✨ Adds SharedState.

.swiftpm/xcode/xcshareddata/xcschemes/SharedState.xcscheme

@@ -0,0 +1,71 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Scheme
+   LastUpgradeVersion = "1520"
+   version = "1.7">
+   <BuildAction
+      parallelizeBuildables = "YES"
+      buildImplicitDependencies = "YES">
+      <BuildActionEntries>
+         <BuildActionEntry
+            buildForTesting = "YES"
+            buildForRunning = "YES"
+            buildForProfiling = "YES"
+            buildForArchiving = "YES"
+            buildForAnalyzing = "YES">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SharedState"
+               BuildableName = "SharedState"
+               BlueprintName = "SharedState"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </BuildActionEntry>
+      </BuildActionEntries>
+   </BuildAction>
+   <TestAction
+      buildConfiguration = "Debug"
+      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
+      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      shouldUseLaunchSchemeArgsEnv = "YES">
+      <TestPlans>
+         <TestPlanReference
+            reference = "container:Tests/SharedStateTests/SharedState.xctestplan"
+            default = "YES">
+         </TestPlanReference>
+      </TestPlans>
+   </TestAction>
+   <LaunchAction
+      buildConfiguration = "Debug"
+      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
+      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      launchStyle = "0"
+      useCustomWorkingDirectory = "NO"
+      ignoresPersistentStateOnLaunch = "NO"
+      debugDocumentVersioning = "YES"
+      debugServiceExtension = "internal"
+      allowLocationSimulation = "YES">
+   </LaunchAction>
+   <ProfileAction
+      buildConfiguration = "Release"
+      shouldUseLaunchSchemeArgsEnv = "YES"
+      savedToolIdentifier = ""
+      useCustomWorkingDirectory = "NO"
+      debugDocumentVersioning = "YES">
+      <MacroExpansion>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "SharedState"
+            BuildableName = "SharedState"
+            BlueprintName = "SharedState"
+            ReferencedContainer = "container:">
+         </BuildableReference>
+      </MacroExpansion>
+   </ProfileAction>
+   <AnalyzeAction
+      buildConfiguration = "Debug">
+   </AnalyzeAction>
+   <ArchiveAction
+      buildConfiguration = "Release"
+      revealArchiveInOrganizer = "YES">
+   </ArchiveAction>
+</Scheme>

.swiftpm/xcode/xcshareddata/xcschemes/swift-nibbles-Package.xcscheme

@@ -76,6 +76,20 @@
                ReferencedContainer = "container:">
             </BuildableReference>
          </BuildActionEntry>
+         <BuildActionEntry
+            buildForTesting = "YES"
+            buildForRunning = "YES"
+            buildForProfiling = "YES"
+            buildForArchiving = "YES"
+            buildForAnalyzing = "YES">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SharedState"
+               BuildableName = "SharedState"
+               BlueprintName = "SharedState"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </BuildActionEntry>
       </BuildActionEntries>
    </BuildAction>
    <TestAction
@@ -100,6 +114,16 @@
                ReferencedContainer = "container:">
             </BuildableReference>
          </TestableReference>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "SharedStateTests"
+               BuildableName = "SharedStateTests"
+               BlueprintName = "SharedStateTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
       </Testables>
    </TestAction>
    <LaunchAction

Package.resolved

@@ -0,0 +1,14 @@
+{
+  "pins" : [
+    {
+      "identity" : "swift-concurrency-extras",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/pointfreeco/swift-concurrency-extras",
+      "state" : {
+        "revision" : "bb5059bde9022d69ac516803f4f227d8ac967f71",
+        "version" : "1.1.0"
+      }
+    }
+  ],
+  "version" : 2
+}

Package.swift

@@ -15,9 +15,13 @@ let package = Package(
         .library(name: "Extensions", targets: ["Extensions"]),
         .library(name: "Fuse", targets: ["Fuse"]),
         .library(name: "Identified", targets: ["Identified"]),
+        .library(name: "SharedState", targets: ["SharedState"]),
         .library(name: "Stash", targets: ["Stash"]),
         .plugin(name: "Create TCA Feature", targets: ["Create TCA Feature"])
     ],
+    dependencies: [
+        .package(url: "https://github.com/pointfreeco/swift-concurrency-extras", from: "1.1.0"),
+    ],
     targets: [
         .target(name: "Exchange"),
         .testTarget(name: "ExchangeTests", dependencies: ["Exchange"]),
@@ -31,6 +35,9 @@ let package = Package(
         .target(name: "Identified"),
         .testTarget(name: "IdentifiedTests", dependencies: ["Identified"]),
 
