Depency liveValue cannot be accessed in static library

[thd-bs]

Description

When using a static library that defines a new DependencyValues property and defines its TestDependencyKey and DependencyKey conformance in two separate extensions, force-accessing the liveValue from a test using DependencyValues.withValue(\.context, .live) does not work and falls back on the testValue.

This only happens with a static library. Doing this with a dynamic framework works as expected.
It also seems to happen just by running the app and using the dependency (which would make sense).

It seems in _liveValue(_ key: Any.Type) function of the OpenExistential file, that the cast to DependencyKey fails.
Probably linked to how static libraries are compiled, not sure you can do anything about it on your side. Maybe if DependencyKey did not extend TestDependencyKey ? But that's maybe not an option.

I have attached a sample vanilla project that reproduces this issue.

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

Accessing the liveValue from a static library splitting its DependencyKey/TestDependencyKey conformance in 2 extension works.

Actual behavior

Accessing the liveValue from a static library splitting its DependencyKey/TestDependencyKey conformance in 2 extension fallbacks to the testValue.

Steps to reproduce

DependencyIssue.zip

1. Download the attached project.
2. Run the test, it will fail because the testValue is accessed instead of the liveValue, as expected by the forced live context.
3. Run the app, the testValue should also be used.

If you do the same by repackaging the DoSomethingStatic library as a dynamic framework, it works as expected.

The Composable Architecture version information

0.45.0

Destination operating system

iOS 16.1

Xcode version information

Xcode 14.1

Swift Compiler version information

swift-driver version: 1.62.15 Apple Swift version 5.7.1 (swiftlang-5.7.1.135.3 clang-1400.0.29.51)
Target: arm64-apple-macosx12.0

[mbrandonw]

Hi @thd-bs, thank you very much for the easily reproducible sample project. I'm seeing the same. I also tried this with the version of _openExistential that works in 5.6 with the same effect. 🫤

One thing that does fix it is to move the DependencyKey conformance to the same file as the TestDependencyKey conformance. But then that's a bummer because it means you couldn't separate the interface from implementation if you want to use static libraries, but at least it does workaround the issue.

This does feel like a Swift/Xcode bug, but it needs more exploration. Maybe a discussion needs to be opened on the Swift forums or an issue on the Swift repo.

[tgrapperon]

From a quick test, it seems that the compiler thinks that it doesn't need the file and simply doesn't compile it. Adding something in the same file as .liveValue and referencing it in your app's target seems to be enough to make this file build, but I didn't manage to make it works for tests:

extension DoSomethingClient: DependencyKey {
  public static var token: Void = ()

  public static let liveValue: Self = .init(
    doSomething: { print("Do something") }
  )
}

And in your app:

let _ = DoSomethingClient.token

When building a dynamic framework, it doesn't makes assumptions about clients and the file is compiled unconditionally. I don't know if there is a specific attribute to add that could help in this situation.

[mbrandonw]

@tgrapperon Wild stuff. So seems certainly an Apple/Swift bug.

[mbrandonw]

I filed an issue in the Swift repo and feedback on this.

[mbrandonw]

This seems to be expected behavior of static libraries. See here. That comment also provides a few potential work arounds.

Since this isn't a bug with the library I'm going to convert it to a discussion.

[mbrandonw]

FWIW, adding -ObjC to the "other link flags" of the app target does fix it for me.

[thd-bs]

Thanks a lot for the follow-up and the workaround.

[mbrandonw]

I was told to file a feedback to make Xcode/linker automatically add -ObjC when Swift code is detected in a static library. I recommend others duplicate it: FB11788325 (gist).

[acosmicflamingo]

I created this discussion before @tgrapperon pointed me to this ticket. This explains quite a bit :)

[acosmicflamingo]

@tgrapperon should I add something like this to Sources/Dependencies/DependencyValues.swift?

          runtimeWarn(
            """
            "@Dependency(\\.\(function))" has no live implementation, but was accessed from a \
            live context.

            \(dependencyDescription)

            Every dependency registered with the library must conform to "DependencyKey", and \
            that conformance must be visible to the running application.

            To fix, make sure that "\(typeName(Key.self))" conforms to "DependencyKey" by \
            providing a live implementation of your dependency, and make sure that the \
            conformance is linked with this current application.

            If your dependency is actually providing a live implementation and conformance occurs   <==
            within a static library, using the linker flag '-ObjC' will fix the problem.            <==
            """,
            file: DependencyValues.currentDependency.file ?? file,
            line: DependencyValues.currentDependency.line ?? line
          )