Dependency overrides in preview traits don't work

[tamasjager]

Description

Hi! I've recently been going through the Sharing related PointFree episodes, in particular this one. When trying to force live dependencies in previews like so

#Preview(
  "Live",
  traits: .dependency(\.context, .live)
) {
  FactFeatureView()
}

the previewValue is still being used (both for the built-in defaultAppStorage, defaultFileStorage and the custom ones).

Tested on Xcode 16.1 and 16.3 with both the unmodified sample code (which uses sharing 1.0.0) and by creating a new project which uses sharing 2.5.0.

(Note that something like

#Preview(
    "Live",
    traits: .dependencies {
        $0.defaultAppStorage = .standard
        $0.defaultFileStorage = .fileSystem
        $0[FactClient.self] = .liveValue
    }
) {
    ContentView()
}

also doesn't seem to work.)

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

Dependencies should be overridable through preview traits.

Actual behavior

Previews use the previewValue no matter the overrides.

Reproducing project

https://github.com/pointfreeco/episode-code-samples/tree/main/0307-sharing-pt3

Sharing version information

2.5.0

Destination operating system

No response

Xcode version information

Xcode 16.3

Swift Compiler version information

[mbrandonw]

Hi @tamasjager, we previously deprecated preview traits (pointfreeco/swift-dependencies#323) because they didn't quite work the way we expected. Then we un-deprecated them (pointfreeco/swift-dependencies#353) because we thought we had a workaround. But we've since then found more problems with preview traits, and you have found a problem too, so we feel that they just may need to be re-deprecated.

A way forward is to simply use prepareDependencies right at the top of your preview:

#Preview {
  let _ = prepareDependencies {
    $0.context = .live
  }
  …
}

Since we plan on deprecating this API, I am going to convert this issue to a discussion. Please feel free to continue the conversation over there!

[tamasjager]

Thank you @mbrandonw! Would it make sense to update the text version of the episode(s) with this for future readers? I understand you guys have a lot of projects running simultaneously, I'm happy to open a PR in case that part of the website is open source.

[OrangeAppleCreations]

I was experiencing the same issue. Your suggestion helped Brandon and still being able to give any Preview a title. Do you expect traits to be the future and is it to insecure at the moment?

[mbrandonw]

Thank you @mbrandonw! Would it make sense to update the text version of the episode(s) with this for future readers? I understand you guys have a lot of projects running simultaneously, I'm happy to open a PR in case that part of the website is open source.

Hi @tamasjager, can you remind me of which episode we use the trait? Most likely the transcript is private, but I can add a correction callout to the transcript.

I was experiencing the same issue. Your suggestion helped Brandon and still being able to give any Preview a title. Do you expect traits to be the future and is it to insecure at the moment?

Hi @OrangeAppleCreations, I don't know what Apple has in store for previews, traits, etc., but the #Preview macro currently has a few issues that make it not always possible to use in place of PreviewProvider. Not only the issue brought up in this discussion, but also since #Preview is a macro you cannot refer to any macro generated code inside #Preview.

The prepareDependencies tool we have in the library was designed specifically for entry points of apps, such as @main, previews and test cases, and so it will always be ok to use in both #Preview's and PreviewProviders.

[tamasjager]

Hi @tamasjager, can you remind me of which episode we use the trait? Most likely the transcript is private, but I can add a correction callout to the transcript.

It's this one towards the very end 🙏

[OrangeAppleCreations]

Thank you @mbrandonw!! I understand that this macro-generated code is not for inspection (yet). Thankfully you guys have options and I can continue on my path. Good luck and until next time.