Initial Commit

.gitignore

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+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc

Package.swift

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+// swift-tools-version: 6.1
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "ArtifactState",
+    products: [
+        // Products define the executables and libraries a package produces, making them visible to other packages.
+        .library(
+            name: "ArtifactState",
+            targets: ["ArtifactState"]),
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package, defining a module or a test suite.
+        // Targets can depend on other targets in this package and products from dependencies.
+        .target(
+            name: "ArtifactState"),
+        .testTarget(
+            name: "ArtifactStateTests",
+            dependencies: ["ArtifactState"]
+        ),
+    ]
+)

Sources/ArtifactState/State.swift

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+
+actor ObservationState {
+    public static var observers: [[any BindingProtocol]] = []
+}
+
+public func observing<T>(into: inout [any BindingProtocol], body: () -> T) -> T {
+    ObservationState.observers.append([])
+    defer { into = ObservationState.observers.popLast()! }
+    return body()
+}
+
+public protocol BindingProtocol {
+    func notify()
+    func subscribe(key: ObjectIdentifier, handler: @escaping () -> Void)
+    func unsubscribe(key: ObjectIdentifier)
+}
+
+public class Binding<T: Equatable>: BindingProtocol {
+    public private(set) var listeners: [ObjectIdentifier: () -> Void] = [:]
+    public var wrappedValue: T { fatalError() }
+    
+    public func notify() {
+        listeners.forEach { $1() }
+    }
+    
+    public func subscribe(key: ObjectIdentifier, handler: @escaping () -> Void) {
+        listeners[key] = handler
+    }
+    
+    public func unsubscribe(key: ObjectIdentifier) {
+        listeners.removeValue(forKey: key)
+    }
+}
+
+@propertyWrapper
+public class State<T: Equatable>: Binding<T> {
+    public var projectedValue: State<T> { self }
+    private var value: T
+    public override var wrappedValue: T {
+        get {
+            if ObservationState.observers.count > 0 {
+                ObservationState.observers[ObservationState.observers.count - 1].append(self)
+            }
+            return value
+        }
+        set {
+            if value != newValue {
+                value = newValue
+                notify()
+            }
+        }
+    }
+    
+    public init(wrappedValue: T) {
+        self.value = wrappedValue
+    }
+}
+
+public class DerivedState<T: Equatable>: Binding<T> {
+    private var value: T
+    public override var wrappedValue: T {
+        get {
+            if ObservationState.observers.count > 0 {
+                ObservationState.observers[ObservationState.observers.count - 1].append(self)
+            }
+            return value
+        }
+    }
+    var deps: [any BindingProtocol] = []
+    var derivation: () -> T
+    private var id: ObjectIdentifier!
+    
+    public init(_ derivation: @escaping () -> T) {
+        self.derivation = derivation
+        value = observing(into: &deps) {
+            derivation()
+        }
+        super.init()
+        id = ObjectIdentifier(self)
+        deps.forEach {
+            $0.subscribe(key: id) { [self] in
+                recompute()
+            }
+        }
+    }
+    
+    deinit {
+        deps.forEach { $0.unsubscribe(key: id) }
+    }
+    
+    func recompute() {
+        deps.forEach { $0.unsubscribe(key: id) }
+        value = observing(into: &deps) {
+            derivation()
+        }
+        deps.forEach {
+            $0.subscribe(key: id) { [self] in
+                recompute()
+            }
+        }
+    }
+}

Tests/ArtifactStateTests/ArtifactStateTests.swift

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+import XCTest
+@testable import ArtifactState
+
+final class ArtifactStateTests: XCTestCase {
+    func testExample() throws {
+        // XCTest Documentation
+        // https://developer.apple.com/documentation/xctest
+
+        // Defining Test Cases and Test Methods
+        // https://developer.apple.com/documentation/xctest/defining_test_cases_and_test_methods
+    }
+}