Add Namespace API (#214)

Author: Kyle-Ye

Sources/OpenSwiftUICore/Data/Namespace.swift

@@ -0,0 +1,72 @@
+//
+//  Namespace.swift
+//  OpenSwiftUICore
+//
+//  Audited for iOS 18.0
+//  Status: Complete
+//  ID: 79F323039D8AB6E63210271E57AD5E86 (SwiftUICore)
+
+/// A dynamic property type that allows access to a namespace defined
+/// by the persistent identity of the object containing the property
+/// (e.g. a view).
+@frozen
+@propertyWrapper
+public struct Namespace: DynamicProperty, Sendable {
+    @usableFromInline
+    var id: Int
+
+    package init(id: Int) {
+      self.id = id
+    }
+
+    @inlinable
+    public init() { id = 0 }
+
+    private struct Box: DynamicPropertyBox {
+
+        typealias Property = Namespace
+
+        var id: Int
+
+        mutating func reset() {
+            id = 0
+        }
+
+        mutating func update(property: inout Property, phase: ViewPhase) -> Bool {
+            let oldID = id
+            if oldID == 0 {
+                id = UniqueID().value
+            }
+            property.id = id
+            return oldID == 0
+        }
+    }
+
+    public static func _makeProperty<V>(
+        in buffer: inout _DynamicPropertyBuffer,
+        container: _GraphValue<V>,
+        fieldOffset: Int,
+        inputs: inout _GraphInputs
+    ) {
+        buffer.append(Box(id: 0), fieldOffset: fieldOffset)
+    }
+
+    public var wrappedValue: Namespace.ID {
+        guard id != .zero else {
+            Log.runtimeIssues("Reading a Namespace property outside View.body. This will result in identifiers that never match any other identifier.")
+            return Namespace.ID(id: UniqueID().value)
+        }
+        return Namespace.ID(id: id)
+    }
+
+    /// A namespace defined by the persistent identity of an
+    /// `@Namespace` dynamic property.
+    @frozen
+    public struct ID: Hashable {
+        package private(set) var id: Int
+
+        package init(id: Int) {
+            self.id = id
+        }
+    }
+}