted value or the provided default value.
The wrappedValue brings it all together, by always returning value, but also persisting any new values into persistent storage.
The setter is annotated with nonmutating, which is required if you want to edit state from your views or any other immutable type. Without it, you get the following error:
Cannot assign to property: 'self' is immutable
This Persisted property lets us handle plain values like ints, doubles, strings, bools etc. as well as more complex Codable types.
For instance, here we use the wrapper to persist a codable User:
struct User: Codable {
var age: Int
}
struct ContentView: View {
@Persisted(key: "value", defaultValue: User(age: 1))
private var value: User
var body: some View {
Button("\(value.age)") {
value = User(age: value.age + 1)
}.buttonStyle(.bordered)
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
Conclusion
SwiftUI’s @AppStorage and @SceneStorage are handly property wrappers for persisting data for the entire app or a specific scene. However, their inability to handle Codable is limiting.
The Persisted property wrapper that we implemented in this post can help. You can find it in the SwiftUIKit library. Feel free to try it out and let me know what you think.
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