Inconsistent Shared override in #Preview between iOS and macOS

[teleki]

Description

I want to override shared data for the preview. The following code works fine on iOS, but has no effect on macOS. I am on the latest released versions.

The attached screen shots have both the code and the output in the preview.

The iOS version correctly shows ["Mock"] while the macOS version just shows an empty array: [].

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

I expect that I can override the shared data.

Actual behavior

I can override the shared data for #Preview in iOS, but not in macOS.

Reproducing project

iOS: SharingTest.zip

macOS: SharingTestMac.zip

Sharing version information

2.7.2

Destination operating system

iOS 18.6

Xcode version information

16.4

Swift Compiler version information

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

[stephencelis]

As far as I can tell this is one (of many) quirks of the #Preview macro. If you use the old style of preview provider it works as expected:

struct ContentViewPreview: PreviewProvider {
  static var previews: some View {
    @Shared(.runtimeData) var runtimeData = .mock
    ContentView()
      .frame(width: 300, height: 600)
  }
}

Alternately, you could force a mutation of the @Shared value:

 #Preview {
-  @Previewable @Shared(.runtimeData) var runtimeData = .mock
+  @Previewable @Shared(.runtimeData) var runtimeData
+  let _ = $runtimeData.withLock { $0 = .mock }
   ContentView()
     .frame(width: 300, height: 600)
 }

Because this seems to be a bug/behavior with #Preview and not our library, I'm going to convert this to a discussion, but if you file feedback with Apple please let us know!