Documentation Wrong/Misleading Regarding Sendable Conformance

[philipbel]

Description

Reading on TCA/Dependencies, I stumbled upon this requirement for dependencies to be Sendable. Am I missing something?

DependencyKey (via TestDependencyKey) requires that Value conform to Sendable.

Reading the Protocol-based dependencies section of the documentation, this example is wrong (or misleading at the very least):

struct LiveAudioPlayer: AudioPlayer { // 🛑 Type 'any AudioPlayer' does not conform to the 'Sendable' protocol
  let audioEngine: AVAudioEngine // 🛑 Static property 'liveValue' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6
  // ...
}
// ...
private enum AudioPlayerKey: DependencyKey {
  static let liveValue: any AudioPlayer = LiveAudioPlayer()
  // ...
}

LiveAudioPlayer is not Sendable and can't be easily made Sendable, as AVAudioEngine is a class.

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

Documentation should clearly state the Sendable requirements and provide guidelines how to isolate to a global actor, if that makes sense (too new to TCA, not even sure whether isolating to MainActor is possible).

Actual behavior

Dependencies must be Sendable or the compiler generates a warning (presumably an error in Swift 6).

Steps to reproduce

1. Copy-paste the provided examples
2. Generates a warning

Dependencies version information

1.3.2

Destination operating system

iOS 17

Xcode version information

15.4

Swift Compiler version information

▶ xcrun swiftc --version
swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: x86_64-apple-macosx14.0

[mbrandonw]

Hi @philipbel, it is true that dependencies are required to be sendable because ultimately they are stored in a @TaskLocal. However, a lot of the docs were written before concurrency warnings got stricter and before the dust had started to settle on which APIs in Apple's ecosystem would be concurrency safe and which would not.

If AVAudioEngine is truly never going to send sendable then we will need to change the example a bit. The modification is quite easy, you will just wrap the AVAudioEngine in an actor or mutex/lock to protect it, and then access it within the implementation of the dependency.

Since Swift 6 is still not officially out, and who knows how sendability is going to settle in Apple frameworks, I am going to convert this to a discussion for now. As we get closer to the final release of Swift 6 we will update the docs accordingly and make sure it is clear that dependencies must be sendable.

[daksh-dls]

It is out now and please update the docs @mbrandonw
Thanks!

[PereAlmendro]

Hi @mbrandonw @stephencelis

Is there any official solution about dependencies conforming Sendable for swift 6 ?

Thanks in advance.

[mbrandonw]

Dependencies must always conform to Sendable and it has always been that way and will never change. The @TaskLocal forces that and there is no way around it.

The only thing we could do now is update the AudioEngine example in the docs, but regardless… dependencies must always be sendable.