Introduce @SharedReader #2979
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,130 @@ | ||
| import ComposableArchitecture | ||
| import SwiftUI | ||
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| private let readMe = """ | ||
| This application demonstrates how to use the `@SharedReader` tool to introduce a piece of \ | ||
| read-only state to your feature whose true value lives in an external system. In this case, \ | ||
| the state is the number of times a screenshot is taken, which is counted from the \ | ||
| `userDidTakeScreenshotNotification` notification. | ||
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| Run this application in the simulator, and take a few screenshots by going to \ | ||
| *Device › Trigger Screenshot* in the menu, and observe that the UI counts the number of times \ | ||
| that happens. | ||
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| The `@SharedReader` state will update automatically when the screenshot notification is posted \ | ||
| by the system, and further you can use the `.publisher` property on `@SharedReader` to listen \ | ||
| for any changes to the data. | ||
| """ | ||
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| @Reducer | ||
| struct SharedStateNotifications { | ||
| @ObservableState | ||
| struct State: Equatable { | ||
| var fact: String? | ||
| @SharedReader(.screenshotCount) var screenshotCount = 0 | ||
| } | ||
| enum Action { | ||
| case factResponse(Result<String, Error>) | ||
| case onAppear | ||
| } | ||
| @Dependency(\.factClient) var factClient | ||
| var body: some ReducerOf<Self> { | ||
| Reduce { state, action in | ||
| switch action { | ||
| case let .factResponse(.success(fact)): | ||
| state.fact = fact | ||
| return .none | ||
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| case .factResponse(.failure): | ||
| return .none | ||
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| case .onAppear: | ||
| return .run { [screenshotCount = state.$screenshotCount] send in | ||
| for await count in screenshotCount.publisher.values { | ||
| await send(.factResponse(Result { try await factClient.fetch(count) })) | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| struct SharedStateNotificationsView: View { | ||
| let store: StoreOf<SharedStateNotifications> | ||
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| var body: some View { | ||
| Form { | ||
| Section { | ||
| AboutView(readMe: readMe) | ||
| } | ||
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| Text("A screenshot of this screen has been taken \(store.screenshotCount) times.") | ||
| .font(.headline) | ||
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| if let fact = store.fact { | ||
| Text("\(fact)") | ||
| } | ||
| } | ||
| .navigationTitle("Long-living effects") | ||
| .task { await store.send(.onAppear).finish() } | ||
| } | ||
| } | ||
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| extension PersistenceReaderKey where Self == NotificationReaderKey<Int> { | ||
| static var screenshotCount: Self { | ||
| NotificationReaderKey( | ||
| initialValue: 0, | ||
| name: MainActor.assumeIsolated { | ||
| UIApplication.userDidTakeScreenshotNotification | ||
| } | ||
| ) { value, _ in | ||
| value += 1 | ||
| } | ||
| } | ||
| } | ||
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| struct NotificationReaderKey<Value: Sendable>: PersistenceReaderKey { | ||
| let name: Notification.Name | ||
| private let transform: @Sendable (Notification) -> Value | ||
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| init( | ||
| initialValue: Value, | ||
| name: Notification.Name, | ||
| transform: @Sendable @escaping (inout Value, Notification) -> Void | ||
| ) { | ||
| self.name = name | ||
| let value = LockIsolated(initialValue) | ||
| self.transform = { notification in | ||
| value.withValue { [notification = UncheckedSendable(notification)] in | ||
| transform(&$0, notification.wrappedValue) | ||
| } | ||
| return value.value | ||
| } | ||
| } | ||
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| func load(initialValue: Value?) -> Value? { nil } | ||
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| func subscribe( | ||
| initialValue: Value?, | ||
| didSet: @Sendable @escaping (Value?) -> Void | ||
| ) -> Shared<Value>.Subscription { | ||
| let token = NotificationCenter.default.addObserver( | ||
| forName: name, | ||
| object: nil, | ||
| queue: nil, | ||
| using: { notification in | ||
| didSet(self.transform(notification)) | ||
| } | ||
| ) | ||
| return Shared.Subscription { | ||
| NotificationCenter.default.removeObserver(token) | ||
| } | ||
| } | ||
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| static func == (lhs: Self, rhs: Self) -> Bool { | ||
| lhs.name == rhs.name | ||
| } | ||
| func hash(into hasher: inout Hasher) { | ||
| hasher.combine(name) | ||
| } | ||
| } | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,34 @@ | ||
| final class DefaultSubscript<Value>: Hashable { | ||
| var value: Value | ||
| init(_ value: Value) { | ||
| self.value = value | ||
| } | ||
| static func == (lhs: DefaultSubscript, rhs: DefaultSubscript) -> Bool { | ||
| lhs === rhs | ||
| } | ||
| func hash(into hasher: inout Hasher) { | ||
| hasher.combine(ObjectIdentifier(self)) | ||
| } | ||
| } | ||
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| extension Optional { | ||
| subscript(default defaultSubscript: DefaultSubscript<Wrapped>) -> Wrapped { | ||
| get { self ?? defaultSubscript.value } | ||
| set { | ||
| defaultSubscript.value = newValue | ||
| if self != nil { self = newValue } | ||
| } | ||
| } | ||
| } | ||
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| extension RandomAccessCollection where Self: MutableCollection { | ||
| subscript( | ||
| position: Index, default defaultSubscript: DefaultSubscript<Element> | ||
| ) -> Element { | ||
| get { self.indices.contains(position) ? self[position] : defaultSubscript.value } | ||
| set { | ||
| defaultSubscript.value = newValue | ||
| if self.indices.contains(position) { self[position] = newValue } | ||
| } | ||
| } | ||
| } |
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I think something like this could be moved into library code, but for now I've kept it in this case study.