Dependency-propagating Schedulers

[kabiroberai]

Update: There's now a PR for this: see #86.

Scheduler APIs currently fail to propagate dependencies across the schedule boundary:

mainQueue.schedule {
  // dependencies from outside this closure aren't brought inside
}

values
  .receive(on: mainQueue)
  .sink { ... } // same issue

If swift-dependencies didn't claim to support schedulers at all this would maybe be acceptable, but there's even a mainQueue dependency so it's extra strange that using it doesn't propagate deps. Plus, it'd be nice if we eased the transition from Combine to Concurrency by somewhat supporting Dependencies in the latter.

One approach to solve this is to add a dependencies modifier to Scheduler, similar to animation/transaction:

extension Scheduler {
  /// Propagates dependencies across the scheduler boundary.
  ///
  /// - Parameter update: Enables transforming the propagated dependencies. No-ops by default.
  func dependencies(
    _ update: @escaping (inout DependencyValues) -> Void = { _ in }
  ) -> AnySchedulerOf<Self>
}

Under the hood this would wrap the schedule calls in withEscapedDependencies to propagate the continuation.

We could also make all built-in CombineSchedulers (e.g. UIScheduler) forward dependencies by default, or at least do so for the mainQueue dependency. Would be happy to put up a PR to 1) add Scheduler.dependencies and 2) forward dependencies on mainQueue — curious to hear other people's thoughts.

[stephencelis]

@kabiroberai swift-dependencies depends on combine-schedulers and swift-clocks, and not the other way around, so I'm not sure it's really possible for us to propagate dependencies automatically without combining these libraries.

Your Scheduler.dependencies operation is definitely interesting, though! We'll have a chat about it soon 😄

[kabiroberai]

Yeah agreed that the UIScheduler feature would be a bit tricky and might require some sort of dependency inversion. On the other hand, since we declare the mainQueue dependency inside this package I think we can make that propagate dependencies, and if you have Dependencies as a dependency (hah) you shouldn't be using UIScheduler.shared directly anyway.

[mbrandonw]

Hey @kabiroberai, we think this looks pretty good, so we will probably move forward with it, but we do need a bit more time to review it. We'll be in touch!