Fix exclusive access problem in preview traits. (#306)

* Fix exclusive access problem in preview traits.

* fix

Author: mbrandonw

Sources/Dependencies/Traits/PreviewTrait.swift

@@ -47,7 +47,9 @@
     public static func dependencies(
       _ updateValuesForPreview: (inout DependencyValues) -> Void
     ) -> PreviewTrait {
-      updateValuesForPreview(&previewValues)
+      var copy = previewValues
+      defer { previewValues = copy }
+      updateValuesForPreview(&copy)
       return PreviewTrait()
     }
   }

Tests/DependenciesTests/PreviewTraitsTests.swift

@@ -0,0 +1,21 @@
+#if canImport(Testing) && (os(iOS) || os(macOS) || os(tvOS) || os(watchOS))
+  import Dependencies
+  import Testing
+  import SwiftUI
+
+  @Suite
+  @MainActor
+  struct PreviewTraitsTests {
+    @Test
+    @available(iOS 18, macOS 15, tvOS 18, watchOS 11, visionOS 2, *)
+    func dependency() {
+      _ = PreviewTrait.dependency(\.date.now, Date(timeIntervalSince1970: 1_234_567_890))
+      withDependencies {
+        $0.context = .preview
+      } operation: {
+        @Dependency(\.date.now) var now
+        #expect(now == Date(timeIntervalSince1970: 1_234_567_890))
+      }
+    }
+  }
+#endif