+        .target(name: "SharedState", dependencies: ["Fuse", .product(name: "ConcurrencyExtras", package: "swift-concurrency-extras"),]),
+        .testTarget(name: "SharedStateTests", dependencies: ["SharedState"]),
+
         .target(name: "Stash"),
         .testTarget(name: "StashTests", dependencies: ["Stash"]),
 

Sources/SharedState/SharedState.swift

@@ -0,0 +1,128 @@
+//
+// MIT License
+//
+// Copyright (c) 2024 Connor Ricks
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+import Combine
+import ConcurrencyExtras
+import Foundation
+import Fuse
+
+public typealias SharedStateUpdateBlock<T> = (_ old: T, _ new: T) async -> Void
+
+
+/// A box that encapsulates an object allowing others to subscribe to and monitor changes to the state.
+///
+/// Useful when you have multiple services or features that wish to subscribe to changes.
+///
+/// Frequently use in PointFree's TCA architecture to subsribe long-running effects to shared state changes.
+///
+/// ```
+/// let settings = SharedState(Settings(theme: .dark), onChange: { old, new in
+///     datastore.saveTheme(new)
+/// })
+///
+/// for await theme in settings {
+///     update(for settings)
+/// }
+/// 
+/// for await theme in settings[stream: \.theme] {
+///     update(for theme)
+/// }
+/// ```
+@dynamicMemberLookup
+public struct SharedState<T: Equatable & Sendable>: @unchecked Sendable {
+
+    // MARK: Properties
+
+    private let state: LockIsolated<T>
+    private let subject = PassthroughSubject<T, Never>()
+    private let onChange: SharedStateUpdateBlock<T>?
+
+    // MARK: Initializers
+
+    public init(_ value: T, onChange: SharedStateUpdateBlock<T>? = nil) {
+        self.state = LockIsolated(value)
+        self.onChange = onChange
+    }
+
+    // MARK: Stream
+
+    public subscript<Value>(dynamicMember keyPath: KeyPath<T, Value>) -> Value {
+        self()[keyPath: keyPath]
+    }
+
+    public subscript<Value: Equatable>(
+        stream keyPath: KeyPath<T, Value>,
+        bufferingPolicy: AsyncStream<Value>.Continuation.BufferingPolicy = .unbounded
+    ) -> AsyncStream<Value> {
+
+        return subject
+            .map(keyPath)
+            .filter { state.value[keyPath: keyPath] != $0 }
+            .values(bufferingPolicy: bufferingPolicy)
+            .eraseToStream()
+    }
+
+    // MARK: Publisher
+
+    public subscript<Value: Equatable>(
+        publisher keyPath: KeyPath<T, Value>
+    ) -> AnyPublisher<Value, Never> {
+        return subject
+            .map(keyPath)
+            .filter { state.value[keyPath: keyPath] != $0 }
+            .eraseToAnyPublisher()
+    }
+
+    public var publisher: AnyPublisher<T, Never> {
+        subject.eraseToAnyPublisher()
+    }
+
+    // MARK: Methods
+
+    public func callAsFunction() -> T {
+        state.value
+    }
+
+    private func set(_ newValue: T) async {
+        let oldValue = state.value
+        state.withValue {
+            $0 = newValue
+            subject.send(newValue)
+        }
+        await onChange?(oldValue, newValue)
+    }
+
+    public func stream(bufferingPolicy: AsyncStream<T>.Continuation.BufferingPolicy = .unbounded) -> AsyncStream<T> {
+        subject
+            .filter { state.value != $0 }
+            .values(bufferingPolicy: bufferingPolicy).eraseToStream()
+    }
+
+    public func mutate(_ operation: (inout T) async throws -> Void) async rethrows {
+        var value = self()
+        try await operation(&value)
+        await set(value)
+        // Yielding in order to allow subscribers time to react before proceeding onwards.
+        await Task.yield()
+    }
+}

Tests/SharedStateTests/SharedState.xctestplan

@@ -0,0 +1,25 @@
+{
+  "configurations" : [
+    {
+      "id" : "3AD1685E-93AE-4C99-B1DD-6BFF0C4A1EC1",
+      "name" : "Configuration 1",
+      "options" : {
+
+      }
+    }
+  ],
+  "defaultOptions" : {
+
+  },
+  "testTargets" : [
+    {
+      "parallelizable" : true,
+      "target" : {
+        "containerPath" : "container:",
+        "identifier" : "SharedStateTests",
+        "name" : "SharedStateTests"
+      }
+    }
+  ],
+  "version" : 1
+}