Enable SwiftUI environment-based store injection for @AppSetting

Development/Design Concepts/DESIGN-Container Migration.md

@@ -676,7 +676,7 @@ public class UserDefaultsMigrationManager {
 public protocol UserDefaultsContainer {
     static var version: Int { get }
     static var prefix: String { get }
-    static var store: UserDefaults { get }
+    static var store: any UserDefaultsStore { get }
 
     /// Migration function executed during version transitions
     static func migrate(from oldVersion: Int, to newVersion: Int)
@@ -722,4 +722,4 @@ extension MySettings: UserDefaultsContainer {
     @Setting(key: "user_name") var username: String = "anonymous"  
     // Explicit key: "MyApp.user_name"
 }
-```
+```

README.md

@@ -58,7 +58,43 @@ Or in Xcode:
 - **Observable** — Built-in `Combine` publishers and `AsyncSequence` streams
 - **Customizable** — Namespaced keys and pluggable encoders/decoders
 
-## Quick Start
+## SwiftUI Integration
+
+```swift
+import Settings
+import SwiftUI
+
+// Special mockable Settings container.
+// Uses UserDefaults.standard per default.
+extension AppSettingValues {
+    @Setting public var score: Int = 0
+}
+
+struct AppSettingsView: View {
+    @AppSetting(\.$score) var score
+
+    var body: some View {
+        Form {
+            TextField("Enter your score", value: $score, format: .number)
+                .textFieldStyle(.roundedBorder)
+                .padding()
+
+            Text("Your score was \(score).")
+        }
+    }
+}
+
+#if DEBUG
+import SettingsMock
+
+#Preview {
+    AppSettingsView()
+        .environment(\.userDefaultsStore, UserDefaultsStoreMock())
+}
+#endif
+```
+
+## Custom Settings Container
 
 ```swift
 import Settings
@@ -74,25 +110,6 @@ AppSettings.username = "Alice"
 print(AppSettings.theme) // "light"
 ```
 
-
-## SwiftUI Integration
-
-```swift
-struct SettingsView: View {
-    @State private var theme = AppSettings.theme
-
-    var body: some View {
-        Picker("Theme", selection: $theme) {
-            Text("Light").tag("light")
-            Text("Dark").tag("dark")
-        }
-        .onChange(of: theme) { _, newValue in
-            AppSettings.theme = newValue
-        }
-    }
-}
-```
-
 ## Projected Value ($propertyName)
 
 Access metadata and observe changes:

Sources/Settings/MacroSetting/Attribute.swift

@@ -12,6 +12,8 @@ public protocol __Attribute<Container>: SendableMetatype {
 
     static func reset()
 
+    static var defaultValue: Value { get }
+
     static func registerDefault()
 
     static var stream: AsyncThrowingStream<Value, Error> { get }

Sources/Settings/MacroSetting/AttributeOptional.swift

@@ -1,8 +1,7 @@
 import Combine
 import Foundation
 
-public protocol __AttributeOptional<Container>: __Attribute {
-    associatedtype Value
+public protocol __AttributeOptional<Container>: __Attribute where Value: ExpressibleByNilLiteral {
     associatedtype Wrapped = Value
     associatedtype Encoder = Never
     associatedtype Decoder = Never
@@ -16,6 +15,7 @@ public protocol __AttributeOptional<Container>: __Attribute {
 
 extension __AttributeOptional {
     public static func registerDefault() { /* nothing */ }
+    public static var defaultValue: Value { nil }
 }
 
 // MARK: - Read / Write

Sources/Settings/MacroSettings/Settings_Container.swift

@@ -1,10 +1,8 @@
 import Foundation
+import os
 
 public protocol __Settings_Container {
-    associatedtype Store: UserDefaultsStore
-    associatedtype Observer: Cancellable
-
-    static var store: Store { get }
+    static var store: any UserDefaultsStore { get }
     static var prefix: String { get }
 
     // MARK: - UserDefaults Operations
@@ -35,7 +33,7 @@ public protocol __Settings_Container {
     static func observer(
         forKey: String,
         update: @escaping @Sendable (Any?, Any?) -> Void
-    ) -> Observer
+    ) -> any Cancellable
 
 }
 
@@ -65,7 +63,7 @@ extension __Settings_Container {
 
 extension __Settings_Container {
     public static var prefix: String { "" }
-    public static var store: Foundation.UserDefaults { .standard }
+    public static var store: any UserDefaultsStore { Foundation.UserDefaults.standard }
 }
 
