Could we configure whether @Dependency sets the value during initialization or on access?

[gsabran]

My dependencies behave like shared singleton, and outside of tests/previews they are never changed. Probably pretty common.

When running tests in parallel, I found that I often have to convert my code to set the dependency during initialization and keep a reference to it, instead of using the @Dependency property wrapper that will read the current value on access. The issue with the later is that when tests are running in parallel, the current shared value for a given class might not be the one it had when initialized anymore.

For my use case, I would always prefer for the dependency to be set during initialization, instead of on access. This would solve most of my testing issues. I was wondering if this is something that could be supported without breaking existing apis.

ie I often have to convert this

class MyClass {
  @Dependency(\.networkingService) var networkService
  
  // when called from a test that runs in parallel with other tests, it can be hard to get this to use the desired value
  // or at least it requires some amount of extra `withDependencies`
  func makeApiCall() async throws {
    networkService.call(...)
  }
}

to:

class MyClass {
  init() {
    @Dependency(\.networkingService) var networkService
    self.networkService = networkService
  }
  let networkService: NetworkService
  
  func makeApiCall() async throws {
    networkService.call(...)
  }
}

The later works, but adds some amount of boilerplate I'd love to avoid. Each time I go add it this is after spending time debugging failing / flakey tests that the later behavior would have most often avoided. So I'd like to be able to make this a default.

[mbrandonw]

Hi @gsabran, the library buckets dependencies on a per-test basis and so there should be no bleeding out between tests running in parallel. However, you do have to use the .dependencies test/suite trait as mentioned in the docs:

@Suite(.dependencies)
struct MySuite {
  // …
}