Main actor-isolated static property 'liveValue' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode

[bitomule]

Hello!

I have started a new app and I'm facing some issues with Dependencies and a dependency marked with MainActor. I have a CoreData client marked as MainActor and I'm trying to use it in a dependency like this:

protocol ObserveDuplicateGroupsUseCase {
    func execute() async -> AsyncThrowingStream<[[String]], Error>
}

@MainActor
struct ObserveDuplicateGroupsUseCaseLive: ObserveDuplicateGroupsUseCase, Sendable {
    let client: CoreDataClient
    
    func execute() async -> AsyncThrowingStream<[[String]], Error> {
        // Code fetching data here
    }
}

private enum ObserveDuplicateGroupsUseCaseKey: DependencyKey {
    @MainActor
    static let liveValue: any ObserveDuplicateGroupsUseCase = ObserveDuplicateGroupsUseCaseLive(
        client: CoreDataClient.buildDefault()
    )
}

This logs a warning "Main actor-isolated static property 'liveValue' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode" and removing the MainActor from liveValue transform warning into this error: "Call to main actor-isolated static method 'buildDefault()' in a synchronous nonisolated context".

What's the best way to handle a case like this?

[mbrandonw]

Hi @bitomule, currently @Dependency can be used from basically anywhere, and that means liveValue (and testValue/previewValue) must be non-isolated. You cannot have a dependency that is @MainActor bound because nothing is stopping you from doing:

// Not @MainActor
class Model { 
  @Dependency(\.mainActorClient) var …
}

This would require initializing a @MainActor thing from a non-necessarily @MainActor context.

Theoretically the tools in the library could start holding onto their isolation as (any Actor)?, but it's a big unknown of what that would do to ergonomics, and I don't think Swift's isolation tools are even up for the challenge. You would have static information of the type of isolation for a dependency, but that doesn't say anything about the instance of isolation. Non-global actors can have multiple instances and each instance is a unique isolation domain.

However, typically one does not need to make the dependency itself @MainActor, and instead it can hold onto something that is @MainActor and provide @MainActor endpoints for accessing its state. I would suggest exploring ways of designing your dependency that does not require a @MainActor initializer, and then things should work just fine.