 // MARK: - Default UserDefaults Operations
@@ -159,7 +157,7 @@ extension __Settings_Container {
     public static func observer(
         forKey key: String,
         update: @escaping @Sendable (Any?, Any?) -> Void
-    ) -> some Cancellable {
+    ) -> any Cancellable {
         self.store.observer(forKey: key, update: update)
     }
 }

Sources/Settings/MacroSettings/UserDefaultsStore.swift

@@ -4,7 +4,7 @@ public protocol Cancellable: Sendable {
     func cancel()
 }
 
-public protocol UserDefaultsStore {
+public protocol UserDefaultsStore: Sendable {
 
     associatedtype Observer: Cancellable
 

Sources/Settings/SwiftUI/AppSetting.swift

@@ -1,6 +1,7 @@
 #if canImport(SwiftUI)
 import SwiftUI
 import Combine
+import os
 
 /// A default container for UserDefaults that uses a configurable store.
 ///
@@ -47,13 +48,19 @@ import Combine
 /// // Reset to standard
 /// AppSettingValues.resetStore()
 /// ```
-@MainActor
 public struct AppSettingValues: __Settings_Container {
-
-    fileprivate static var currentStore: any UserDefaultsStore = UserDefaults.standard
-
-    public static var store: any UserDefaultsStore {
-        currentStore
+    private static let _store = OSAllocatedUnfairLock<any UserDefaultsStore>(initialState: UserDefaults.standard)
+    public internal(set) static var store: any UserDefaultsStore {
+        get {
+            _store.withLock { store in
+                store
+            }
+        }
+        set {
+            _store.withLock { store in
+                store = newValue
+            }
+        }
     }
 }
 
@@ -136,26 +143,34 @@ public struct AppSetting<Attribute: __Attribute>: @MainActor DynamicProperty
 where Attribute.Value: Sendable {
 
     @State private var value: Attribute.Value
-    @State private var cancellable: AnyCancellable?
+    @State private var observer: Observer = Observer()
     @Environment(\.userDefaultsStore) private var environmentStore
 
     /// The current UserDefaults value.
     public var wrappedValue: Attribute.Value {
-        get { value }
+        get {
+            value
+        }
         nonmutating set {
-            value = newValue
             Attribute.write(value: newValue)
         }
     }
 
     /// Provides a binding to the UserDefaults value.
     public var projectedValue: Binding<Attribute.Value> {
-        $value
+        Binding(
+            get: {
+                value
+            },
+            set: { value in
+                Attribute.write(value: value)
+            }
+        )
     }
 
     /// Creates a new AppSetting property wrapper for the specified attribute.
     public init(_ attribute: Attribute.Type) {
-        self._value = State(initialValue: Attribute.read())
+        self._value = .init(initialValue: Attribute.defaultValue)
     }
 
     /// Creates a new AppSetting property wrapper from a UserDefaults projected value.
@@ -165,7 +180,7 @@ where Attribute.Value: Sendable {
     /// @MyAppSetting(MyAppSettingValues.$username) var username
     /// ```
     public init(_ proxy: __AttributeProxy<Attribute>) {
-        self._value = State(initialValue: Attribute.read())
+        self._value = .init(initialValue: Attribute.defaultValue)
     }
 
     /// Creates a new AppSetting property wrapper using a key path to an
@@ -186,7 +201,7 @@ where Attribute.Value: Sendable {
     public init(
         _ keyPath: KeyPath<AppSettingValues, __AttributeProxy<Attribute>>
     ) where Attribute.Container == AppSettingValues {
-        self._value = State(initialValue: Attribute.read())
+        self._value = .init(initialValue: Attribute.defaultValue)
     }
 
     /// Creates a new AppSetting property wrapper using a key path to a property
@@ -207,22 +222,37 @@ where Attribute.Value: Sendable {
     public init(
         _ keyPath: KeyPath<Attribute.Container, __AttributeProxy<Attribute>>
     ) {
-        self._value = State(initialValue: Attribute.read())
+        self._value = .init(initialValue: Attribute.defaultValue)
     }
 
     /// Called by SwiftUI to set up the publisher subscription.
     public mutating func update() {
-        AppSettingValues.currentStore = self.environmentStore
+        AppSettingValues.store = environmentStore
+        if observer.cancellable == nil {
+            observer.start(binding: $value)
+        }
+    }
+}
+
+extension AppSetting {
+
+    @MainActor
+    final class Observer {
+        var cancellable: AnyCancellable? = nil
+
+        init() {}
 
-        if cancellable == nil {
-            cancellable = Attribute.publisher
+        func start(binding: Binding<Attribute.Value>) {
+            if cancellable == nil {
+                cancellable = Attribute.publisher
                 .catch { _ in Just(Attribute.read()) }
                 .receive(on: DispatchQueue.main)
-                .sink { [wrappedValue = $value] newValue in
-                    wrappedValue.wrappedValue = newValue
+                .sink { newValue in
+                    binding.wrappedValue = newValue
                 }
+            }
         }
     }
+
 }
-
 #endif

Sources/SettingsClient/main.swift

@@ -9,7 +9,7 @@ import Combine
 // }
 //
 // struct Container<Prefix: ConstString>: __Container {
-//     static var store: UserDefaults { Foundation.UserDefaults.standard }
+//     static var store: any UserDefaultsStore { Foundation.UserDefaults.standard }
 //     static var prefix: String { Prefix.value }
 //
 //     static func clear() {
@@ -133,6 +133,11 @@ try await main1()
 // MARK: - App
 import SwiftUI
 
+extension AppSettingValues {
+    @Setting var user: String?
+    @Setting var theme: String = "default"
+}
+
 // @main
 struct SettingsView: View {
     @Environment(\.userDefaultsStore) var settings

Sources/SettingsMock/UserDefaultsMock.swift

@@ -190,6 +190,7 @@ public final class UserDefaultsStoreMock: NSObject, UserDefaultsStore, Sendable
     public func set(_ url: URL?, forKey key: String) { set(url as Any?, forKey: key) }
 
     public func register(defaults newDefaults: [String : Any]) {
+        print("register(defaults(\(newDefaults)")
         for (key, newDefault) in newDefaults {
             // Only accept property-list-serializable defaults
             guard let copy = plistDeepCopy(newDefault) else { continue }

Tests/SettingsMockTests/UserDefaultsStoreMockTests.swift

@@ -10,7 +10,7 @@ struct UserDefaultsStoreMockTests {
     }
 
     struct TestContainer<Prefix: ConstString>: __Settings_Container {
-        static var store: UserDefaultsStoreMock { UserDefaultsStoreMock.standard }
+        static var store: any UserDefaultsStore { UserDefaultsStoreMock.standard }
         static var prefix: String { Prefix.value }
 
         static func clear() {

Tests/SettingsTests/AttributeCodingTests/AttributeCodingTests.swift

@@ -9,7 +9,7 @@ struct AttributeCodingTests {
     }
 
     struct TestContainer<Prefix: ConstString>: __Settings_Container {
-        static var store: UserDefaults { UserDefaults.standard }
+        static var store: any UserDefaultsStore { UserDefaults.standard }
         static var prefix: String { Prefix.value }
 
         static func clear() {

Tests/SettingsTests/AttributeCodingTests/AttributeCustomCodableTypesTests.swift

@@ -11,7 +11,7 @@ struct AttributeCustomCodableTypesTests {
     }
 
     struct TestContainer<Prefix: ConstString>: __Settings_Container {
-        static var store: UserDefaults { UserDefaults.standard }
+        static var store: any UserDefaultsStore { UserDefaults.standard }
         static var prefix: String { Prefix.value }
 
         static func clear() {

Tests/SettingsTests/AttributeTests/AttributeTests.swift

@@ -10,7 +10,7 @@ struct AttributeTests {
     }
 
     struct TestContainer<Prefix: ConstString>: __Settings_Container {
-        static var store: UserDefaults { UserDefaults.standard }
+        static var store: any UserDefaultsStore { UserDefaults.standard }
         static var prefix: String { Prefix.value }
 
         static func clear() {

Tests/SettingsTests/AttributeTests/DefaultRegistrarTests.swift

@@ -23,7 +23,7 @@ struct DefaultRegistrarTests {
 
         struct TrackedContainer: __Settings_Container {
             static let tracker = RegistrationTracker()
-            static var store: UserDefaults { .standard }
+            static var store: any UserDefaultsStore { Foundation.UserDefaults.standard }
             static var prefix: String { "DefaultRegistrarTest_" }
             static var suiteName: String? { nil }
 
@@ -107,7 +107,7 @@ protocol ConstString {
 }
 
 struct TestContainer<Prefix: ConstString>: __Settings_Container {
-    static var store: UserDefaults { .standard }
+    static var store: any UserDefaultsStore { Foundation.UserDefaults.standard }
     static var prefix: String { Prefix.value }
     static var suiteName: String? { nil }
 

Tests/SettingsTests/UserDefaultObserverTests/UserDefaultObserverTests.swift

@@ -1,10 +1,6 @@
 import Foundation
 import Testing
 
-// This is ugly, but UserDefaults is documented to be thread-safe, so this
-// should be OK.
-extension UserDefaults: @retroactive @unchecked Sendable {}
-
 extension UserDefaults {
     func observeKey<Value: Sendable>(_ key: String, valueType _: Value.Type)
         -> AsyncStream<Value?>